The most active in nucleophilic substitution reactions should be haloalkanes RF, RCl, RBr And R.I., since in their molecules, upon substitution, stable leaving group anion X¯, which is one of the halide ions, that is, the anion of a strong acid. This is confirmed by numerous examples of substitution of halogen atoms in haloalkanes, for example, by hydroxy, alkoxy, amino, cyano, and nitro groups. On the contrary, amines should have the least reactivity, since ammonia and amines are very weak acids and, accordingly, their conjugate bases, that is, anions ¯ NH2,¯ NHR,¯ NR 2 highly reactive and therefore not stable (easily attaches a proton). The hydroxyl group in alcohols can also be replaced in reactions with many nucleophiles, however, under more severe conditions. The alkoxy group is even more difficult to replace. Hydroxyl and alkoxy groups are replaced only in an acidic medium, in which the leaving particle is not an anion, but a molecule (respectively, water or alcohol). The amino group is sufficiently resistant to substitution, cases of its nucleophilic substitution are rare, reactions proceed under very harsh conditions and only for ammonium salts. For this reason, the widest range S N-reactions of haloalkanes (ch. 3.2).
Nucleophilic substitution reactions at sp The 3-hybridized carbon atom is the most studied in organic chemistry. Just as in the case of radical substitution, here it is supposed to break the -bond in the molecule of the original substance, also called the substrate, and the formation of a new -bond in the reaction product. However, nucleophilic substitution refers to reactions of the ionic type, so the molecule of the starting material ( RX) must be polarized, and the substituent X must have a sufficiently high effective electronegativity. The general scheme of the reaction can be represented as follows:
Attack agent Y, called a nucleophile, due to the lone pair of electrons attacks the positively charged center in the substrate molecule. The reaction is accompanied by a heterolytic cleavage of the -bond in the substrate molecule, the substituent X leaves with a pair of electrons. A new covalent bond is formed by a pair of nucleophilic reagent in a coordination way.
A wide variety of particles can be nucleophilic reagents, but they must necessarily have an unshared electron pair. These are, for example, anions HO¯ , RO¯ , ¯ NH2, F¯ , Cl¯ , Br¯ , I¯ , CN¯ , H¯ , ¯ CH2-R and neutral molecules H 2 O, ROH, NH 3, RNH 2, RR'NH, H 2 S, RSH. Nucleophilic properties are also possessed by such compounds as unsaturated and aromatic hydrocarbons.
Substrates can be polar molecules having a carbon atom with an effective positive charge and a substitutable group X. The carbon atom here is called the electrophilic center. Group X also called leaving group or nucleofuge, has a high electronegativity and can leave both as an anion and as an uncharged molecule.
In nucleophilic substitution reactions, depending on the nature of the substrate, nucleophile, leaving group, and reaction conditions, several different mechanisms can be realized. For such reactions, the most common mechanism is the bimolecular nucleophilic substitution, denoted S N 2, and monomolecular nucleophilic substitution, denoted S N 1.
Mechanism of bimolecular nucleophilic substitution
The reaction is bimolecular, because it occurs when two particles collide: a nucleophile and a substrate molecule. The reaction rate in this case depends on the concentration of the substrate and on the concentration of attacking nucleophilic particles. The nucleophile attacks the positively charged center of the substrate molecule from the electrostatically more favorable "rear" side, since in this case it is not affected by the same charge of the nucleofuge. The reaction is a one-step process. Connection C–Y formed at the same time as the rupture C–X connections.
Energy required to break C–X connection, is delivered due to the synchronous process of connection formation C–Y. As soon as the group Y is included in the transition state, the group X must go, since a carbon atom cannot have more than eight electrons in its outer level. In the transition state, the original sp 3-hybridization of the carbon atom changes to sp 2 - hybridization with approximately perpendicular R- orbital. In the transition state, the nucleophilic reagent, the central carbon atom, and the nucleofuge are in a straight line; therefore, if the approach of the nucleophile from the side opposite the leaving group is impossible, for example, due to the structural features of the substrate, the bimolecular reaction also becomes impossible. The three non-reactive substrate groups and the central carbon atom are approximately coplanar, that is, they are in the same plane. They will be strictly coplanar if the incoming and outgoing groups are the same. In other cases, it is possible as an earlier transition state (the bonds of the central carbon atom have not yet adopted a trigonal configuration, the bond order S...X more communication order C...Y) and later.
The stereochemistry of the process of bimolecular nucleophilic substitution can be easily observed using the hydrolysis of an optically active substrate as an example. Three non-reacting groups, when attacked, seem to “turn inside out”. Therefore, sometimes they say about the carbon atom that it “turns out”, but most often they use the term “reversal of the configuration” of the carbon atom, meaning a change in the spatial arrangement of the groups attached to it. Indeed, if the groups X And Y had the same chemical nature (for example, in the isotope exchange reaction upon substitution 35 Cl on 37 Cl), then it would turn out that the reaction product rotates a beam of plane polarized light in the opposite direction compared to the original substance and is its mirror image. This process is compared to turning an umbrella inside out in the wind. This change in configuration is also known as "Walden inversion". All reactions of bimolecular nucleophilic substitution are accompanied by Walden inversion, regardless of the structure of the substrate.
Mechanism of monomolecular substitution
The ideal mechanism S N 1 includes two stages:
The first stage is the slow ionization of the substrate, and it is this stage that determines the rate of the reaction. The dissociation of molecules into free ions is preceded by a transition state in which the bond length increases S–X and the gradual movement of the electron pair to the leaving group. Then an ion pair is formed. Its decomposition into ions occurs almost always with the participation of polar solvent molecules. Practical mechanism S N 1 is carried out easily only in polar solvents. The second stage is the rapid interaction of the intermediate carbocation with the nucleophile.
Thus, the rate of the entire reaction as a whole depends only on the rate of the slowest first stage, in which only substrate molecules take part. Therefore, the reaction is monomolecular, and its rate depends only on the concentration of the initial substrate.
The particle formed as a result of the substitution process X¯ can slow down the reaction rate due to its reversibility. Therefore, in many cases it is possible to add salts containing anions X¯, slow down the reaction. This decrease in the reaction rate caused by the addition X¯ , called common ion effect.
In general, for S N 1-reaction rate should not depend on the nature of the nucleophile and its concentration.
The stereochemistry of monomolecular nucleophilic substitution is less unambiguous than substitution S N 2-type. Ideally, if the process involves the formation of a free carbocation, then the latter should be planar, that is, have a flat configuration corresponding to sp 2 hybridization of orbitals. The nucleophile must attack the carbocation at the same rate from both sides of the plane, which will lead to the formation of two new substrate molecules that are enantiomers with respect to each other. The result is a racemic mixture.
For many reactions, it is fairly easy to assert that, under given conditions, they follow the mechanism of either S N 1 or S N 2. However, in some cases the reaction mechanism is much more difficult to characterize. There are intermediate cases, the so-called "border" region of mechanisms, that is, the reaction mechanism is neither "pure" S N 1, nor "clean" S N 2, but belongs to the intermediate type. This can be represented by the following diagram:
where II is a tight ion pair, III is a loose ion pair, IV and V are dissociated ions, each of which is surrounded by solvent molecules.
Thus, S N 1 and S N 2-reactions can be explained by the ion-pair mechanism. The substrate dissociates with the formation of an intermediate ion pair, which then turns into products. Difference between mechanisms S N 1 and S N 2 is that in the first case, dissociated ions (IV and V) are subjected to nucleophilic attack, and in the second case, the nucleophile attacks systems I, II, and, possibly, III. Since, in the general case, a substrate can be subjected to nucleophilic attack at any stage of the transformation according to the above scheme, it can most often only be argued that one or another mechanism is close to S N 1 or S N 2.
Factors affecting the mechanism and rate of nucleophilic substitution
The same factors can affect the rate of reactions proceeding in a “pure” environment in completely different ways. S N 1or S N 2-mechanism. Therefore, some of them contribute to the flow of reactions according to a monomolecular mechanism, and some - according to a bimolecular one.
1. Influence of substrate structure. An increase in the spatial volume of substituents at the reactive center of the substrate reduces the rate of bimolecular substitution, since the reactive center becomes less accessible to attack by the nucleophile. In this case, during the transition from bromomethane to bromoethane, the rate S N 2-reaction decreases by 145 times, and to 2-bromopropane - by 18,000 times.
bromomethane bromoethane 2-bromopropane
However, the speed S N 1-reactions in this series will increase, since the influence of the electronic effects of substituents in the substrate in most cases is much stronger in monomolecular substitution reactions. Therefore, it is obvious that during the transition from primary systems to secondary and tertiary ones, the rate by this mechanism should increase. This can be explained by the increase in the stability of alkyl cations:
,
depending, in particular, on the number of methyl groups around the positively charged carbon atom, which have an electron-donor inductive effect and therefore compensate for the charge of the reaction center. In the same series, the magnitude of the superconjugation effect also increases R- orbitals of a carbocationic carbon atom with electrons S–N connections. Therefore, a high rate of nucleophilic substitution reactions can be characteristic of both primary and tertiary alkyl halides. In the first case, due to the ease of interaction on S N 2-mechanism (free access of the reaction center, no steric obstacles), in the second - according to S N 1-mechanism (ease of dissociation of substrates, stability of the resulting carbocation). Secondary alkyl halides in most cases should react according to a mixed mechanism, and their reaction rate will be relatively low, since there are obstacles to the flow of both monomolecular and bimolecular substitution.
The introduction of substituents into the substrate molecule will have a different effect on the rate of monomolecular and bimolecular reactions. Electron donor substituents should further stabilize the resulting cation and, consequently, increase the rate of monomolecular substitution.
The influence of the electronic effects of substituents on the rate of a bimolecular reaction is not so unambiguous. But most S N 2-reactions are accelerated by electron-withdrawing substituents - in these cases, the reaction rate is determined by the ease of interaction of the nucleophilic particle with the positively charged reaction center of the substrate. In other cases, the rate of a bimolecular reaction depends on the ability of the nucleofuge group to split off from the reaction center, and the effect of the nature of the substituents will be opposite - the same as in monomolecular reactions.
2. Influence of the nature of the nucleophile. In nucleophilic substitution reactions, virtually any neutral or negatively charged species with a lone pair of electrons can be a nucleophile. The rates of unimolecular reactions do not depend on the nature of the nucleophile, since it does not take part in the limiting step, therefore, the nucleophilicity of the reagent, that is, the ability to provide an electron pair for the formation of a covalent bond when interacting with a positively charged center in the substrate, affects only the rate S N 2 reactions. For these reactions occurring in solution, several basic principles can be noted that determine the effect of the nucleophile on the rate.
First, the nucleophilicity of an anion is always higher than that of the corresponding neutral molecule. So Oh stronger than H2O; ¯ NH2 stronger than NH3 etc.
Second, when comparing nucleophiles whose attacking atoms are in the same period of the periodic table, the order of nucleophilicity is approximately the same as the order of basicity. That's why
R 3 C¯> R2N¯> RO¯> F¯.
Electron donor substituents increase the nucleophilicity of the reagent, so RO¯nucleophilicity is higher than that of HO¯; at RSH- than H 2 S etc. Unstable anions, in particular carbanions, have a high nucleophilicity, since the formation of an anion for a carbon atom is energetically unfavorable (due to its low electronegativity), and therefore such particles have a high potential energy.
Thirdly, nucleophilicity increases from top to bottom in groups of the periodic system (with increasing atomic radius), although basicity decreases in this series. So, the usual order of nucleophilicity of halides is as follows: I¯ >Br¯ >Cl¯ >F¯. Similarly, any sulfur-containing nucleophile is stronger than the corresponding oxygen-containing analogue, and the same is true for compounds containing phosphorus and nitrogen. This is due to the ease of polarization of larger atoms and ions and the lower solvation energy of these ions.
Fourth, the freer the nucleophiles, the greater the rate; solvation slows down the reaction rate. Thus, protic solvents (see below) reduce the nucleophilic strength of the reagent due to the formation of hydrogen bonds if the nucleophilic center is a strongly electronegative element ( F, O, N).
However, these rules cannot take into account all the factors affecting the nucleophilicity of the reagent. Thus, steric hindrances often play a certain role. For example, tert-butylate ion (CH 3) 3 CO¯ is a stronger base than HO or C2H5O¯, but a much less strong nucleophile, since its large spatial volume makes it difficult to approach the substrate closely.
Thus, the activity of the most common nucleophiles decreases in the series (for S N 2-reactions in protic solvents):
RS¯ >C2H5S¯ >I¯ >CN¯ >HO¯ >Br¯ >C2H5O¯ >Cl¯ >CH3COO¯ >H2O.
3. Influence of solvents and catalysts. In monomolecular substitution, at the first stage, ions are formed from a neutral substrate molecule; they are easily solvated by molecules of polar solvents, especially protic ones. Therefore, protic highly polar solvents will promote the reaction according to the mechanism S N 1.
A nucleophilic particle takes part in the rate-limiting step of the bimolecular substitution. Therefore, the use of a protic solvent will lead to its deactivation due to the formation of hydrogen bonds with the hydrogen atoms of the solvent and slow down the reaction. In aprotic solvents, nucleophilic reagents retain high reactivity. In addition, by solvating cations, aprotic solvents promote the dissociation of reagent molecules into ions and, therefore, increase the nucleophilic strength of the reagent. Thus, polar aprotic solvents contribute to S N 2 reactions. Therefore, an acidic environment is generally not conducive to bimolecular reactions, and neutral or basic environments are preferred for these reactions, since strong nucleophiles are usually strong bases.
Lewis acids, i.e. boron, aluminum, iron, zinc, cadmium, mercury, copper and others halides, are used as catalysts in nucleophilic substitution reactions. These substances are capable of accepting anions from solution due to the valence orbitals of the metal, and their use can only slow down the process of bimolecular substitution, but promotes S N 1-process, since it facilitates the dissociation of the substrate, while the catalyst does not interact with the resulting carbocations of the substrate.
4. Influence of the nature of the leaving group. During the reaction, the leaving group is split off along with a pair of electrons.
In reactions carried out by the mechanism of monomolecular substitution, the easier it is for the leaving group to split off, the faster the reaction will go, since it is the breaking of the bond S–X and is the rate-limiting step of the reaction carried out according to the mechanism S N 1. The ease of cleavage is affected not only by the bond dissociation energy, but also by the stability of the nucleofuge group as a free particle. For example, upon detachment of halide ions, the stability of these anions decreases in the series I¯ >Br¯ >Cl¯ >F¯. However, this order is observed in an aprotic medium. On the contrary, in protic solvents or in the presence of acid catalysts, anions of weak acids are most easily eliminated, therefore splitting order will be reversed (HF the weakest acid of all halogenated acids). These reactions do not require strong nucleophiles, but do require substrates to have good leaving groups, so most monomolecular reactions proceed in an acidic environment.
For S N 2-reactions, the nature of the leaving group does not have a noticeable effect on the rate, since the limiting stage here is the formation of a transition state, and the elimination of the substituted group occurs, as a rule, quickly. But groups like Oh,OR,NH2 are difficult to remove, since the carbon-oxygen or carbon-nitrogen bond is quite strong.
Thus, the influence of various factors on the direction and rate of nucleophilic substitution reactions can be reduced to the following main provisions.
Factors contributing to leakageS N 1-reactions:
1) the formation of a stable carbocation,
2) the use of a highly polar protic solvent and acid catalysts,
3) the stability of the leaving group.
The nucleophilicity of the attacking particle is not essential.
Factors contributing to leakage S N 2 reactions:
1) the availability of the electrophilic center of the substrate,
2) the use of an aprotic solvent,
3) high nucleophilicity of the reagent.
The nature of the leaving group is not essential.
Nucleophilic substitution reactions(English) nucleophilic substitution reaction ) - substitution reactions, in which the attack is carried out by a nucleophile - a reagent carrying an unshared electron pair. The leaving group in nucleophilic substitution reactions is called nucleofuge.
All nucleophiles are Lewis bases.
General view of nucleophilic substitution reactions:
R−X + Y− → R−Y + X− (anionic nucleophile) R−X + Y−Z → R−Y + X−Z (neutral nucleophile)
Highlight reactions aliphatic(widespread) and aromatic(not common) nucleophilic substitution.
Aliphatic nucleophilic substitution reactions play an extremely important role in organic synthesis and are widely used both in laboratory practice and in industry.
A coherent theory describing the mechanism of nucleophilic substitution reactions, summarizing the available facts and observations, was developed in 1935 by English scientists Edward Hughes and Christopher Ingold.
Aliphatic nucleophilic substitution reactions
Reactions S N 1
reaction mechanism S N 1 or monomolecular nucleophilic substitution reactions(English) substitution nucleophilic unimolecular ) includes the following stages:
Conditional energy profile of the reaction S N 1
Speed reaction S N 1(in a simplified form) does not depend on the concentration of the nucleophile and is directly proportional to the concentration of the substrate:
Reaction rate = k ×Since a carbocation is formed during the reaction, its attack (under ideal conditions without taking into account the influence of substituents) by the nucleophile can occur from both sides, which leads to racemization of the resulting product.
It is important to keep in mind that S N 1 the mechanism is realized only in the case of relative stability of the intermediate carbocation, therefore, only tertiary ((R) 3 C-X) and secondary ((R) 2 CH-X) alkyl derivatives usually react along this path.
Reactions S N 2
Conditional energy profile of the reaction S N 2
reaction mechanism S N 2 or bimolecular nucleophilic substitution reactions(English) substitution nucleophilic bimolecular ) occurs in one stage, without intermediate formation of intermediate . In this case, the attack of the nucleophile and the elimination of the leaving group occur simultaneously:
R−X + Y − → − → R−Y + X −
An example of a reaction S N 2 is the hydrolysis of ethyl bromide:
The conditional energy profile of the bimolecular nucleophilic substitution reaction is shown in the diagram.
Speed reaction S N 2 depends on both the concentration of the nucleophile and the concentration of the substrate:
Reaction rate = k × × [Y]Since during the reaction the attack by the nucleophile can occur only from one side, the result of the reaction is a stereochemical inversion of the resulting product.
CH 3 -CHBr-CH 3 + HO - → CH 3 -CHOH -CH 3 + Br - Reaction rate = k 1 × + k 2 × ×
Often a mixed mechanism provokes the use ambident nucleophiles, that is, nucleophiles having at least two atoms - donors of electron pairs (for example: NO 2 - , CN - , NCO - , SO 3 2- etc.)
If the substrate contains a substituent located near the attacked atom and carrying a free electron pair, it can significantly increase the rate of the nucleophilic substitution reaction and affect its mechanism (configuration retention). In this case, one speaks of anchimeric assistance neighboring group (for example: COO -, COOR, OCOR, O -, OR, NH 2, NHR, NR 2, etc.)
An example of anchimeric assistance is the hydrolysis of 2-bromopropionate:
Despite the formal (in terms of one-stage) mechanism S N 2, the product formed during the reaction has the same optical configuration as the original.
Reactions S N i
reaction mechanism S N i or intramolecular nucleophilic substitution reactions(English) substitution nucleophilic internal ) proceeds in several stages by analogy with the mechanism S N 1, however, part of the leaving group attacks the substrate, splitting off from the rest.
General reaction scheme:
1. Substrate ionization:
2. Nucleophilic attack:
At the first stage, the substrate dissociates with the formation of the so-called. contact ion pair. The components of such a pair are very close to each other, so the nucleophile is forced to attack from the same side where the leaving group was before.
Reactions proceeding according to the mechanism S N i are extremely rare. One example is the interaction of alcohol with SOCl 2:
It can be seen from the diagram that in the reactions S N i the configuration of the reaction center remains unchanged.
Factors affecting reactivity
Influence of the nature of the nucleophile
The nature of the nucleophile has a significant effect on the rate and mechanism of the substitution reaction. The factor that quantitatively describes this effect is nucleophilicity - a relative value that characterizes the ability of a reagent to influence the rate of a chemical reaction of nucleophilic substitution.
Nucleophilicity - value kinetic, i.e., there is only affects the reaction rate. In this it is fundamentally different from basicity, which is thermodynamic value , and determines the equilibrium position.
Ideally, the nature of the nucleophile does not affect the rate of the S N 1 reaction, since the rate-limiting step of this process does not depend on this. At the same time, the nature of the reagent can affect the course of the process and the final product of the reaction.
For S N 2 reactions, the following principles can be distinguished by which the influence of the nature of the nucleophile is determined:
- A negatively charged nucleophile (eg NH 2 -) is always stronger than its conjugate acid (NH 3), provided that it also exhibits nucleophilic properties.
- When comparing nucleophiles whose attacking atoms are in the same period of the Periodic Table. D. I. Mendeleev, a change in their strength corresponds to a change in their basicity:
- From top to bottom in the periodic table, nucleophilicity generally increases:
- Exception from the previous paragraph:
- The freer the nucleophile, the stronger it is.
- If there are free electron pairs in the position adjacent to the attacked atom, the nucleophilicity increases ( α effect):
It should be borne in mind that the nucleophilicity of various reagents is compared with respect to some chosen standard, provided that the reaction conditions are identical (thermodynamic parameters and solvent). In practice, for S N 2 reactions, the Swen-Scott equation is used:
,Where:
- rate constants of the reaction of the substrate with a given nucleophile and water (or another standard, for example, methanol);
- the parameter of the sensitivity of the substrate to a change in the nucleophile (as a standard nucleophile, when S = 1, choose CH 3 Br or CH 3 I);
is the nucleophilicity parameter.
Leaving group influence
The factor that quantitatively describes the influence of the leaving group is nucleofugity- relative value characterizing the ability of the nucleofuge to influence the rate of the chemical reaction of nucleophilic substitution.
To describe nucleofugity, it is usually difficult to choose one parameter that would exhaustively determine the dependence of the reaction rate on the nature of the leaving group. Often, as a measure of nucleofugity for reactions S N 1 serve as solvolysis constants.
Empirically, one can be guided by the following rule - the easier the leaving group splits off, the more stable it is as an independent particle.
Good nucleofuges are the following groups:
Solvent effect
Obviously, for reactions S N 1, the higher the polarity of the solvent, the higher the rate of the substitution reaction (for neutral substrates). If the substrate carries a positive charge, an inverse relationship is observed - an increase in the polarity of the solvent slows down the reaction. Comparing protic and aprotic solvents, it should be noted that if the solvent is able to form a hydrogen bond with the leaving group, it increases the rate for neutral substrates.
For reactions S N 2 the effect of the solvent is more difficult to assess. If in the transition state the charge is distributed similarly to the initial state or is reduced, aprotic polar solvents slow down the reaction. If such a charge occurs only in the transition state, polar solvents accelerate the reaction. Protic polar solvents are able to form a bond with anions, which makes the reaction difficult.
The size of the attacking atom also affects the reaction rate in aprotic solvents: small atoms are more nucleophilic.
Summarizing the above, we can empirically note that for most substrates, with increasing solvent polarity, the rate S N 1 reactions are growing and S N 2- decreases.
Sometimes the effect of a solvent is evaluated by considering its ionizing power ( Y) using Winstein-Grunwald equation(1948):
where: - solvolysis rate constants of a standard substrate (used as a standard tert-butychloride) in a specified and standard solvent (80% vol. ethanol is used as a standard).
Substrate sensitivity parameter to the ionizing strength of the solvent.
Meaning Y for some solvents: water: 3.493; formic acid: 2.054; methanol: −1.090; ethanol (100%): -2.033; dimethylformamide: −3.500
There is also an alternative I is a parameter introduced in 1969 by Drugar and DeCrook. It is similar Y-factor, but was chosen as a standard S N 2 reaction between three n-propylamine and methyl iodide at 20°C.
Typical aliphatic nucleophilic substitution reactions
| Name | Reaction |
|---|---|
| Nucleophiles: H 2 O, HO -, ROH, RO - | |
| Hydrolysis of alkyl halides | |
| Hydrolysis of acyl halides | |
| Hydrolysis of esters | |
| Alkylation with alkyl halides | |
| Formation and transesterification of ethers | |
| Formation and transesterification of esters | |
| Nucleophiles: RCOOH, RCOO - | |
| Alkylation reactions | |
| Acylation reactions | |
| Nucleophiles: H 2 S, SH -, SR - | |
| Nucleophiles: NH 3 , RNH 2 , R 2 NH | |
| Alkylation of amines | |
| Acylation of amines | |
| Nucleophiles: halogens and halogen derivatives | |
| Halogen exchange reaction | |
| Obtaining alkyl halides from alcohols | |
| Preparation of alkyl halides from ethers and esters | |
| Preparation of acyl halides | |
| Other nucleophiles | |
| Reactions with metals and organometallic compounds | |
| Reactions with an active CH 2 group | |
| Reactions involving the acetylenic group | |
Aromatic nucleophilic substitution reactions
For aromatic systems, electrophilic substitution reactions are more characteristic. As a rule, they enter into nucleophilic substitution reactions only in the case of the action of a strong nucleophile or under rather harsh conditions.
S N Ar reactions (Arene mechanism)
reaction mechanism S N Ar or aromatic nucleophilic substitution reactions(English) substitution nucleophilic aromatic ) is the most important among the reactions of nucleophilic substitution of aromatic compounds and consists of two stages. At the first stage, the nucleophile is added, and at the second stage, the nucleofuge is cleaved off. Otherwise, the mechanism S N Ar called the mechanism attachment-detachment:
Reaction mechanism of aromatic nucleophilic substitution
The intermediate complex formed during the reaction, sometimes quite stable, is called Meisenheimer complex(Meisenheimer).
For a more efficient and gentle reaction according to the mechanism S N Ar the presence of electron-withdrawing substituents (NO 2 , CN, COR, etc.) in the aromatic ring is necessary to stabilize the intermediate.
Reactions S N 1
Reactions with mechanism S N 1 for aromatic compounds, they are extremely rare and, in fact, are characteristic only of diazonium salts:
When aryl halides that do not contain substituents interact with strong bases (for example: NaNH 2), substitution proceeds along aryne mechanism- through the stage of formation of dehydrobenzene:
Before considering each of the classes of acid derivatives separately, it is useful to give a general picture of their behavior, within which it will be easier to consider the rather numerous individual features.
Each derivative is almost always prepared - directly or indirectly - from the corresponding carboxylic acid and can be converted back to the carboxylic acid by simple hydrolysis. An important role in the chemistry of acid derivatives is played by their transformations into each other and into the original acid. In addition, each class has its own characteristic reactions.
Derivatives of carboxylic acids, like the acids themselves, contain a carbonyl group. This group is preserved in the products of most reactions of these compounds and does not undergo visible changes. However, the very presence of this group in the molecule determines the characteristic reactivity of these compounds, and this fact is key to understanding their chemistry.
Acyl compounds (carboxylic acids and their derivatives) usually undergo nucleophilic substitution reactions in which the or groups are replaced by other basic groups. Substitution proceeds much more easily than substitution at a saturated carbon atom; in fact, many of these reactions do not occur at all in the absence of a carbonyl group, such as substitution with
To explain the properties of acyl compounds, it is necessary to turn again to the structure of the carbonyl group. We have already met this group
in the study of aldehydes and ketones (Secs. 19.1 and 19.9) and know what reactions can be expected in general for it.
The carbonyl group carbon is bonded to three other bond atoms; since these bonds use -orbitals (Sec. 2.23), they lie in a plane at an angle of 120° (2.094 rad) to each other. The remaining -orbital of the carbon atom overlaps with the -orbital of the oxygen atom to form a -bond; carbon and oxygen are thus linked by a double bond. The part of the molecule immediately adjacent to the carbon atom of the carbonyl group is planar; oxygen, the carbonyl carbon and the two atoms associated with it lie in the same plane
Both electronic and steric factors make the carbonyl group particularly susceptible to nucleophilic attack at the carbonyl carbon. These factors are as follows: (a) the tendency of oxygen to gain electrons, even if this gives oxygen a negative charge; b) the relative ease of the transition state during the transformation of a trigonal reagent into a tetrahedral intermediate. The same factors make acyl compounds open to nucleophilic attack.
However, acyl compounds differ from aldehydes and ketones in the nature of the second reaction step. A tetrahedral intermediate, derived from an aldehyde or ketone, adds a proton, and an addition product is formed. The tetrahedral intermediate formed from the acyl compound eliminates the group, resulting again in a trigonal compound, and the result of the reaction is substitution.
One can understand why these two classes of compounds behave differently. The ease with which a group is eliminated depends on its basicity: the weaker the base, the more easily this group leaves. For acid chlorides, acid anhydrides, esters and amides, the groups are respectively: a very weak base a moderately weak base and strong bases we know are the strongest bases (note the very low acidity and As a result, in reactions with aldehydes and ketones, addition always occurs instead of elimination.
(see scan)
(see scan)
Thus, the nucleophilic substitution in the acyl group occurs in two stages with the intermediate formation of a tetrahedral intermediate. Usually the overall speed is determined by the speed of both stages, but the first stage is more important.
The rate of the first stage (the formation of a tetrahedral intermediate) is determined by the same factors as the reaction of addition to aldehydes and ketones (Sec. 19.9): it is favored by the effect of electron withdrawal, which stabilizes the resulting negative charge; it is prevented by the presence of bulky groups that create spatial obstacles in the transition state. The ease of the second stage depends on the basicity of the leaving group
Homolytic (radical) reactions
For example, halogenation of alkanes (chain reaction)
CH 4 + Cl 2 hν → CH 3 Cl + HCl (1 stage);
CH 3 Cl + Cl 2 hν → CH 2 Cl 2 + HCl (stage 2);
CH 2 Cl 2 + Cl 2 hν → CHCl 3 + HCl (stage 3);
CHCl 3 + Cl 2 hν → CCl 4 + HCl (stage 4).
Attention! In the substitution reactions of alkanes, hydrogen atoms are most easily replaced at tertiary carbon atoms, then at secondary ones, and, last of all, at primary ones.
CH 3 - CH 2 - CH - CH 3 + Cl 2 hν → a mixture of haloalkanes.
1; 4 - primary; 3 - secondary; 2 - tertiary.
Heterolytic (ionic)
The heterolytic decay of a covalent polar bond leads to the formation of nucleophiles (anions) and electrophiles (cations):
b) H 2 O → H + + OH -
The formed ions enter into further transformations, for example:
CH 3 + + OH - → CH 3 OH
electrophile nucleophile
Ionic reactions are divided according to the nature of the reagent acting on the molecule into electrophilic and nucleophilic.
Electrophile E(loving electrons) – this is a particle that attacks the carbon atom of an organic compound, taking away an electron pair from it (it is an electron acceptor). Examples of particles - electrophiles: H 3 O + , H + , HCl, HNO 3 , NO 2 + , AlCl 3 and others
Nucleophile N(loving the nucleus) is a particle that attacks the carbon atom, providing it with an electron pair (it is an electron donor). Such particles, as a rule, have basic properties. These include: OH -, Cl -, S 2-, NH 3, H 2 O, R-OH, CH 3 O - and others
Nucleophilic reactions- these are reactions of organic substances with nucleophiles, i.e. anions or molecules that provide an electron pair to form a new bond:
CH 3 Br (substrate) + NaOH (nucleophile reagent) → CH 3 OH + NaBr
Electrophilic reactions– reactions of organic compounds with electrophilic reagents, i.e. cations or molecules that have an empty orbital ready to accept an electron pair to form a new bond
C 6 H 6 (substrate) + HO: - NO 2 + (electrophile reagent) → C 6 H 5 - NO 2 + H -OH
Examples of nucleophilic reactions
Nucleophilic substitution:
Not for all reactions it is possible to clearly determine the mechanism by which they proceed, since pure S N 1 or S N 2 are just ideal (limiting) model cases. It should be remembered that the same substrate can react with the same nucleophile, depending on the reaction conditions and solvent, as by the mechanism S N 1, and S N 2.
For example, the rate of hydrolysis of 2-bromopropane is described taking into account the mixed mechanism of its occurrence:
CH 3 -CHBr-CH 3 + HO - → CH 3 -CHOH -CH 3 + Br -
4. Organic alkanes compounds whose molecules consist of carbon and hydrogen are classified as hydrocarbons. If a hydrocarbon molecule contains only simple sigma bonds, and their composition corresponds to the general formula C n H 2 n + 2, then they are classified as limiting, or paraffins. Carbon atoms in alkanes found. in the state of sp 3 hybridization and tetravalent. Alkanes form a series of homologues, in which each subsequent member differs from the previous one by homologous. The difference is the CH2 group.
Isomerism: 1) isomerism of the carbon skeleton;
2) isomerism of the position of the substituent in the carbon chain
Nomenclature: by nomencl. IUPAC names of saturated hydrocarbons are characterized by the suffix -en-. The first four hydrocarbons have trivial names, and starting from the fifth, they are based on the name of the Latin numeral in accordance with the number of at.carbon in the molecule. The names of hydrocarbon radicals are built by replacing the suffix -an on –ill.
General methods of obtaining:
1. Wurtz reaction (reaction of halocarbons with alkaline Me-Li, Na, K)
CH3Br+2Na +Br-CH32NaBr+CH3-CH3
CH3-Cl+2Na+Cl-CH3CH3-CH3+2NaCl
2. Hydrogenation of unpredicted hydrocarbons
CH2=CH2-ethylene
CH2=CH2+H2 Pt,t CH3-CH3-ethane
Physical properties:
1. from C1-C4 gases (b.z)
2. from C5-C22-liquid (spec.z)
3.>C22-solid in-va (b.z)
They are well studied up to C100. Their boiling or melting temperatures in the homologous series increase monotonously with each new –CH2-group (a vivid example of the transition from quantity to quality).
Chemical properties:
1.R-tion mix-I at.N:
A) direct halogenation (F2.Cl2.Br2)
Cl2+CH4 hv HCL+CH3Cl+CH2Cl2+CHCl3+CCl4
Fluorination (explosive)
Chlorination (in the light)
Chlorination mechanism (chain, radical)
Cl2 hv 2Cl .
Cl . +CH4HCl+CH3
CH3+Cl . CH3Cl-chain open
CH3 . +Cl2Cl . +CH3ClHCl+CH2Cl . etc- chain growth
Bromination (heating, receiving light)
B) nitration (-No2 gr.) - district Konovalava
CH3-CH3+NO3 140C H2O+CH3-CH2NO2 nitroethane
C) Sulfochlorination (SO3,Cl2)
CH3-CH3+SO3+CL2 hv HCl+CH3-CH2-SO3Cl sulphachloroethane
D) Kreting
CH3-CH2-CH2-CH3 Pt,t CH2=CH2+CH3-CH3
Use in agriculture: distribution of used oils as herbicides for the destruction of dicotyledonous weeds in crops of cereals and corn. Oil waste has acquired great practical importance in connection with the discovery of the possibility of using them as organic. Substrates in the cultivation of some strains of yeast cultures for the production of dry protein-vitamin concentrates (DVK).
Sp3 hybridization
Occurs when mixing one s- and three p-orbitals, forming four sp3-hybrid orbitals of equal shape and energy. They can form four σ-bonds with other atoms or be filled with lone pairs of electrons.
The axes of sp3-hybrid orbitals are directed to the vertices of a regular tetrahedron. The tetrahedral angle between them is 109°28", which corresponds to the lowest electron repulsion energy. Sp3 orbitals can also form four σ-bonds with other atoms or be filled with lone pairs of electrons. This state is typical for carbon atoms in saturated hydrocarbons and, accordingly, in alkyl radicals and their derivatives.
Examples of compounds that are characterized by sp 3 hybridization: NH 3, POCl 3, SO 2 F 2, SOBr 2, NH 4+, H 3 O +. Also, sp 3 hybridization is observed in all saturated hydrocarbons (alkanes, cycloalkanes) and other organic compounds: CH 4, C 5 H 12, C 6 H 14, C 8 H 18, etc. The general formula of alkanes is: C n H 2n+ 2. The general formula of cycloalkanes is: C n H 2n. In saturated hydrocarbons, all chemical bonds are single, so only σ-overlapping is possible between the hybrid orbitals of these compounds.
sp 3 - Hybridization is characteristic of carbon atoms in saturated hydrocarbons (alkanes) - in particular, in methane
Fig.2 Scheme of the electronic structure of the methane molecule
6.Alkenes are organic compounds whose molecules consist of at.carbon and hydrogen, and in addition to simple sigma-st. content Also double pi-st. Their composition corresponds to the general formula CnH2n. which means cash. In their comp. Molek. Deficiency 2 at. Hydrogen versus alkanes.
Ethylene CH2=CH2
Electronic nature of the double bond: From the point of view of electronic representations, the double bond is carried out by two pairs of electrons belonging to the two bonded carbon atoms. In this case, one pair of electrons forms an ordinary covalent σ-bond, while the second pair of electrons forms a bond of a different nature, the so-called π-bond. The special configuration of the π-bond electron clouds determines the fixation of the directions of the remaining four covalent σ-bonds at these two carbon atoms. These bonds turn out to lie in the same plane and are located at angles of 120 ° relative to each other and relative to the direction of the σ-bond between carbon atoms bound by a double bond. A double bond is not energetically twice as strong as a single bond. The corresponding bond energies for C-C and C=C are 79.3 and 140.5 kcal/mol.
Isomerism:
1.carbon skeleton
CH2=CH-CH2-CH2-CH3 pentene-1
CH2=C(CH3)-CH2-CH3 2-methylbutene-1
2.deputy position
CH2=CH-CH(CH3)-CH3 3-methylbutene-1
3.double bond position in the hydrocarbon chain
CH3-CH=CH-CH2-CH3 pentene-2
4. geometric (cis-, trans-)
Nomenclature:
They use the IUPAC nomenclature. A distinctive feature is the need to choose in the case of alkenes as the main carbon-carbon chain, which includes a double light, but a character. For alkanes, suffixes -vn and for alkenes should be replaced with -en. For example:
Ways to get:
1. Cracking of alkanes. It consists in the thermal decomposition of alkanes with a longer chain of carbon atoms to a mixture of alkanes and unsaturated hydrocarbons with a short chain and hydrogen at 500-700 C and high pressure:
2. Dehydration of alcohols. It proceeds in the presence of a catalyst - aluminum oxide and water-removing agents with obligatory additional heating and in accordance with the scheme (according to the rule of A. Zaitsev: the splitting of water from alcohols occurs with the participation of a hydroxyl group due to the hydrogen atom of the neighboring and least hydrogenated at.carbon) :
3. Dehalogenation of dihalogen derivatives of hydrocarbons proceeds in the presence of active divalent metals (Mg, Zn) when heated, according to the scheme:
4. The reduction of alkynes (saturation of the triple bond with active hydrogen), depending on the type of catalyst used, leads to the formation of cis- or trans-alkenes according to the scheme):
5. Dehydrohalogenation of monohalohydrocarbons with alcohol alkali proceeds at a temp. Boiling alcohol
Sp2 hybridization
Occurs when mixing one s- and two p-orbitals. Three hybrid orbitals are formed with axes located in the same plane and directed to the vertices of the triangle at an angle of 120 degrees. The non-hybrid p-atomic orbital is perpendicular to the plane and, as a rule, participates in the formation of π-bonds
Carbon atoms in the sp 2 -hybrid state form such allotropic forms as graphite, graphene, fullerenes, and other nanostructures.
sp 2 -Hybridization is characteristic of C, N, O, etc. atoms with a double bond (sp 2 -atoms are highlighted in red): H 2 C=C H 2 (animation, 21.3 Kb), H 2 C=C HR, R2 C=N R,
R -N=N -
R, R2 C=O, R -N=O, as well as for cations of the type R 3 C + and free radicals R 3 C.
Electronic model of the ethylene molecule.
Cis-, trans-isomerism on the example of butene -2.
Cis-butene2, trans-butene2
Geometric, or cis-trans-isomerism, is a type of spatial isomerism, depending on the different arrangement of atoms with respect to the plane of the double bond. A dis-isomer is an isomer in which the same atoms (or atomic groups) are located on one side of the plane of the double bond.
8.Alkynes-organic.compounds, the molecules of which comp. Their carbon and hydrogen atoms, and in addition to simple sigma bonds, also contain at least two double pi bonds; one triple bond is present. .hydrogen in the molecule.
Acetylene C2H2(); propyne()
The nature of the triple bond:
In alkynes, the −С≡С− bond is linear (angle 180°) and is in the same plane. Carbon atoms are linked by one σ- and two π-bonds, the maximum electron density of which is located in two mutually perpendicular planes. The length of the triple bond is approximately 0.121 nm, the binding energy is 836 kJ/mol.
The diagram above shows the molecular orbitals of ethylene and acetylene.
Isomerism:
1.carbon skeleton
2.Deputy position
3. Position of the triple bond
Nomenclature: IUPAC. At the same time, the ending-in characterizing the presence of a triple bond:
Ways to get:
1. Dehydrogenation of alkenes
CH2=CH2 ethylene Kt,t H2+HC=CH austylene
2.Double dehydrogemogenation of dihalohydrocarbons (-2HX)
3. Carbide method (only for acytylene)
A)>60% in the chemical industry
B)>30% in technology>3000C
Chemical properties:
1. Substitution reactions of H atoms at at. C with triple St.
A) substitution = for metal (Na, k, Cu)
B) substitution for halogen (Cl, Br)
2. Reactions involving pi-st.
A) join
Connection with water (Kucherova river)
3.R-ii polymerization
A) dimerization
Question 9. Sp-hybridization. electronic model of the mol of acetylene. qualitative reaction to acetylene. sp-hybridization (characteristic of alkynes). Occurs when one s- and one p-orbitals are mixed. Two equivalent sp-atomic orbitals are formed, located linearly at an angle of 180 degrees and directed in different directions from the nucleus of the carbon atom. The two remaining non-hybrid p-orbitals are located in mutually perpendicular planes and participate in the formation of π-bonds, or are engaged in lone pairs of electrons. The C atoms in acetylene are united by three common pairs of electrons, i.e. they are connected by a triple C===C bond. The structure of the mol of acetylene: n-s === s-n. Based on the SP-hybrid state of ATC at the triple bond, the structure of the acetylene molecule can be represented as the result of the overlap of 2 hybrid (s and p x) orbitals from each neighboring ATC. In this case, the hybrid sp-orbitals are located on the same straight line, forming an angle of 180. tet) as a consequence of very weak connection C-H.
Production methods: dehydrogenation of alkenes
Carbide method (for acetylene)
1. Substitution reaction of atH at at C with triple light: a) substitution for Me (Cu, Hg)
(dilution of alkynes Cu-qualitative dilution for these alkynes)
Substitution for halogen (chlorine, bromine)
Question 10. Him sv-va alkenes and alkynes.
Alkenes.1.R-ii additions: a) halogens
This p-ia is a qualitative re-it on a double bond
B) hydrogen halides
Markovnikov's rule: any electrophilic particle joins the molecule of an unsymmetrical alkene at the point of rupture of the p-bond to a more hydrogenated at C, and the process proceeds through the stage of formation of the most stable carbocation.
D) oxidative hydroxylation (R-Wagner)
This p-th is often used to detect a double bond (qualitative p-th)
2. polymerization-p-tion of combining a large number of identical or different molecules into one new large molecule.
3.ATN substitution
Alkynes 1. R-substitution of at H at atC of the triple bond: a) substitution for Me (K, Na, Cu)
B) substitution for halogen
R-ia, taking into account the P-connection a) p-ia of accession
B) connection with NON (Kucherov district)
3. polymerization region: a) dimerization I
B) dimerization II
Their significance: from the individual unsaturated ultraviolet, the biogenic value of ethylene CH2=CH2 should be noted. The processes of ripening of fruits and berries are accompanied by the obligatory formation of ethylene in their peel. The ability of ethylene to stimulate the processes of root formation and cause leaf fall in some plants is noted. Ethylene leads to a noticeable acceleration of their ripening time. The simplest alkyne-acetylene has a similar property.
Question 11. Cycloalkanes are cyclic saturated hydrocarbons. СnH2n-general formula. (see their structure in tete)
The Bayer stress theory of cycles suggests that C atoms in cycloalkanes form a flat closed cycle, in which the bond angles of a simple C-C bond differ from the tetrahedral, least stressed bond angle in a methane molecule. Moreover, the stronger the bond angle in cycloalkanes differs from the tetrahedral one, the stronger their molecular cyclic skeletons are strained. According to Bayer, the voltage value should decrease from cyclopropane to cyclopentane, and then increase again in cyclohexane. The conformation of a molecule is the spatial arrangement of atoms in a molecule of a certain configuration, due to rotation around one or more single sigma bonds. Rings in cycloalkanes (with the exception of cyclopropane) are nonplanar. So, cyclobutane has a slightly swollen shape - one of the carbon atoms is located above or below the plane in which the other three atoms are located, cyclopentane has an envelope or twist conformation, cyclohexane can exist in two chair conformations, upon transition between which (through the bath conformation ) all axial substituents become equatorial and vice versa. For larger rings, the number of conformations increases; therefore, such compounds exist in the form of several interconvertible conformers. Thus, 4 stable conformations are possible for cycloheptane: distorted chair (twist-chair), chair, bath, distorted bath (twist-bath), for cyclooctane - 11 conformations.
Him sv-va: (lecture and textbook)
Q12 Arena- these are hydrocarbon derivatives of benzene, including benzene itself (С6H6). Benzene was first discovered by Faraday.
The simplest representatives (single-core arenas):
Multicore Arenas: Naphthalene C 10 H 8, anthracene C 14 H 10 and etc.
The concept of aromaticity: the term "aromatic compounds" arose long ago due to the fact that some representatives of this series of substances have a pleasant smell. However, at present, a completely different meaning is being put into the concept of "aromaticity". Aromaticity of a molecule means its increased stability due to the delocalization of π-electrons in a cyclic system. Aromatic compounds include benzo and in-va, reminiscent of it in their chemical behavior.
Hückel's rule according to the cat, any organic compound that satisfies the following conditions will be aromatic: 1. the presence of a closed and planar (flat) cycle. 2. continuity of conjugation of P-electrons of all P-bonds. match the formula: 4n+2(n-integer).
Receiving Methods: 1. Dehydrogenation of the corresponding cycloalkanes proceeds successfully over a Pt catalyst at t approx. 300C.
2.aromatization of alkanes
3.trimerization of alkynes
Theory of electrophilic substitution: electrophilic phenomena with a deficit of electrons, as well as acids. Electrophilic substitution reactions are substitution reactions in which the attack is carried out by an electrophile particle that is positively charged or has a deficit of electrons.
1.p-ii substitution of at.H of the benzene ring.
A) halogenation
B) alkylation (r-iya Friedel-Crafts)
2.p-ii connection on the benzene ring
A) hydrogenation.
B) chlorination
3.oxidation of alkyl derivatives of benzene.
Question 13. Alkadienes. Dienes-org.compound-Ia, the cat molecule is composed of at. C and H, and in addition to simple b-bonds, they also contain two double P-sv. Their general formula is СnH2n-2.
Classification dienes is based on the mutual arrangement of 2 double C=C bonds in their molecules. On this basis, they are divided into the next group: 1. cumulated - dienes with a neighboring arrangement of two P-bonds, which have the general formula: R-CH \u003d C \u003d CH2. The simplest representative is allene CH2=C=CH2, which is why they are also called allenes. 2. conjugated dienes with alternating bonds and the general formula: R-CH=CH-CH=CH2. 3. Isolated dienes with a distance between P-St. exceeding one simple b-St., with a total. formula: R-CH=CH-(CH2)n-CH=CH2, where n=1.2 or more.
IUPAC nomencl.: the choice of the main chain of carbon atoms and the numbering of atoms is carried out so that the positions of double bonds are indicated by the smallest numbers, and to indicate the number of double bonds, suff.-diene is used. For example:
The simplest member of conjugated dienes is butadiene: CH2=CH-CH=CH2. Four at.C in butadiene are united by common pairs of electrons, which form two, alternating with a simple b-sv, double P-sv. This is a common distinguishing feature of the structure of the entire class of conjugated dienes. Hydrocarbons with conjugated double bonds are obtained by: 1) dehydrogenation of alkanes contained in natural gas and refinery gases by passing them over a heated catalyst
CH 3 –CH 2 –CH 2 –CH 3 –– ~600°С; Cr 2 O 3, Al 2 O 3- CH 2 \u003d CH–CH \u003d CH 2 + 2H 2
2) dehydrogenation and dehydration of ethyl alcohol by passing alcohol vapor over heated catalysts (method of academician S.V. Lebedev
2CH 3 CH 2 OH - ~ 450 ° С; ZnO, Al2O3 CH 2 \u003d CH–CH \u003d CH 2 + 2H 2 O + H 2,
He was the first to obtain butadiene rubber on the basis of butadiene.
The interaction of two or more neighboring p-bonds with the formation of a single p-electron cloud, resulting in the transfer of the mutual influence of atoms in this system, is called the conjugation effect.
Consider the reactions of halogenation and hydrohalogenation of conjugated dienes. 
Divinyl and isoprene enter into polymerization and copolymerization (ie joint polymerization) with other unsaturated compounds, forming rubbers. Rubbers are elastic high-molecular materials (elastomers), from which rubber is obtained by vulcanization (heating with sulfur). polymerization reactions. Diene hydrocarbons have an extremely important feature: they easily enter into polymerization reactions with the formation of rubber-like high-molecular products. Polymerization reactions proceed with the attachment of molecules to each other in the 1,4- or 1,2-position, as well as with the simultaneous attachment in the 1,4- and 1,2-positions.
Physical Properties
Butadiene is a gas (bp -4.5°C), isoprene is a liquid boiling at 34°C, dimethylbutadiene is a liquid boiling at 70°C. Isoprene and other diene hydrocarbons are able to polymerize into rubber. Natural rubber in its purified state is a polymer with the general formula (C5H8)n and is obtained from the milky sap of some tropical plants.
Question14.reactions of polymerization of diene ultraviolet.
R-ion polymerization leads to the formation of polymers from monomer molecules as a result of breaking the main valences of weak P-bonds and sequentially binding the resulting radicals to each other. Polymerization of diene hydrocarbons. Obtaining synthetic rubber is the main field of application of diene hydrocarbons (mainly butadiene and isoprene). Natural rubber polymer isoprene:n=1000-3000
Synthetic rubber on an industrial scale for the first time according to the method of S. V. Lebedev: It was found that multiple addition of monomeric butadiene-1,3 can occur in positions 1,4- and 1,2- with the formation of a polymer chain that contains double connections. in the presence of metallic sodium.
Rubber is of great importance in the national economy.
polymerization reactions. Diene hydrocarbons have an extremely important feature: they easily enter into polymerization reactions with the formation of rubber-like high-molecular products. Polymerization reactions proceed with the attachment of molecules to each other in the 1,4- or 1,2-position, as well as with the simultaneous attachment in the 1,4- and 1,2-positions. This is how a fragment of the formula for the polymerization product of divinyl (butadiene-1,3) looks like if the attachment of molecules to each other goes to position 1,4.
An analogue of isoprene-chloroprene- easily polymerizes into polychloroprene of the structure:
n (H 2 C \u003d CCl-CH \u003d CH 2) → (-H 2 C-CCl \u003d CH-CH 2 -) 2n
Question 15. Halogenated hydrocarbons– org.compounds formed by replacing one or carried cat.H in a hydrocarbon molecule with a halogen. if, for example, in the molecule of propane, cyclohexane, benzene, only one at. H is replaced by halogen, then we will get a new and trace class of org.
Classification: 1 . According to the number of at.H in the molecule, u / c, substituted for halogen, they are classified into mono-, di-, tri-, tetrahalogen derivatives .. CH3CH2Br-ethyl bromide (mono) CH2C12-methylene chloride (di) ) CC14-carbon tetrachloride (tetra) there are also polyhalogens . 2. Depending on the character of at.C, halogen atoms are connected to the cat, primary R-CH2-Hal, secondary R 2 CH-Na1 and tertiary R 3 C-Na1 halogen derivatives are distinguished. 3. Depending on the relative position of the halogen atoms, they are divided into geminal (when both halogen atoms are at the same C atom) - R-CHC1 2 and vicinal (the halogen atom is found at neighboring C at) -R-CH (C1 )-CH2C1 4. Depending on the type and character of the structure of the skeleton, org molecules: aliphatic (saturated and unsaturated), cycloaliphotic and aromatic. The IUPAC name for a halocarbon is based on the name of the longest straight chain. Carbon atoms are numbered in such a way that the substituent, which is written first in the name, gets the lower number, and the substituents themselves are listed in alphabetical order. The chains of carbon atoms in halogen derivatives of alkenes and alkynes are numbered from the end to which the multiple bond is closer. CHC13-trichloromethane, CH 2 (C1) -CH 2 (C1) -1,2-dichloroethane For some of the simplest halogen derivatives of hydrocarbons, names are retained, which are based on the name of the hydrocarbon residue CH 3 Cl - methyl chloride, CH 3 J - methyl iodide, C 2 H 5 Br - ethyl bromide.
The inductive effect (I-effect) is the transfer of the electronic influence of substituents along the chain of σ-bonds. This effect is transmitted along the chain of σ-bonds with gradual attenuation and, as a rule, after three or four bonds it no longer manifests itself. The direction of the inductive effect of the substituent is qualitatively assessed by comparing with the C-H bond, assuming it to be nonpolar, and the inductive effect of hydrogen to be zero. Electrodrawing substituents reduce the electron density in the system of σ-bonds, and they are called electron-withdrawing substituents. Electron-giving substituents increase the electron density in the chain of σ-bonds in comparison with the hydrogen atom, i.e., they exhibit the +I effect and are electron-donating. These include atoms with low electronegativity (for example, metals), as well as negatively charged atoms or groups that have an excess of electron density, which they tend to redistribute to neighboring bonds. This effect affects the reactivity of org molecules, determining both the rate of p-ii and the direction of attack of the reagent.
Production methods: 1. industrial photochemical halogenation (chlorination or bromination) of alkenes under the action of UV radiation CH4 + C12HC1 + CH3C1-chloromethane. 2. addition of halogens and hydrogen halides at the multiple bond a) CH2 = CH-CH3 (propene) + Br2 CH2 (Br) -CH (Br) -CH3-1,2-dibromopropane b) CH2 = CH2 (ethene) + HC1CH3CH2C1-chloroethane
Chemical properties: 1. hydrolysis solution: R-Hal + MeOH (H 2 O) R-OH + MeHal Nucleophilic substitution of halogen, as established, proceeds through two SN2 mechanisms - second-order nucleophilic substitution (bimolecular) and SN1 – nucleophilic substitution of the first order (monomolecular). The order of the reaction corresponds to the number of reagents, the concentration of which determines the reaction rate.1) SN2 - substitution is most typical for primary alkyl halides. Substitution occurs through an intermediate (activated complex) in one step.
2) SN1 is a mechanism typical of tertiary alkyl halides and allyl-type halides, in which the dissociation of the C−Hal bond in the first stage leads to stable carbocations.
R-and hydrogen halide elimination, nucleophilic substitution in other reactions (see account s129
Question16.Comparative characterization of the chemical sv-in alif-them and aroma-their halocarbons
Question 17. Alcohols and phenols
*
Alcohols are such hydroxyl compounds, in which the OH-gr. is never connected to at. C of the benzene ring. СnH 2 n +1 OH-general formula. Classification: Polyatomic (2 or more he-gr.) And monoatomic (one OH-gr) divided into primary, secondary, tertiary.
Isomerism: all types of carbon skeleton, from the position of he-gr. in the carbon chain (pentanol-2, pentanol-3). Nomenclature yupak: addition to the name of the parent-th u/v suff.-ol. If there are older f-ii in alcohol, then OH-gr is denoted by a prefix (oxy) numbering is carried out closer to the end where it-gr is located.
Ways to get: 1. hydration of alkenes (i.e. + water) under the action of t and H3PO4: CH2 \u003d CH2 (ethylene) + NOH CH3-CH2OH-ethanol. 2.hydrolysis of monohalohydrocarbons CH3-CH2Br+ + H2O HBr+CH3-CH2OH(ethanol). 3. oxidation of alkanes (-water) CH3-CH2-CH3+O2 CH3-CH-CH3-propanol-2
Chemical sv-va: 1.r-ii substitution of atH in OH-g
2.OH-gr substitution
3.dehydration
4.oxidation
*
Phenols are hydroxyl compounds; in a cat, OH-gr. is always connected to at. C of the benzene ring.
Nomenclature:
Obtaining methods:1. Dow process
2. R-I Sergeeva
Chi.sv-va: 1.r-and substitute OH-gr no
2.r-and substitute at. H in OH-gr
3.R-and replacing atn benz rings
Question 18. Polyhydric alcohols and phenols.
* Polyhydric alcohols are those containing 2 or more functional OH-g in the cos-ve mol-ly. Depending on the number of OH-gr, they are divided into two-, three-, four-atomic ones. Dihydric alcohols (glycols) are unstable, at the moment they lose water moles and turn into aldehydes, ketones and to-you.
Chemical St. Islands. They react with alkalis to form salts. For example, ethylene glycol reacts not only with alkali metals, but also with heavy metal hydroxides: 
Glycols with alcohols form products of mono-(alcohol ethers) and disubstitution (ethers):
Physical properties: colorless syrupy liquids of a sweetish taste, soluble in water, poorly soluble in organic solvents; have high boiling points. For example, tboil ethylene glycol 198°C, density= 1.11 g/cm 3 ; tboil of glycerin = 290°С, raft=1.26g/cm 3 .
quality response.
* Phenols are hydroxyl compounds, in the cat OH-gr. is always connected to at. C of the benzene ring.
According to the number of OH-groups, all phenols are subdivided into one-, two-, three-atomic ones.
Chemical properties 1.p-and substitute OH-g no
2.r-and substitute at. H in OH-gr
3.R-and replacing atN of the benz-th ring.
a) mutual With alkalis
b) r-ii substitute. At H benz-th ring
Physical properties: Most phenols are colorless solids. Phenol melts at t°=41°C. The presence of water in phenol lowers its melting point. A mixture of phenol with water at room temperature is liquid, has a characteristic odor. When heated to 70 ° C, it dissolves completely. Phenol is an antiseptic, its aqueous solution is used for disinfection and is called carbolic acid.
Qualitative response: In water p-rach odnoat. phenols interact with iron (III) chloride to form complex phenolates that have a purple color; the color disappears after the addition of hydrocyanic acid. 6C 6 H 5 OH + FeCl 3 \u003d H 3 + 3HCl
Question 19 p-tion on polyhydric alcohols and primary amino group
Qualitative response to polyat. alcohols: Substituting atN in glycols for heavy metal ions leads to the image of brightly colored blue intracomplex chelate-type compounds. Freshly precipitated copper hydroxide with glycols gives:
Qualitative reaction to the primary amino group: Alkylation- When amino acids react with an excess of alkyl halide, exhaustive alkylation of the amino group occurs and internal salts are formed. 
Question 20
Question 21
Aldehydes-carbonyl-e comp., content-ie aldehyde-yu gr.
Ketones are organic compounds in which the carbonyl group is bonded to two hydrocarbon radicals.
Question 22. Monocarboxylic acids. Isomerism, nomenclature, methods of obtaining. The structure of the carboxyl group, chemical St. Islands.
Monocarboxylic acids - monobasic carboxylic acids contain one carboxyl group associated with a hydrocarbon radical (saturated, unsaturated, aromatic).
Ways to get: 1. Oxidation of the corresponding aldehydes.
2. Hydrolysis of hymenial trihalohydrocarbons.
3. Hydrolysis of nitriles.
The structure of the carboxyl group: 
Chemical properties: 1.R-ii substitute. At H OH-gr. Interaction with alkalis. (p-I neutralization).
2.R-ii substitute. OH-gr. a) Images of esters:
b) image-e of anhydrides:
c) image-e of acid halides:
3.Loss of OH-g
4. R-th by radical R
Question 23. How does the acidity of carboxylic acids depend on the size and character of the radical. How does the presence of acceptor substituents and their position in the mole affect? Justify the answer.
Question 24. Functional derivatives of carboxylic acids: salts, esters, anhydrides and acid halides, amides, nitriles. Receiving and St. Islands.
Carbonic to-you show high reactivity. They enter into regions with various things and form a variety of compounds, among the cat. big value have functional derivatives, i.e. compounds obtained as a result of r-th on the carboxyl group.
1. Formation of salts. a) when interacting with metals: 2RCOOH + Mg ® (RCOO) 2Mg + H2
b) in reactions with metal hydroxides: 2RCOOH + NaOH ® RCOONa + H2O
2. Formation of esters R "–COOR": (r-th esterification) Esters of lower carb. to-t and the simplest monoat. alcohols - volatile colorless. liquid with a characteristic fruity odor. Diff. esters of higher carb. to-t - colorless. Tv. things, melt temp. depends both on the lengths of the carbon chains of the acyl and alcohol residues and on their structure. 
3. Formation of anhydrides. Acetic anhydride is a colorless, transparent mobile liquid with a pungent odor. .
Acetic anhydride is often used in acylation reactions.
4. Formation of acid halides. Acid halides are highly reactive substances and are widely used in organic synthesis to introduce an acyl group (acylation reaction). 
5. Formation of amides. Formic amide to-you are a liquid, amides of all other to-t are white crystals of things. The lower amides are highly soluble in water. Aqueous solutions of amides give a neutral solution for litmus .
The most important property of amides is their ability to hydrolyze in the presence of acids and alkalis 
6. Formation of nitriles(dehydration of amides)
25. Dicarboxylic acids- organ. Compounds containing two carboxyl groups in their molecules. Formula:HOOC-R-COOH. Ways to get. When ethylene glycol is oxidized, oxalic acid HO-CH 2 CH 2 -OH is formed
Hydrolysis of dinitriles: N \u003d C-CH 2 CH 2 -C \u003d NNH 2 OC-CH 2 CH 2 -CONH 2 HOOC-CH 2 CH 2 -COOH
Oxidation of cyclic ketones with conc nitric acid
Chemical properties HOOC -R- COOH + CH 3 OHHOOC - R- C-O-CH 3 + H 2 O
HOOC-R-C-OCH 3 + CH 3 OH CH 3 -O-C-R-C-O-CH 3
Heating oxalic and malonic to-t with the release of CO 2 and the formation of monocarboxylic to-t
HOOC-COOH HCOOK + CO 2 HOOC-CH 2 -COOH CH 3 - COOH +CO 2
Malonic acid and its diethyl ether.
Terephthalic acid - solid crystalline, high melting point, white in color; obtained by oxidation of n-xylene
Unlimited to-you contain in their composition one or more multiple bonds. They are characterized by all known p-ii for the carboxyl group and all p-ii inherent in the compounds. ethylene series. Acrylic acid - the 1st member of the homomol of the non-precursor series, is obtained by the oxidation of acrolein
In the industry from ethylene oxide and hydrocyanic to-you
Mutual influence P– connections and A - the position of the radical organ and P- the C=O bond of the carboxyl group leads to the polarization of the first one, which causes the orientation of the attached halohydrocarbons against the Markovnikov rule:
Acrylonitrile is a product of a large-tonnage chemical synthesis; teach by dehydration of oxynitrile or catalyte by adding hydrocyanic acid to acetylene at 80 C
Fumaric and maleic acids are isomeric compounds obtained by dehydration of malic acid.
Geometric isomers also form long-chain higher fatty acids. For example, liquid and oily oleic acid, in the process of slow heating in the presence of catalytic amounts of NO 2, isomerizes with the image of an already solid under normal conditions trans-isomer - elaidine acid structure:
26. The most important functional groups in org chemistry. Alcohols are derivatives of hydrocarbons, containing one or more hydroxyl groups-OH at saturated carbon atoms in the molecule. An oxygen-containing organ of the compound. By structure, they are divided into saturated (alkanols) CH 3 OH methanol, aromatic - phenyl methanol, unsaturated: alkenols CH 2 \u003d CH-CH 2 OH propen-2-ol-1, alkinols NS C-CH 2 OH Propin-2-ol-1 qualities p-I on monohydric primary alcohols - copper oxide (calcined copper wire). Alcohols are oxidized to aldehydes, and a layer of reduced copper is formed on the wire.
CH3 - CH2 - OH + CuO \u003d CH3 - COH + Cu + H2O on polyatomic
The simplest qualitative reaction to alcohols is the oxidation of alcohol with copper oxide. To do this, alcohol vapor is passed over hot copper oxide. Then the resulting aldehyde is captured with fuchsine sulfuric acid, the solution turns purple:
CH 3 -CH 2 -OH + CuO -t-> CH 3 -CHO + Cu + H 2 O Alcohols are identified by the Lucas test - conc. solution of hydrochloric acid and zinc chloride. When a secondary or tertiary alcohol is passed into such a solution, an oily precipitate of the corresponding alkyl chloride is formed:
CH 3 -CHOH-CH 3 + HCl -ZnCl 2 -> CH 3 -CHCl-CH 3 ↓ + H2O
Primary alcohols do not react. Another well-known method is the iodoform test:
CH 3 -CH 2 -OH + 4I 2 + 6NaOH --> CHI 3 ↓ + 5NaI + HCOONa + 5H 2 O Qualitative reactions to polyhydric alcohols.
The most well-known qualitative reaction to polyhydric alcohols is their interaction with copper (II) hydroxide. The hydroxide dissolves, forming a dark blue chelate complex. Please note that, unlike aldehydes, polyhydric alcohols react with copper (II) hydroxide without heating. For example, when glycerin is added, copper (II) glycerate is formed: 
Carbonyl compounds (oxo compounds) - these are hydrocarbon derivatives containing a carbonyl group C=O in the molecule
Oxosoed are divided into aldehydes and ketones. Aldehydes are an organ of a compound, the molecules of which contain an aldehyde group associated with a hydrocarbon radical C = O R-I with an ammonia solution of silver oxide (1) and an alkaline solution of copper sulfate (2) are high-quality solutions.
CH 3 -CHO + 2OH -t->CH 3 -COOH + 2Ag ↓ + 4NH 3 + H 2 O silver mirror reaction
During the reaction, methane acid is oxidized to carbonic acid, which decomposes into carbon dioxide and water:
HCOOH + 2OH -t-> CO 2 + 2H 2 O + 4NH 3 + 2Ag↓
In addition to the silver mirror reaction, there is also a reaction with copper (II) hydroxide Cu(OH) 2 . To do this, aldehyde is added to freshly prepared copper (II) hydroxide and the mixture is heated:
CuSO 4 + 2NaOH --> Na 2 SO 4 + Cu(OH) 2 ↓
CH 3 -CHO + 2Cu(OH) 2 -t-> CH 3 -COOH + Cu 2 O↓ + 2H 2 O
Copper oxide (I) Cu 2 O precipitates - a red precipitate. Another method for determining aldehydes is the reaction with an alkaline solution of potassium tetraiodomercurate (II), known to us from a previous article as Nessler's reagent:
CH 3 -CHO + K 2 + 3KOH --> CH 3 -COOK + Hg↓ + 4KI + 2H 2 carboxylic acids- these are derivatives of hydrocarbons, containing the function of the carboxyl group COOH. Formula: Depending on the structure of the hydrocarbon radical, carboxylic acids are divided into saturated R = alkyl, unsaturated - derivatives of unsaturated hydrocarbons, aromatic. Nitrogen-containing organic compounds. Amines are derivatives of ammonia (NH 3), in the molecule of which one, two or three hydrogen atoms are replaced by hydrocarbon radicals. Amino acids are hydrocarbon derivatives containing amino groups (-NH 2) and carbox groups Qualitative reactions to amines.
There are no qualitative reactions to amines (with the exception of aniline). You can prove the presence of an amine by staining blue litmus. If amines cannot be detected, then it is possible to distinguish the primary amine from the secondary one by interacting with nitrous acid HNO 2. First you need to cook it, and then add the amine:
NaNO 2 + HCl --> NaCl + HNO 2 Primary give nitrogen N 2: CH 3 -NH 2 + HNO 2 --> CH 3 -OH + N 2 + H 2 O Secondary - alkylnitrosamines - substances with a pungent odor (for example, dimethylnitrosoamine ):
CH 3 -NH-CH 3 + HNO 2 --> CH 3 -N (NO) -CH 3 + H 2 O Tertiary amines do not react with HNO 2 under mild conditions. Aniline forms a precipitate when bromine water is added:
C 6 H 5 NH 2 + 3Br 2 --> C6H 2 NH 2 (Br) 3 ↓ + 3HBr
Hydroxy acids
Hydroxy acids are called organic carboxylic acids,
groups. The number of carboxyl groups determines the basicity of the hydroxy acid. By
the number of hydroxyls, including those that are part of the carboxyl groups,
determine the atomicity of hydroxy acids.
The simplest hydroxy acids are commonly referred to as their natural
source.
For example:
Lactic acid is a monobasic, diatomic acid;
was discovered by Sheele in sour milk, from which it got its name.
HOOC-CH2-CH COOH
Oh-apple, dibasic, tribasic
(found in apples).
HOOC-CH-CH COOH
HO OH-tartaric acid, dibasic, tetrahydric
(was isolated from the "tartar" - waste obtained during the manufacture
and aging of grape wines).
HOOC-CH2-C CH2COOH
Citric acid, tribasic,
tetraatomic, was isolated from lemon leaves.
Very often, hydroxy acids are named as hydroxy derivatives.
corresponding carboxylic acids. Depending on the position
hydroxy groups in relation to carboxyl distinguish α-, β-, γ-, etc.
hydroxy acids (hydroxy - according to IUPAC).
How to get
1. Hydrolysis of halogenated acids.
This is a convenient way to synthesize α-hydroxy acids, due to the availability of α-
halogenated acids.
2. Obtaining from aldehydes, ketones(cyanohydrin synthesis, obtaining α-
hydroxy acids).
3. Recovery of oxo acids.
4. Isamino acids.
5. From unsaturated acids.
6. Oxidation of hydroxyaldehydes (aldols) and glycols.
For example, with silver oxide in an ammonia solution, hydroxy acids are obtained
with a different structure depending on the position of the respective groups
in the original connection:
7. Oxidation of acids with a tertiary carbon atom located in α-
position to the carboxyl.
8. Hydrolyzlactones.
cyclohexanone-caprolactone 6-hydroxyhexanoic acid
Many of the hydroxy acids are obtained by specific methods or
extracted from plant and animal products.
One of the most common and important mechanisms of organic transformations is nucleophilic substitution at a saturated carbon atom. As a result of this process, $Z$ leaving groups in $RZ$ organic substrates containing $C_sp3-Z$ bonds are replaced by $Nu$ nucleophilic reagents: in such a way that non-shared pairs of nucleophiles in the $RNu$ reaction products become electronic pairs of $\sigma$-bonds $C-Nu$, and electron pairs of $s$-bonds $C-Z$ become lone pairs of split-off leaving groups:
Leaving groups $Z$ are often called nucleofuges ("mobile in the form of nucleophiles"). A good leaving group has high nucleofugity, a poor leaving group has low nucleofugity. The groups with high nucleofugity include the triflate (OTf) group, which leaves in the form of $Z^-=CF_3SO_3^-$ anions, as well as fluorosulfonate $FSO_3^-$, p-toluenesulfonate or tosylate (OTs-), etc. low nucleofugity groups include the acetate group, $(RCOO^-)$ carboxylate ions, and $F^-$.
Nucleophilic substitution reactions are classified according to the change in charges in the substrates or nucleophiles and according to the type of substitution mechanisms.
Classification of nucleophilic substitution reactions according to the charge criterion
According to the charging characteristic, such reactions are divided into four groups.
Cationic substrates - neutral nucleophiles
$Nu: + RZ^+ \to Nu^+-R + Z:^-$
For example:
Cationic substrates - anionic nucleophiles
$Nu:^- + RZ^+ \to NuR + Z:$
For example:
Interaction of neutral substrates with neutral nucleophiles
$Nu: + RZ \to Nu^+-R + Z^-$
For example:
Interactions of neutral substrates with anionic nucleophiles
$Nu:^- + RZ \to NuR + Z:^-$
For example:
Replacing one halogen with another
Isotopic and group exchanges
Remark 1
It follows from the above list of reactions that, with the help of various nucleophilic substitution reactions, it is possible to synthesize practically any class of compounds of the aliphatic series.
Classification of nucleophilic substitution reactions according to the type of reaction mechanism
Depending on the types of mechanisms of nucleophilic substitution reactions, they can be divided into bimolecular ones, which are denoted as $S_N2$. As well as monomolecular, which are designated as $S_N1$.
In addition, organic reactions can be divided into three categories:
- isomerization and rearrangement,
- dissociation and recombination,
- substitutions.
In this classification, reactions proceeding by the SN2 mechanism belong to the third category, and reactions proceeding by the SNl mechanism belong to the second:
Significance of nucleophilic substitution
The study of the mechanisms of nucleophilic substitution plays an exceptional role in the development of ideas about the reactions of organic chemistry, and at the same time they represent the most detailed types of transformations studied. Research into the mechanisms of nucleophilic aliphatic substitution began in the mid-1930s by two prominent scientists, K. K. Ingold and E. D. Hughes. They own brilliant fundamental works that make up the golden fund of organic chemistry. Subsequently, the studies of Ingold and Hughes were significantly modified and their theories underwent a number of changes. But the proposal by these scientists to classify substitution mechanisms into $S_N2-$ and $S_N1-$ types is still relevant and fair.
