Alkenes (olefins, ethylene hydrocarbons C n H 2n
Homologous series.
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ethene (ethylene) | |
The simplest alkene is ethylene (C 2 H 4). According to the IUPAC nomenclature, the names of alkenes are formed from the names of the corresponding alkanes by replacing the suffix “-ane” with “-ene”; The position of the double bond is indicated by an Arabic numeral.
Hydrocarbon radicals formed from alkenes have the suffix "-enil". Trivial names: CH 2 =CH- "vinyl", CH 2 =CH-CH 2 - "allyl".
The carbon atoms at the double bond are in a state of sp² hybridization and have a bond angle of 120°.
Alkenes are characterized by isomerism of the carbon skeleton, double bond positions, interclass and spatial.
Physical properties
The melting and boiling points of alkenes (simplified) increase with molecular weight and length of the carbon backbone.
Under normal conditions, alkenes from C 2 H 4 to C 4 H 8 are gases; from pentene C 5 H 10 to hexadecene C 17 H 34 inclusive - liquids, and starting from octadecene C 18 H 36 - solids. Alkenes are insoluble in water, but are highly soluble in organic solvents.
Dehydrogenation of alkanes
This is one of the industrial methods for producing alkenes
Hydrogenation of alkynes
Partial hydrogenation of alkynes requires special conditions and the presence of a catalyst
A double bond is a combination of sigma and pi bonds. A sigma bond occurs when sp2 orbitals overlap axially, and a pi bond occurs when there is lateral overlap.
Zaitsev's rule:
The abstraction of a hydrogen atom in elimination reactions occurs predominantly from the least hydrogenated carbon atom.
13. Alkenes. Structure. sp 2 hybridization, multiple coupling parameters. Reactions of electrophilic addition of halogens, hydrogen halides, hypochlorous acid. Hydration of alkenes. Morkovnikov's rule. Mechanisms of reactions.
Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n
One s- and 2 p-orbitals mix and form 2 equivalent sp2-hybrid orbitals located in the same plane at an angle of 120.
If a bond is formed by more than one pair of electrons, then it is called multiple.
A multiple bond is formed when there are too few electrons and bonding atoms for each bond-forming valence orbital of the central atom to overlap with any orbital of the surrounding atom.
Electrophilic addition reactions
In these reactions, the attacking particle is an electrophile.
Halogenation:
Hydrohalogenation
Electrophilic addition of hydrogen halides to alkenes occurs according to Markovnikov’s rule
Markovnikov rule
Addition of hypochlorous acid to form chlorohydrins:
Hydration
The addition of water to alkenes occurs in the presence of sulfuric acid:
Carbocation- a particle in which a positive charge is concentrated on the carbon atom; the carbon atom has a vacant p-orbital.
14. Ethylene hydrocarbons. Chemical properties: reactions with oxidizing agents. Catalytic oxidation, reaction with peracids, oxidation reaction to glycols, with cleavage of the carbon-carbon bond, ozonation. Wacker process. Substitution reactions.
Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n
Oxidation
Oxidation of alkenes can occur, depending on the conditions and types of oxidizing reagents, both with the cleavage of the double bond and with the preservation of the carbon skeleton.
When burned in air, olefins produce carbon dioxide and water.
H 2 C=CH 2 + 3O 2 => 2CO 2 + 2H 2 O
C n H 2n+ 3n/O 2 => nCO 2 + nH 2 O – general formula
Catalytic oxidation
In the presence of palladium salts, ethylene is oxidized to acetaldehyde. Acetone is formed from propene in the same way.
When alkenes are exposed to strong oxidizing agents (KMnO 4 or K 2 Cr 2 O 7 in H 2 SO 4), the double bond breaks when heated:
When alkenes are oxidized with a dilute solution of potassium permanganate, dihydric alcohols are formed - glycols (E.E. Wagner reaction). The reaction takes place in the cold.
Acyclic and cyclic alkenes, when reacting with peracids RCOOOH in a non-polar environment, form epoxides (oxiranes), therefore the reaction itself is called the epoxidation reaction.
Ozonation of alkenes.
When alkenes interact with ozone, peroxide compounds are formed, which are called ozonides. The reaction of alkenes with ozone is the most important method for the oxidative cleavage of alkenes at the double bond
Alkenes do not undergo substitution reactions.
Wacker process-the process of producing acetaldehyde by direct oxidation of ethylene.
The Wacker process is based on the oxidation of ethylene with palladium dichloride:
CH 2 = CH 2 + PdCl 2 + H 2 O = CH 3 CHO + Pd + 2HCl
15. Alkenes: chemical properties. Hydrogenation. Lebedev's rule. Isomerization and oligomerization of alkenes. Radical and ionic polymerization. The concept of polymer, oligomer, monomer, elementary unit, degree of polymerization. Telomerization and copolymerization.
Hydrogenation
Hydrogenation of alkenes directly with hydrogen occurs only in the presence of a catalyst. Hydrogenation catalysts include platinum, palladium, and nickel.
Hydrogenation can also be carried out in the liquid phase with homogeneous catalysts
Isomerization reactions
When heated, isomerization of alkene molecules is possible, which
can lead to both double bond movement and skeletal changes
hydrocarbon.
CH2=CH-CH2-CH3 CH3-CH=CH-CH3
Polymerization reactions
This is a type of addition reaction. Polymerization is the reaction of sequential combination of identical molecules into larger molecules, without isolating any low-molecular-weight product. During polymerization, a hydrogen atom is added to the most hydrogenated carbon atom located at the double bond, and the rest of the molecule is added to the other carbon atom.
CH2=CH2 + CH2=CH2 + ... -CH2-CH2-CH2-CH2- ...
or n CH2=CH2 (-CH2-CH2-)n (polyethylene)
A substance whose molecules undergo a polymerization reaction is called monomer. A monomer molecule must have at least one double bond. The resulting polymers consist of a large number of repeating chains having the same structure ( elementary units). The number showing how many times a structural (elementary) unit is repeated in a polymer is called degree of polymerization(n).
Depending on the type of intermediate particles formed during polymerization, there are 3 polymerization mechanisms: a) radical; b) cationic; c) anionic.
The first method produces high-density polyethylene:
The reaction catalyst is peroxides.
The second and third methods involve the use of acids (cationic polymerization) and organometallic compounds as catalysts.
In chemistry oligomer) - a molecule in the form of a chain of small number of identical constituent links.
Telomerization
Telomerization is the oligomerization of alkenes in the presence of chain transfer agents (telogens). As a result of the reaction, a mixture of oligomers (telomeres) is formed, the end groups of which are parts of telogen. For example, in the reaction of CCl 4 with ethylene, the telogen is CCl 4 .
CCl 4 + nCH 2 =CH 2 => Cl(CH 2 CH 2) n CCl 3
The initiation of these reactions can be carried out by radical initiators or g-radiation.
16. Alkenes. Reactions of radical addition of halogens and hydrogen halides (mechanism). Addition of carbenes to olefins. Ethylene, propylene, butylenes. Industrial sources and main uses.
Alkenes readily add halogens, especially chlorine and bromine (halogenation).
A typical reaction of this type is the discoloration of bromine water
CH2=CH2 + Br2 → CH2Br-CH2Br (1,2-dibromoethane)
Electrophilic addition of hydrogen halides to alkenes occurs according to Markovnikov’s rule:
Markovnikov rule: When adding protic acids or water to unsymmetrical alkenes or alkynes, hydrogen is added to the most hydrogenated carbon atom
A hydrogenated carbon atom is one that has hydrogen attached to it. Most hydrogenated - where there is most H
Carbene addition reactions
CR 2 carbenes: - highly reactive short-lived species that can easily add to the double bond of alkenes. As a result of the carbene addition reaction, cyclopropane derivatives are formed
Ethylene is an organic chemical described by the formula C 2 H 4. Is the simplest alkene ( olefin)compound. Under normal conditions, it is a colorless flammable gas with a faint odor. Partially soluble in water. Contains a double bond and therefore belongs to unsaturated or unsaturated hydrocarbons. Plays an extremely important role in industry. Ethylene is the most produced organic compound in the world: Ethylene oxide; polyethylene, acetic acid, ethyl alcohol.
Basic chemical properties(don’t teach me, just let them be there just in case, in case they can write it off)
Ethylene is a chemically active substance. Since there is a double bond between the carbon atoms in the molecule, one of them, which is less strong, is easily broken, and at the site of the bond break the attachment, oxidation, and polymerization of molecules occurs.
Halogenation:
CH 2 =CH 2 + Br 2 → CH 2 Br-CH 2 Br
Bromine water becomes discolored. This is a qualitative reaction to unsaturated compounds.
Hydrogenation:
CH 2 =CH 2 + H - H → CH 3 - CH 3 (under the influence of Ni)
Hydrohalogenation:
CH 2 =CH 2 + HBr → CH 3 - CH 2 Br
Hydration:
CH 2 =CH 2 + HOH → CH 3 CH 2 OH (under the influence of a catalyst)
This reaction was discovered by A.M. Butlerov, and it is used for the industrial production of ethyl alcohol.
Oxidation:
Ethylene oxidizes easily. If ethylene is passed through a solution of potassium permanganate, it will become discolored. This reaction is used to distinguish between saturated and unsaturated compounds. Ethylene oxide is a fragile substance; the oxygen bridge breaks and water joins, resulting in the formation of ethylene glycol. Reaction equation:
3CH 2 =CH 2 + 2KMnO 4 + 4H 2 O → 3HOH 2 C - CH 2 OH + 2MnO 2 + 2KOH
C 2 H 4 + 3O 2 → 2CO 2 + 2H 2 O
Polymerization (production of polyethylene):
nCH 2 =CH 2 → (-CH 2 -CH 2 -) n
Propylene(propene) CH 2 = CH-CH 3 - unsaturated (unsaturated) hydrocarbon of the ethylene series, flammable gas. Propylene is a gaseous substance with a low boiling point t boil = −47.6 °C
Typically, propylene is isolated from oil refining gases (during cracking of crude oil, pyrolysis of gasoline fractions) or associated gases, as well as from coal coking gases.
Chemical properties of alkanes
Alkanes (paraffins) are non-cyclic hydrocarbons in whose molecules all carbon atoms are connected only by single bonds. In other words, there are no multiple - double or triple bonds - in alkane molecules. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).
Due to saturation, alkanes cannot undergo addition reactions.
Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the C-H bonds in their molecules are extremely low-polar. In this regard, for alkanes, reactions proceeding through the radical substitution mechanism, denoted by the symbol S R, are more typical.
1. Substitution reactions
In reactions of this type, carbon-hydrogen bonds are broken
RH + XY → RX + HY
Halogenation
Alkanes react with halogens (chlorine and bromine) when exposed to ultraviolet light or high heat. In this case, a mixture of halogen derivatives with varying degrees of substitution of hydrogen atoms is formed - mono-, ditri-, etc. halogen-substituted alkanes.
Using methane as an example, it looks like this:
By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that a specific halogen derivative of methane predominates in the composition of the products.
Reaction mechanism
Let us analyze the mechanism of the free radical substitution reaction using the example of the interaction of methane and chlorine. It consists of three stages:
- initiation (or chain nucleation) is the process of formation of free radicals under the influence of external energy - irradiation with UV light or heating. At this stage, the chlorine molecule undergoes homolytic cleavage of the Cl-Cl bond with the formation of free radicals:
Free radicals, as can be seen from the figure above, are atoms or groups of atoms with one or more unpaired electrons (Cl, H, CH 3, CH 2, etc.);
2. Chain development
This stage involves the interaction of active free radicals with inactive molecules. In this case, new radicals are formed. In particular, when chlorine radicals act on alkane molecules, an alkyl radical and hydrogen chloride are formed. In turn, the alkyl radical, colliding with chlorine molecules, forms a chlorine derivative and a new chlorine radical:
3) Break (death) of the circuit:
Occurs as a result of the recombination of two radicals with each other into inactive molecules:
2. Oxidation reactions
Under normal conditions, alkanes are inert towards such strong oxidizing agents as concentrated sulfuric and nitric acids, potassium permanganate and dichromate (KMnO 4, K 2 Cr 2 O 7).
Combustion in oxygen
A) complete combustion with excess oxygen. Leads to the formation of carbon dioxide and water:
CH 4 + 2O 2 = CO 2 + 2H 2 O
B) incomplete combustion due to lack of oxygen:
2CH 4 + 3O 2 = 2CO + 4H 2 O
CH 4 + O 2 = C + 2H 2 O
Catalytic oxidation with oxygen
As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.
For example, methane, depending on the nature of the catalyst, can be oxidized into methyl alcohol, formaldehyde or formic acid:
3. Thermal transformations of alkanes
Cracking
Cracking (from English to crack - to tear) is a chemical process occurring at high temperatures, as a result of which the carbon skeleton of alkane molecules breaks down with the formation of alkene molecules and alkanes with lower molecular weights compared to the original alkanes. For example:
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH 2 -CH 2 -CH 3 + CH 3 -CH=CH 2
Cracking can be thermal or catalytic. To carry out catalytic cracking, thanks to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.
Dehydrogenation
The elimination of hydrogen occurs as a result of the cleavage of C-H bonds; carried out in the presence of catalysts at elevated temperatures. When methane is dehydrogenated, acetylene is formed:
2CH 4 → C 2 H 2 + 3H 2
Heating methane to 1200 °C leads to its decomposition into simple substances:
CH 4 → C + 2H 2
When the remaining alkanes are dehydrogenated, alkenes are formed:
C 2 H 6 → C 2 H 4 + H 2
When dehydrogenating n-butane, butene-1 and butene-2 are formed (the latter in the form cis- And trance-isomers):
Dehydrocyclization
Isomerization
Chemical properties of cycloalkanes
The chemical properties of cycloalkanes with more than four carbon atoms in their rings are, in general, almost identical to the properties of alkanes. Oddly enough, cyclopropane and cyclobutane are characterized by addition reactions. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:
Chemical properties of alkenes
1. Addition reactions
Since the double bond in alkene molecules consists of one strong sigma and one weak pi bond, they are fairly active compounds that easily undergo addition reactions. Alkenes often undergo such reactions even under mild conditions - in the cold, in aqueous solutions and organic solvents.
Hydrogenation of alkenes
Alkenes are capable of adding hydrogen in the presence of catalysts (platinum, palladium, nickel):
CH 3 -CH = CH 2 + H 2 → CH 3 -CH 2 -CH 3
Hydrogenation of alkenes occurs easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process occurs at a higher temperature and lower pressure.
Halogenation
Alkenes easily undergo addition reactions with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow bromine solutions lose their color, i.e. become discolored.
CH 2 =CH 2 + Br 2 → CH 2 Br-CH 2 Br
Hydrohalogenation
As is easy to see, the addition of a hydrogen halide to a molecule of an unsymmetrical alkene should, theoretically, lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:
However, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with Markovnikov’s rule:
The addition of a hydrogen halide to an alkene occurs in such a way that a hydrogen is added to a carbon atom with a greater number of hydrogen atoms (more hydrogenated), and a halogen is added to a carbon atom with a fewer number of hydrogen atoms (less hydrogenated).
Hydration
This reaction leads to the formation of alcohols, and also proceeds in accordance with Markovnikov’s rule:
As you can easily guess, due to the fact that the addition of water to an alkene molecule occurs according to Markovnikov’s rule, the formation of a primary alcohol is possible only in the case of ethylene hydration:
CH 2 =CH 2 + H 2 O → CH 3 -CH 2 -OH
It is through this reaction that the bulk of ethyl alcohol is carried out in large-scale industry.
Polymerization
A specific case of an addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through the free radical mechanism:
Oxidation reactions
Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:
C n H 2n + (3/2) nO 2 → nCO 2 + nH 2 O
Unlike alkanes, alkenes are easily oxidized. When alkenes are exposed to an aqueous solution of KMnO 4, discoloration occurs, which is a qualitative reaction to double and triple CC bonds in molecules of organic substances.
Oxidation of alkenes with potassium permanganate in a neutral or weakly alkaline solution leads to the formation of diols (dihydric alcohols):
C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)
In an acidic environment, the double bond is completely broken and the carbon atoms that formed the double bond are converted into carboxyl groups:
5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)
If the double C=C bond is located at the end of the alkene molecule, then carbon dioxide is formed as a product of oxidation of the outermost carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, easily oxidizes itself in an excess of oxidizing agent:
5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)
The oxidation of alkenes in which the C atom at the double bond contains two hydrocarbon substituents produces a ketone. For example, the oxidation of 2-methylbutene-2 produces acetone and acetic acid.
The oxidation of alkenes, in which the carbon skeleton is broken at the double bond, is used to determine their structure.
Chemical properties of alkadienes
Addition reactions
For example, the addition of halogens:
Bromine water becomes discolored.
Under normal conditions, the addition of halogen atoms occurs at the ends of the 1,3-butadiene molecule, while the π-bonds are broken, bromine atoms are added to the extreme carbon atoms, and the free valences form a new π-bond. Thus, a “movement” of the double bond occurs. If there is an excess of bromine, another molecule can be added at the site of the formed double bond.
Polymerization reactions
Chemical properties of alkynes
Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of undergoing addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.
Halogenation
Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are capable of attaching either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds through an electrophilic mechanism sequentially in two stages:
Hydrohalogenation
The addition of hydrogen halide molecules also occurs via an electrophilic mechanism and in two stages. In both stages, the accession proceeds in accordance with Markovnikov’s rule:
Hydration
The addition of water to alkynes occurs in the presence of ruti salts in an acidic medium and is called the Kucherov reaction.
As a result of hydration, the addition of water to acetylene produces acetaldehyde (acetic aldehyde):
For acetylene homologues, the addition of water leads to the formation of ketones:
Hydrogenation of alkynes
Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, and nickel are used as catalysts:
Trimerization of alkynes
When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:
Dimerization of alkynes
Acetylene also undergoes a dimerization reaction. The process takes place in the presence of copper salts as catalysts:
Alkyne oxidation
Alkynes burn in oxygen:
C nH 2n-2 + (3n-1)/2 O 2 → nCO 2 + (n-1)H 2 O
Reaction of alkynes with bases
Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom at the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:
HC≡CH + 2NaNH 2 → NaC≡CNa + 2NH 3 ,
and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:
Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.
It should be noted that all silver and copper acetylenides are explosive substances.
Acetylenides are capable of reacting with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:
CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3
CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr
Chemical properties of aromatic hydrocarbons
The aromatic nature of the bond influences the chemical properties of benzenes and other aromatic hydrocarbons.
The unified 6pi electron system is much more stable than ordinary pi bonds. Therefore, substitution reactions rather than addition reactions are more typical for aromatic hydrocarbons. Arenes undergo substitution reactions via an electrophilic mechanism.
Substitution reactions
Halogenation
Nitration
The nitration reaction proceeds best under the influence of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:
Alkylation
A reaction in which one of the hydrogen atoms at the aromatic ring is replaced by a hydrocarbon radical:
Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric halides or inorganic acids can be used as catalysts.<
Addition reactions
Hydrogenation
Chlorine addition
Proceeds via a radical mechanism upon intense irradiation with ultraviolet light:
A similar reaction can only occur with chlorine.
Oxidation reactions
Combustion
2C 6 H 6 + 15O 2 = 12CO 2 + 6H 2 O + Q
Incomplete oxidation
The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . There is no reaction.
Substituents on the benzene ring are divided into two types:
Let us consider the chemical properties of benzene homologues using toluene as an example.
Chemical properties of toluene
Halogenation
The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. This is often what is observed during its halogenation. We already know that benzene undergoes a substitution reaction with chlorine via an electrophilic mechanism, and to carry out this reaction it is necessary to use catalysts (aluminum or ferric halides). At the same time, methane is also capable of reacting with chlorine, but via a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it is subjected to chlorination, can give either products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as for the chlorination of benzene, or products of substitution of hydrogen atoms in the methyl radical, if it, how chlorine acts on methane under ultraviolet irradiation:
As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.
If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichloro-substituted toluene is possible:
Similarly, when toluene is chlorinated in the light at a higher chlorine/toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:
Nitration
The replacement of hydrogen atoms with a nitro group during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids leads to substitution products in the aromatic ring rather than the methyl radical:
Alkylation
As already mentioned, the methyl radical is an orienting agent of the first kind, therefore its alkylation according to Friedel-Crafts leads to the substitution products in ortho- and para-positions:
Addition reactions
Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):
C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O
Incomplete oxidation
When exposed to an oxidizing agent such as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic core cannot oxidize under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.
Characteristic chemical properties of hydrocarbons: alkanes, alkenes, dienes, alkynes, aromatic hydrocarbons
Alkanes
Alkanes are hydrocarbons in whose molecules the atoms are connected by single bonds and which correspond to the general formula $C_(n)H_(2n+2)$.
Homologous series of methane
As you already know, homologues- these are substances that are similar in structure and properties and differ by one or more $CH_2$ groups.
Saturated hydrocarbons make up the homologous series of methane.
Isomerism and nomenclature
Alkanes are characterized by so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. As you already know, the simplest alkane, which is characterized by structural isomers, is butane:
Let's take a closer look at the basics of IUPAC nomenclature for alkanes:
1. Selecting the main circuit.
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis.
2.
The atoms of the main chain are assigned numbers. The numbering of the atoms of the main chain begins from the end to which the substituent is closest (structures A, B). If the substituents are located at an equal distance from the end of the chain, then numbering starts from the end at which there are more of them (structure B). If different substituents are located at equal distances from the ends of the chain, then numbering begins from the end to which the senior one is closest (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins appears in the alphabet: methyl (—$CH_3$), then propyl ($—CH_2—CH_2—CH_3$), ethyl ($—CH_2—CH_3$ ) etc.
Please note that the name of the substituent is formed by replacing the suffix -an to suffix -il in the name of the corresponding alkane.
3. Formation of the name.
At the beginning of the name, numbers are indicated - the numbers of the carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma ($2.2-$). After the number, the number of substituents is indicated with a hyphen ( di- two, three- three, tetra- four, penta- five) and the name of the deputy ( methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane ( methane, ethane, propane, etc.).
The names of substances whose structural formulas are given above are as follows:
— structure A: $2$ -methylpropane;
— structure B: $3$ -ethylhexane;
— structure B: $2,2,4$ -trimethylpentane;
— structure G: $2$ -methyl$4$-ethylhexane.
Physical and chemical properties of alkanes
Physical properties. The first four representatives of the homologous series of methane are gases. The simplest of them is methane, a colorless, tasteless, and odorless gas (the smell of gas, upon sensing it, you need to call $104$, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people , located next to them, could detect the leak by smell).
Hydrocarbons of composition from $С_5Н_(12)$ to $С_(15)Н_(32)$ are liquids; heavier hydrocarbons are solids.
The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties.
1. Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the equations of the most characteristic reactions.
Halogenation:
$CH_4+Cl_2→CH_3Cl+HCl$.
In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:
$CH_3Cl+Cl_2→HCl+(CH_2Cl_2)↙(\text"dichloromethane (methylene chloride)")$,
$CH_2Cl_2+Cl_2→HCl+(CHСl_3)↙(\text"trichloromethane(chloroform)")$,
$CHCl_3+Cl_2→HCl+(CCl_4)↙(\text"carbon tetrachloride(carbon tetrachloride)")$.
The resulting substances are widely used as solvents and starting materials in organic syntheses.
2. Dehydrogenation (elimination of hydrogen). When alkanes are passed over a catalyst ($Pt, Ni, Al_2O_3, Cr_2O_3$) at high temperatures ($400-600°C$), a hydrogen molecule is eliminated and an alkene is formed:
$CH_3—CH_3→CH_2=CH_2+H_2$
3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as fuel:
$СН_4+2О_2→СО_2+2Н_2O+880 kJ.$
In general, the combustion reaction of alkanes can be written as follows:
$C_(n)H_(2n+2)+((3n+1)/(2))O_2→nCO_2+(n+1)H_2O$
Thermal splitting of hydrocarbons:
$C_(n)H_(2n+2)(→)↖(400-500°C)C_(n-k)H_(2(n-k)+2)+C_(k)H_(2k)$
The process occurs via a free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals:
$R—CH_2CH_2:CH_2—R→R—CH_2CH_2·+·CH_2—R$.
These radicals interact with each other, exchanging a hydrogen atom, to form an alkane molecule and an alkene molecule:
$R—CH_2CH_2·+·CH_2—R→R—CH=CH_2+CH_3—R$.
Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage of oil refining.
When methane is heated to a temperature of $1000°C$, methane pyrolysis begins - decomposition into simple substances:
$CH_4(→)↖(1000°C)C+2H_2$
When heated to a temperature of $1500°C$, the formation of acetylene is possible:
$2CH_4(→)↖(1500°C)CH=CH+3H_2$
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:
5. Aromatization. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:
What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of $sp^3$ hybridization. The molecules of these substances are built using covalent nonpolar $C-C$ (carbon-carbon) bonds and weakly polar $C-H$ (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e. such bonds, the electron density in which can shift under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, because bonds in alkane molecules are not broken by the heterolytic mechanism.
Alkenes
Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Unlimited are alkenes, alkadienes (polyenes), alkynes. Cyclic hydrocarbons containing a double bond in the ring (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the ring (three or four atoms) also have an unsaturated character. The property of unsaturation is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.
Alkenes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n)$.
Its second name is olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from lat. oleum- oil).
Homologous series of ethene
Unbranched alkenes form the homologous series of ethene (ethylene):
$С_2Н_4$ - ethene, $С_3Н_6$ - propene, $С_4Н_8$ - butene, $С_5Н_(10)$ - pentene, $С_6Н_(12)$ - hexene, etc.
Isomerism and nomenclature
Alkenes, like alkanes, are characterized by structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, characterized by structural isomers, is butene:
A special type of structural isomerism is isomerism of the position of the double bond:
$CH_3—(CH_2)↙(butene-1)—CH=CH_2$ $CH_3—(CH=CH)↙(butene-2)—CH_3$
Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.
Cis- isomers differ from trance- isomers by the spatial arrangement of molecular fragments (in this case, methyl groups) relative to the plane of the $π$ bond, and, consequently, by their properties.
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
The IUPAC nomenclature for alkenes is similar to that for alkanes.
1. Selecting the main circuit.
Naming a hydrocarbon begins with identifying the main chain—the longest chain of carbon atoms in the molecule. In the case of alkenes, the main chain must contain a double bond.
2. Numbering of main chain atoms.
The numbering of the atoms of the main chain begins from the end to which the double bond is closest. For example, the correct connection name is:
$5$-methylhexene-$2$, not $2$-methylhexene-$4$, as one might expect.
If the position of the double bond cannot determine the beginning of the numbering of atoms in the chain, then it is determined by the position of the substituents, just as for saturated hydrocarbons.
3. Formation of the name.
The names of alkenes are formed in the same way as the names of alkanes. At the end of the name, indicate the number of the carbon atom at which the double bond begins, and a suffix indicating that the compound belongs to the class of alkenes - -en.
For example:
Physical and chemical properties of alkenes
Physical properties. The first three representatives of the homologous series of alkenes are gases; substances of the composition $С_5Н_(10)$ - $С_(16)Н_(32)$ - liquids; Higher alkenes are solids.
Boiling and melting points naturally increase with increasing molecular weight of compounds.
Chemical properties.
Addition reactions. Let us recall that a distinctive feature of representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed according to the mechanism
1. Hydrogenation of alkenes. Alkenes are capable of adding hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:
$CH_3—CH_2—CH=CH_2+H_2(→)↖(Pt)CH_3—CH_2—CH_2—CH_3$.
This reaction occurs at atmospheric and elevated pressure and does not require high temperature, because is exothermic. When the temperature rises, the same catalysts can cause a reverse reaction—dehydrogenation.
2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent ($CCl_4$) leads to rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihalogen alkanes:
$CH_2=CH_2+Br_2→CH_2Br—CH_2Br$.
3.
$CH_3-(CH)↙(propene)=CH_2+HBr→CH_3-(CHBr)↙(2-bromopropene)-CH_3$
This reaction obeys Markovnikov's rule:
When a hydrogen halide is added to an alkene, the hydrogen is added to the more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen to the less hydrogenated one.
Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:
$(CH_2)↙(ethene)=CH_2+H_2O(→)↖(t,H_3PO_4)CH_3-(CH_2OH)↙(ethanol)$
Note that a primary alcohol (with a hydroxo group on the primary carbon) is only formed when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed.
This reaction also proceeds in accordance with Markovnikov’s rule - a hydrogen cation attaches to a more hydrogenated carbon atom, and a hydroxo group to a less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:
$nCH_2(=)↙(ethene)CH_2(→)↖(UV light, R)(...(-CH_2-CH_2-)↙(polyethylene)...)_n$
This addition reaction occurs via a free radical mechanism.
6. Oxidation reaction.
Like any organic compounds, alkenes burn in oxygen to form $СО_2$ and $Н_2О$:
$СН_2=СН_2+3О_2→2СО_2+2Н_2О$.
In general:
$C_(n)H_(2n)+(3n)/(2)O_2→nCO_2+nH_2O$
Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are added to those atoms between which a double bond existed before oxidation:
Alkadienes (diene hydrocarbons)
Alkadienes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, two double bonds between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Depending on the relative arrangement of double bonds, three types of dienes are distinguished:
- alkadienes with cumulated arrangement of double bonds:
- alkadienes with conjugated double bonds;
$CH_2=CH—CH=CH_2$;
- alkadienes with isolated double bonds
$CH_2=CH—CH_2—CH=CH_2$.
These three types of alkadienes differ significantly from each other in structure and properties. The central carbon atom (the atom that forms two double bonds) in alkadienes with cumulated bonds is in a state of $sp$-hybridization. It forms two $σ$-bonds lying on the same line and directed in opposite directions, and two $π$-bonds lying in perpendicular planes. $π$-Bonds are formed due to the unhybridized p-orbitals of each carbon atom. The properties of alkadienes with isolated double bonds are very specific, because conjugate $π$-bonds significantly influence each other.
p-orbitals forming conjugated $π$-bonds constitute practically a single system (it is called a $π$-system), because p-orbitals of neighboring $π$-bonds partially overlap.
Isomerism and nomenclature
Alkadienes are characterized by both structural isomerism and cis-, trans-isomerism.
Structural isomerism.
— carbon skeleton isomerism:
— isomerism of the position of multiple bonds:
$(CH_2=CH—CH=CH_2)↙(butadiene-1,3)$ $(CH_2=C=CH—CH_3)↙(butadiene-1,2)$
Cis-, trans- isomerism (spatial and geometric)
For example:
Alkadienes are isomeric compounds of the classes of alkynes and cycloalkenes.
When forming the name of an alkadiene, the numbers of double bonds are indicated. The main chain must necessarily contain two multiple bonds.
For example:
Physical and chemical properties of alkadienes
Physical properties.
Under normal conditions, propandiene-1,2, butadiene-1,3 are gases, 2-methylbutadiene-1,3 is a volatile liquid. Alkadienes with isolated double bonds (the simplest of them is pentadiene-1,4) are liquids. Higher dienes are solids.
Chemical properties.
The chemical properties of alkadienes with isolated double bonds differ little from the properties of alkenes. Alkadienes with conjugated bonds have some special features.
1. Addition reactions. Alkadienes are capable of adding hydrogen, halogens, and hydrogen halides.
A special feature of the addition to alkadienes with conjugated bonds is the ability to add molecules both in positions 1 and 2, and in positions 1 and 4.
The ratio of products depends on the conditions and method of carrying out the corresponding reactions.
2.Polymerization reaction. The most important property of dienes is the ability to polymerize under the influence of cations or free radicals. The polymerization of these compounds is the basis of synthetic rubbers:
$nCH_2=(CH—CH=CH_2)↙(butadiene-1,3)→((... —CH_2—CH=CH—CH_2— ...)_n)↙(\text"synthetic butadiene rubber")$ .
Polymerization of conjugated dienes proceeds as 1,4-addition.
In this case, the double bond turns out to be central in the unit, and the elementary unit, in turn, can take on both cis-, so trance- configuration
Alkynes
Alkynes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one triple bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.
Homologous series of ethyne
Straight-chain alkynes form the homologous series of ethyn (acetylene):
$С_2Н_2$ - ethine, $С_3Н_4$ - propine, $С_4Н_6$ - butine, $С_5Н_8$ - pentine, $С_6Н_(10)$ - hexine, etc.
Isomerism and nomenclature
Alkynes, like alkenes, are characterized by structural isomerism: isomerism of the carbon skeleton and isomerism of the position of the multiple bond. The simplest alkyne, which is characterized by structural isomers of the multiple bond position of the alkyne class, is butine:
$СН_3—(СН_2)↙(butine-1)—С≡СН$ $СН_3—(С≡С)↙(butine-2)—СН_3$
Isomerism of the carbon skeleton in alkynes is possible, starting with pentine:
Since a triple bond assumes a linear structure of the carbon chain, geometric ( cis-, trans-) isomerism is impossible for alkynes.
The presence of a triple bond in hydrocarbon molecules of this class is reflected by the suffix -in, and its position in the chain is the number of the carbon atom.
For example:
Compounds of some other classes are isomeric to alkynes. Thus, the chemical formula $C_6H_(10)$ has hexine (alkyne), hexadiene (alkadiene) and cyclohexene (cycloalkene):
Physical and chemical properties of alkynes
Physical properties. The boiling and melting points of alkynes, as well as alkenes, naturally increase with increasing molecular weight of the compounds.
Alkynes have a specific odor. They are more soluble in water than alkanes and alkenes.
Chemical properties.
Addition reactions. Alkynes are unsaturated compounds and undergo addition reactions. Mostly reactions electrophilic addition.
1. Halogenation (addition of a halogen molecule). An alkyne is capable of attaching two halogen molecules (chlorine, bromine):
$CH≡CH+Br_2→(CHBr=CHBr)↙(1,2-dibromoethane),$
$CHBr=CHBr+Br_2→(CHBr_2-CHBr_2)↙(1,1,2,2-tetrabromoethane)$
2. Hydrohalogenation (addition of hydrogen halide). The addition reaction of a hydrogen halide, which occurs via an electrophilic mechanism, also occurs in two stages, and at both stages the Markovnikov rule is satisfied:
$CH_3-C≡CH+Br→(CH_3-CBr=CH_2)↙(2-bromopropene),$
$CH_3-CBr=CH_2+HBr→(CH_3-CHBr_2-CH_3)↙(2,2-dibromopropane)$
3. Hydration (addition of water). Of great importance for the industrial synthesis of ketones and aldehydes is the reaction of addition of water (hydration), which is called Kucherov's reaction:
4. Hydrogenation of alkynes. Alkynes add hydrogen in the presence of metal catalysts ($Pt, Pd, Ni$):
$R-C≡C-R+H_2(→)↖(Pt)R-CH=CH-R,$
$R-CH=CH-R+H_2(→)↖(Pt)R-CH_2-CH_2-R$
Since the triple bond contains two reactive $π$ bonds, alkanes add hydrogen in a stepwise manner:
1) trimerization.
When ethyne is passed over activated carbon, a mixture of products is formed, one of which is benzene:
2) dimerization.
In addition to the trimerization of acetylene, its dimerization is possible. Under the influence of monovalent copper salts, vinyl acetylene is formed:
$2HC≡CH→(HC≡C-CH=CH_2)↙(\text"butene-1-in-3(vinylacetylene)")$
This substance is used to produce chloroprene:
$HC≡C-CH=CH_2+HCl(→)↖(CaCl)H_2C=(CCl-CH)↙(chloroprene)=CH_2$
by polymerization of which chloroprene rubber is obtained:
$nH_2C=CCl-CH=CH_2→(...-H_2C-CCl=CH-CH_2-...)_n$
Oxidation of alkynes.
Ethine (acetylene) burns in oxygen, releasing a very large amount of heat:
$2C_2H_2+5O_2→4CO_2+2H_2O+2600kJ$ The action of an oxygen-acetylene torch is based on this reaction, the flame of which has a very high temperature (over $3000°C$), which allows it to be used for cutting and welding metals.
In air, acetylene burns with a smoky flame, because the carbon content in its molecule is higher than in the molecules of ethane and ethene.
Alkynes, like alkenes, discolor acidified solutions of potassium permanganate; In this case, the multiple bond is destroyed.
Ionic (V.V. Markovnikov’s rule) and radical reaction mechanisms in organic chemistry
Types of chemical reactions in organic chemistry
Reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the entire variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the reactions that occur between inorganic substances, already familiar to you from the course of inorganic chemistry.
Typically, the main organic compound involved in a reaction is called the substrate, and the other component of the reaction is conventionally considered the reactant.
Substitution reactions
Reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.
Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes.
Let us give examples of such reactions.
Under the influence of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:
$CH_4+Cl_2→CH_3Cl+HCl$
Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:
The equation for this reaction can be written differently:
In this form of notation, the reagents, catalyst, and reaction conditions are written above the arrow, and the inorganic reaction products are written below it.
Addition reactions
Reactions in which two or more molecules of reacting substances combine into one are called addition reactions.
Unsaturated compounds such as alkenes or alkynes undergo addition reactions.
Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration and other addition reactions are distinguished. Each of them requires certain conditions.
1. Hydrogenation— reaction of addition of a hydrogen molecule through a multiple bond:
$CH_3(-CH=)↙(\text"propene")CH_2+H_2(→)↖(Pt)CH_3(-CH_2-)↙(\text"propane")-CH_3$
2.Hydrohalogenation— hydrogen halide addition reaction (hydrochlorination):
$(CH_2=)↙(\text"ethene")CH_2+HCl→CH_3(-CH_2-)↙(\text"chloroethane")-Cl$
3.Halogenation- halogen addition reaction:
$(CH_2=)↙(\text"ethene")CH_2+Cl_2→(CH_2Cl-CH_2Cl)↙(\text"1.2-dichloroethane")$
4. Polymerization- a special type of addition reaction in which molecules of a substance with a small molecular weight combine with each other to form molecules of a substance with a very high molecular weight - macromolecules.
Polymerization reactions are processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.
An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the influence of ultraviolet radiation and a radical polymerization initiator $R:$
$(nCH_2=)↙(\text"ethene")CH_2(→)↖(\text"UV light, R")((...-CH_2-CH_2-...)_n)↙(\text" polyethylene")$
The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of shared electron pairs. As a result of this, an orbital common to the two atoms is formed, in which a common electron pair is located. When a bond is broken, the fate of these shared electrons can be different.
Types of reactive particles in organic chemistry
An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. In this case, a covalent bond is formed along exchange mechanism:
$H + H→H:H,$ or $H-H$
Exchange mechanism The formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.
The process opposite to the formation of a covalent bond by the exchange mechanism is the cleavage of the bond, in which one electron is lost to each atom. As a result of this, two uncharged particles are formed, having unpaired electrons:
Such particles are called free radicals.
Free radicals- atoms or groups of atoms that have unpaired electrons.
Reactions that occur under the influence and with the participation of free radicals are called free radical reactions.
In the course of inorganic chemistry, these are the reactions of hydrogen with oxygen, halogens, and combustion reactions. Please note that reactions of this type are characterized by high speed and release of large amounts of heat.
A covalent bond can also be formed by a donor-acceptor mechanism. One of the orbitals of an atom (or anion) containing a lone electron pair overlaps with an unoccupied orbital of another atom (or cation) having an unoccupied orbital, and a covalent bond is formed, for example:
$H^(+)+(:O-H^(-))↙(\text"acceptor")→(H-O-H)↙(\text"donor")$
Breaking a covalent bond results in the formation of positively and negatively charged species; since in this case both electrons from a common electron pair remain with one of the atoms, the second atom has an unfilled orbital:
$R:|R=R:^(-)+R^(+)$
Let's consider the electrolytic dissociation of acids:
$H:|Cl=H^(+)+Cl^(-)$
One can easily guess that a particle having a lone electron pair $R:^(-)$, i.e. a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge. Particles with lone pairs of electrons are called nucleophilic agents (nucleus- nucleus, positively charged part of the atom), i.e. “friends” of the nucleus, positive charge.
Nucleophiles ($Nu$)- anions or molecules that have a lone pair of electrons that interact with parts of the molecules that have an effective positive charge.
Examples of nucleophiles: $Cl^(-)$ (chloride ion), $OH^(-)$ (hydroxide anion), $CH_3O^(-)$ (methoxide anion), $CH_3COO^(-)$ ( acetate anion).
Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to parts of the molecules that have an increased electron density, a negative charge, and a lone electron pair. They are electrophiles, “friends” of the electron, negative charge, or particles with increased electron density.
Electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.
Examples of electrophiles: $NO_2$ (nitro group), -$COOH$ (carboxyl), -$CN$ (nitrile group), -$SON$ (aldehyde group).
Not every particle with an unfilled orbital is an electrophile. For example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have low electron affinity. From this we can conclude that, despite the presence of an unfilled orbital, such particles will not be electrophiles.
Basic reaction mechanisms
We have identified three main types of reacting species - free radicals, electrophiles, nucleophiles - and three corresponding types of reaction mechanisms:
- free radical;
- electrophilic;
- nucleophilic.
In addition to classifying reactions according to the type of reacting particles, in organic chemistry there are four types of reactions based on the principle of changing the composition of molecules: addition, substitution, detachment, or elimination (from Lat. eliminaue- remove, split off) and rearrangements. Since addition and substitution can occur under the influence of all three types of reactive species, several basic reaction mechanisms can be distinguished.
1.Free radical substitution:
$(CH_4)↙(\text"methane")+Br_2(→)↖(\text"UV light")(CH_3Br)↙(\text"bromomethane")+HBr$
2. Free radical addition:
$nCH_2=CH_2(→)↖(\text"UV light,R")(...-CH_2-CH_2-...)_n$
3. Electrophilic substitution:
4. Electrophilic connection:
$CH_3-(CH=)↙(\text"propene")CH_2+HBr(→)↖(\text"solution")(CH_3-CHBr-CH_3)↙(\text"2-bromopropane")$
$CH_3(-C≡)↙(\text"propyne")CH+Cl_2(→)↖(\text"solution")(CH_3-CCl=CHCl)↙(\text"1,2-dichloropropene")$
5. Nucleophilic addition:
In addition, we will consider elimination reactions that occur under the influence of nucleophilic particles - bases.
6. Elimination:
$СH_3-CHBr-CH_3+NaOH(→)↖(\text"alcohol solution")CH_3-CH=CH_2+NaBr+H_2O$
V.V. Markovnikov’s rule
A distinctive feature of alkenes (unsaturated hydrocarbons) is their ability to undergo addition reactions. Most of these reactions proceed according to the mechanism electrophilic addition.
Hydrohalogenation (addition of hydrogen halogen):
$СH_3(-CH-)↙(\text"propene")CH_2+HBr→CH_3(-CHBr-CH_3)↙(\text"2-bromopropane")$
This reaction obeys V.V. Markovnikov’s rule: When a hydrogen halide is added to an alkene, hydrogen is added to the more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen to the less hydrogenated one.
ALKENES
Hydrocarbons, in the molecule of which, in addition to simple carbon-carbon and carbon-hydrogen σ-bonds, there are carbon-carbon π-bonds, are called unlimited. Since the formation of a π bond is formally equivalent to the loss of two hydrogen atoms by the molecule, unsaturated hydrocarbons contain 2p there are fewer hydrogen atoms than the limiting ones, where P - number of π bonds:
A series whose members differ from each other by (2H) n is called isological series. Thus, in the above scheme, the isologs are hexanes, hexenes, hexadienes, hexines, hexatrienes, etc.
Hydrocarbons containing one π bond (i.e. double bond) are called alkenes (olefins) or, according to the first member of the series - ethylene, ethylene hydrocarbons. The general formula of their homologous series is C p H 2l.
1. Nomenclature
In accordance with the IUPAC rules, when constructing the names of alkenes, the longest carbon chain containing a double bond is given the name of the corresponding alkane in which the ending -an replaced by -en. This chain is numbered in such a way that the carbon atoms involved in the formation of the double bond receive the lowest possible numbers:
Radicals are named and numbered as in the case of alkanes.
For alkenes of relatively simple structure, simpler names are allowed. Thus, some of the most frequently occurring alkenes are named by adding the suffix -en to the name of a hydrocarbon radical with the same carbon skeleton:
Hydrocarbon radicals formed from alkenes receive the suffix -enil. The numbering in the radical starts from the carbon atom having a free valence. However, for the simplest alkenyl radicals, instead of systematic names, it is allowed to use trivial ones:
Hydrogen atoms directly bonded to unsaturated carbon atoms forming a double bond are often called vinyl hydrogen atoms,
2. Isomerism
In addition to isomerism of the carbon skeleton, isomerism of the position of the double bond also appears in the series of alkenes. In general, this type of isomerism is isomerism of substituent position (function)- observed in all cases when the molecule contains any functional groups. For the alkane C4H10, two structural isomers are possible:
For the C4H8 alkene (butene), three isomers are possible:
Butene-1 and butene-2 are isomers of the position of the function (in this case, its role is played by a double bond).
Spatial isomers differ in the spatial arrangement of substituents relative to each other and are called cis isomers, if the substituents are located on the same side of the double bond, and trans isomers, if on opposite sides:
3. Structure of a double bond
The energy of cleavage of a molecule at the C=C double bond is 611 kJ/mol; since the energy of the C-C σ bond is 339 kJ/mol, the energy of breaking the π bond is only 611-339 = 272 kJ/mol. π -electrons are much lighter than σ -electrons and are susceptible to the influence of, for example, polarizing solvents or the influence of any attacking reagents. This is explained by the difference in the symmetry of the distribution of the electron cloud of σ- and π-electrons. The maximum overlap of p-orbitals and, therefore, the minimum free energy of the molecule are realized only with a flat structure of the vinyl fragment and with a shortened C-C distance equal to 0.134 nm, i.e. significantly smaller than the distance between carbon atoms connected by a single bond (0.154 nm). As the “halves” of the molecule rotate relative to each other along the double bond axis, the degree of orbital overlap decreases, which is associated with energy consumption. The consequence of this is the absence of free rotation along the double bond axis and the existence of geometric isomers with appropriate substitution at the carbon atoms.
1. Oxidation of alkenes.
1.1 Combustion.
In excess air or oxygen, all alkenes burn to carbon dioxide and water:
CH 3 – CH = CH 2 + 4.5 O 2 3 CO 2 + 3 H 2 O
The combustion of alkenes is not used in internal combustion engines, since during storage of gasoline they become resinous and the resins clog the fuel equipment (injector).
The possibility of combustion of alkenes should be taken into account during transportation and storage at chemical plants.
1.2 Oxidation of alkenes with a calculated amount of atmospheric oxygen in the presence of silver.
Epoxy compounds are used to create adhesives for various purposes.
1.3 Oxidation of alkenes with a one percent solution of potassium permanganate in water - a qualitative reaction to alkenes by E.E. Wagner.
The reaction was first described by E.E. Wagner in the Journal of the Russian Physical and Chemical Society in 1886. The oxidation of alkenes or other unsaturated compounds occurs at room temperature and is accompanied by the disappearance of the violet color of the permanganate ion and the formation of a brown precipitate of manganese dioxide. Regardless of the structure of the alkene (but not alkadiene, for example), the coefficients in the Wagner reaction are always the same (324-322). Below are examples of the oxidation of specific alkenes and show the half-reactions and total ORR in ionic and molecular form:
:
As can be seen in the Wagner reaction, the final organic products are dihydric alcohols. They are also called glycols. For example, 1,2-ethanediol is called ethylene glycol.
1.4 Oxidation of alkenes with strong oxidizing agents in the liquid phase in an acidic environment.
Depending on the structure of alkenes, when oxidized under these conditions, various products are obtained, namely CO 2, carboxylic acids and ketones. The oxidation scheme for alkenes of various structures is shown below.
To illustrate the use of this scheme, an example is given of the oxidation of 2-methylpentene with potassium permanganate in a sulfuric acid medium. According to the oxidation scheme, the final organic products for this alkene are a carboxylic acid and a ketone:
Half-reactions for this process:
Another example: the oxidation of 2-ethylbutene-1 with potassium dichromate in sulfuric acid. In accordance with the rules of the oxidation scheme, in this case a ketone and carbon dioxide are obtained:
Third example: oxidation cis- 3,4,5-trimethylheptene-3 sodium bismuthate in dilute nitric acid. In accordance with the rules of the oxidation scheme, in this case two ketones are obtained:
1.5 Ozonolysis
Ozonolysis is a two-stage process, in the first stage of which ozone is added to an alkene and an ozonide is formed, and in the second stage this ozonide is either slowly destroyed by water with the formation of hydrogen peroxide, aldehydes and ketones, or quickly reduced by zinc dust with the formation of zinc oxide and the same aldehydes and ketones.
Below is an example of ozonolysis of 3-methyl- cis-heptene-3.
Ozonolysis produces two different ketones:
Formaldehyde (methanal) can be obtained as one of the oxidation products if the terminal alkene is taken into the reaction:
2. Addition reactions at the double bond of alkenes.
Both nonpolar and polar molecules can be added to the double bond of alkenes.
Non-polar: H 2, Cl 2, Br 2, J 2. Fluorine F2 does not add to alkenes, but burns them to CF4 and HF:
CH 3 – CH = CH − CH 3 + 12 F 2 → 4 CF 4 + 8 HF
2.1 Addition of hydrogen.
The addition occurs only in the presence of a catalyst. Most often, palladium or platinum is used in industry, which are easily regenerated by calcination. Nickel is practically not used, since under conventional calcination conditions it turns into an oxide, the reduction of which is not economically profitable.
CH 3 – CH = CH 2 + H 2 CH 3 – CH 2 – CH 3
2.2 Addition of chlorine.
Goes to two atoms at a double bond. Dichloro derivatives of alkanes are obtained. The reaction can occur either in an aqueous solution at room temperature or lower temperatures, or in organic solvents, for example, carbon tetrachloride CCl 4 or dichloroethane C 2 H 4 Cl 2:
2.3 Addition of bromine.
It occurs similarly both with bromine water at temperatures up to 0 0 C, and in the same organic solvents. In the latter case, the reaction can take place at temperatures down to – 25 0 C, that is, in the cold.
The reaction with bromine is a qualitative test for the presence of alkenes in gaseous and liquid mixtures, as it is accompanied by the discoloration of orange bromine solutions:
2.4 Reaction with iodine.
The reaction is widely used to determine the total unsaturation of fats, which are derivatives of unsaturated fatty acids containing double bonds, as in alkenes:
The mass of iodine in grams used to completely iodize 100 g of fat is called the iodine number. The higher it is, the more beneficial the fat is for humans, since the body synthesizes hormones only from polyunsaturated fatty acids. Examples of iodine numbers: palm oil - 12, lamb fat - 35, olive oil - 80, soybean oil - 150, herring oil - 200, seal oil - 280
2.5 Reactions with polar molecules.
Polar molecules of type H-A include the following: H-F, H-Cl, H-Br, H-J, H-OH,
H-O-R (alcohols) and carboxylic acids –
The addition of hydrogen chloride and other polar molecules proceeds accordingly, that is, a hydrogen atom from a polar molecule preferentially attaches to a more hydrogenated carbon atom at a double bond, and the A residue to another atom at a double bond.
Thus, the reaction is poorly selective.
As the difference in hydrogenation increases, the selectivity in the reaction increases. Indeed, the difference in hydrogenation of atoms 1 and 2 in propene is one hydrogen atom, and 85% of chlorine goes to the less hydrogenated carbon atom, whereas in
In 2-methylpropene, the difference in hydrogenation between atoms 1 and 2 is already two hydrogen atoms and more than 98% of chlorine goes to atom 2:
The addition of HF, HBr, HJ proceeds similarly:
Otherwise, HBr (and only HBr, and not HCl, HF and HI) is added in the presence of hydrogen peroxide H 2 O 2:
This reaction is called the addition of HBr according to Karas. The selectivity in it practically changes to the opposite in comparison with that for the addition of HBr in the absence of hydrogen peroxide (according to Markovnikov’s rule).
The reaction of alkenes with chlorine at 500 °C is very interesting. Under these conditions, the reaction of addition of chlorine at the double bond is reversible, moreover, the equilibrium in it is strongly shifted towards the starting substances. On the contrary, a much slower but irreversible reaction of radical substitution at the allylic position, that is, next to the double bond, goes to the end:
This reaction is of great practical importance. For example, one of the stages of the large-scale industrial synthesis of glycerol is the chlorination of propene to
3-chloropropene-1.
When water is added to alkenes in the presence of catalytic amounts of sulfuric or phosphoric acids, alcohols are obtained. The joining follows the Markovnikov rule:
When alcohols are added to alkenes, ethers are obtained:
These isomeric ethers can be called both alkanes containing alkoxy substituents and ethers. In the first case, the longest chain of carbon atoms is selected and numbered on the side closest to the alkoxy substituent. For example, for broadcast I chain numbered with numbers in brackets. And the corresponding name is also in brackets. For isomer II, on the contrary, the numbers in brackets number the chain starting from the carbon atom bonded to the oxygen atom. The name in this case is formed as follows: first the simpler radical associated with the oxygen atom is named, then the more complex one, and finally the “ether” is added.
When carboxylic acids are added to alkenes, esters are obtained:
The names of esters are formed as follows: first, the hydrocarbon radical associated with oxygen is named. In this case, atom number 1 is taken to be the carbon atom in contact with oxygen. The longest existing chain is numbered from this atom. Groups of atoms not included in the main chain are considered substituents and are listed according to the usual rules. Then the “ester of such and such acid” is added.
