Esters are derivatives of acids in which the acidic hydrogen is replaced by alkyl (or generally hydrocarbon) radicals.
Esters are divided depending on which acid they are derived from (inorganic or carboxylic).
Among the esters, a special place is occupied by natural esters - fats and oils, which are formed by the trihydric alcohol glycerol and higher fatty acids containing an even number of carbon atoms. Fats are part of plant and animal organisms and serve as one of the energy sources of living organisms, which is released during the oxidation of fats.
The general formula for carboxylic acid esters is:
where R and R" are hydrocarbon radicals (in formic acid esters, R is a hydrogen atom).
General formula for fats:
where R", R", R"" are carbon radicals.
Fats are "simple" and "mixed". The composition of simple fats includes residues of the same acids (i.e. R’ = R "= R""), the composition of mixed fats includes different ones.
The most common fatty acids found in fats are:
1. Butyric acid CH 3 - (CH 2) 2 - COOH
3. Palmitic acid CH 3 - (CH 2) 14 - COOH
4. Stearic acid CH 3 - (CH 2) 16 - COOH
5. Oleic acid C 17 H 33 COOH
CH 3 -(CH 2) 7 -CH === CH-(CH 2) 7 -COOH
6. Linoleic acid C 17 H 31 COOH
CH 3 -(CH 2) 4 -CH \u003d CH-CH 2 -CH \u003d CH-COOH
7. Linolenic acid C 17 H 29 COOH
CH 3 CH 2 CH \u003d CHCH 2 CH \u003d\u003d CHCH 2 CH \u003d CH (CH 2) 4 COOH
Esters are characterized by the following types of isomerism:
1. The isomerism of the carbon chain begins at the acid residue with butanoic acid, at the alcohol residue - with propyl alcohol, for example, ethyl isobutyrate, propyl acetate and isopropyl acetate are isomers of ethyl butyrate.
2. Isomerism of the position of the ester group -CO-O-. This type of isomerism begins with esters containing at least 4 carbon atoms, such as ethyl acetate and methyl propionate.
3. Interclass isomerism, for example propanoic acid is isomeric to methyl acetate.
For esters containing unsaturated acid or unsaturated alcohol, two more types of isomerism are possible: isomerism of the position of the multiple bond and cis-, trans-isomerism.
Esters of lower carboxylic acids and alcohols are volatile, water-insoluble liquids. Many of them have a pleasant smell. So, for example, butyl butyrate smells like pineapple, isoamyl acetate smells like pear, etc.
Esters of higher fatty acids and alcohols are waxy substances, odorless, insoluble in water.
The pleasant aroma of flowers, fruits, berries is largely due to the presence of certain esters in them.
Fats are widely distributed in nature. Along with carbohydrates and proteins, they are part of all plant and animal organisms and constitute one of the main parts of our food.
According to their state of aggregation at room temperature, fats are divided into liquid and solid. Solid fats, as a rule, are formed by saturated acids, liquid fats (they are often called oils) are unsaturated. Fats are soluble in organic solvents and insoluble in water.
1. The reaction of hydrolysis, or saponification. Since the esterification reaction is reversible, therefore, in the presence of acids, the reverse hydrolysis reaction proceeds:
The hydrolysis reaction is also catalyzed by alkalis; in this case, hydrolysis is irreversible, since the resulting acid with alkali forms a salt:
2. Addition reaction. Esters containing an unsaturated acid or alcohol in their composition are capable of addition reactions.
3. Recovery reaction. The reduction of esters with hydrogen leads to the formation of two alcohols:
4. The reaction of the formation of amides. Under the action of ammonia, esters are converted into acid amides and alcohols:
Receipt. 1. Esterification reaction:
Alcohols react with mineral and organic acids to form esters. The reaction is reversible (the reverse process is the hydrolysis of esters).
The reactivity of monohydric alcohols in these reactions decreases from primary to tertiary.
2. Interaction of acid anhydrides with alcohols:
3. Interaction of acid halides with alcohols:
Hydrolysis mechanism:
Liquid fats are converted into solid ones by a hydrogenation reaction. Hydrogen is added at the site of the double bond break in the hydrocarbon radicals of fat molecules:
The reaction proceeds when heated under pressure and in the presence of a catalyst - finely divided nickel. The product of hydrogenation is a solid fat (artificial lard), called tallow, which is used in the production of soap, stearin and glycerin. Margarine - edible fat, consists of a mixture of hydrogenated oils (sunflower, cottonseed, etc.), animal fats, milk and some other substances (salt, sugar, vitamins, etc.).
An important chemical property of fats, like all esters, is the ability to undergo hydrolysis (saponification). Hydrolysis easily proceeds when heated in the presence of catalysts - acids, alkalis, oxides of magnesium, calcium, zinc:
The reaction of hydrolysis of fats is reversible. However, with the participation of alkalis, it comes almost to the end - alkalis convert the resulting acids into salts and thereby eliminate the possibility of interaction of acids with glycerol (reverse reaction).
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The hydrolysis of esters is catalyzed by both acids and bases. Acid hydrolysis of esters is usually carried out by heating with hydrochloric or sulfuric acid in an aqueous or aqueous-alcoholic medium. In organic synthesis, acid hydrolysis of esters is most often used for mono- and dialkyl-substituted malonic esters (Chapter 17). Mono- and disubstituted derivatives of malonic ester, when boiled with concentrated hydrochloric acid, undergo hydrolysis followed by decarboxylation.
For base-catalyzed hydrolysis, an aqueous or aqueous-alcoholic solution of NaOH or KOH is usually used. Best results are obtained using a thin suspension of potassium hydroxide in DMSO containing a small amount of water.
The latter method is preferred for saponification of esters of hindered acids, another modification of this method is the alkaline hydrolysis of hindered esters in the presence of 18-crown-6-polyester:
For preparative purposes, base catalyzed hydrolysis has a number of clear advantages over acid hydrolysis. The rate of basic hydrolysis of esters is typically a thousand times faster than that of acid catalysis. Hydrolysis in an acidic medium is a reversible process, in contrast to hydrolysis in the presence of a base, which is irreversible.
18.8.2.A. Mechanisms of ester hydrolysis
Hydrolysis of esters with pure water is in most cases a reversible reaction, leading to an equilibrium mixture of carboxylic acid and starting ester:
This reaction in acidic and alkaline media is greatly accelerated, which is associated with acid-base catalysis (Chapter 3).
According to K. Ingold, the mechanisms of ester hydrolysis are classified according to the following criteria:
(1) Type of catalysis: acidic (symbol A) or basic (symbol B);
(2) Type of cleavage, showing which of the two -C-O bonds in the ester is cleaved as a result of the reaction: acyl oxygen (index AC) or alkyl oxygen (index AL):
(3) Molecularity of reaction (1 or 2).
From these three criteria, eight different combinations can be made, which are shown in Figure 18.1.
These are the most common mechanisms. Alkaline saponification is almost always of type B AC 2. Acid hydrolysis (as well as esterification) in most cases has an A AC 2 mechanism.
The AAC 1 mechanism is usually observed only in strongly acidic solutions (for example, in conc. H 2 SO 4), and is especially common for esters of sterically hindered aromatic acids.
The mechanism of BAC 1 is still unknown.
The B AL 2 mechanism was found only in the case of exceptionally strong spatially screened acyl groups and neutral hydrolysis of -lactones. The mechanism of A AL 2 is still unknown.
According to the mechanism And AL 1 usually react tertiary-alkyl esters in a neutral or acidic environment. The same substrates under similar conditions can react according to the B AL 1 mechanism, however, upon transition to a slightly more alkaline environment, the B AL 1 mechanism is immediately replaced by the B AC 2 mechanism.
As can be seen from Scheme 18.1, reactions catalyzed by acids are reversible, and from the principle of microscopic reversibility (Chapter 2) it follows that acid-catalyzed esterification also proceeds by similar mechanisms. However, with base catalysis, the equilibrium is shifted towards hydrolysis (saponification), since the equilibrium is shifted due to the ionization of the carboxylic acid. According to the above scheme, in the case of mechanism A AC 1, the COOR and COOH groups are protonated at the alkoxy or hydroxyl oxygen atom. Generally speaking, from the point of view of thermodynamics, the protonation of carbonyl oxygen, the C=O group, is more advantageous, because in this case, the positive charge can be delocalized between both oxygen atoms:
Nevertheless, the solution also contains a tautomeric cation, a necessary intermediate in the A AC 1 mechanism, in small amounts. Both B1 mechanisms (of which B AC 1 is unknown) are in fact not catalytic at all, because the dissociation of the neutral ether occurs at the beginning.
Of the eight Ingold mechanisms, only six have been experimentally proven.
DEFINITION
Compounds of organic nature, which are derivatives of carboxylic acids, formed during the interaction of the latter with alcohols:
The structural formula of esters in general terms:
where R and R' are hydrocarbon radicals.
Hydrolysis of esters
One of the most characteristic abilities for esters (in addition to esterification) is their hydrolysis - splitting under the action of water. In another way, the hydrolysis of esters is called saponification. In contrast to the hydrolysis of salts, in this case it is practically irreversible. Distinguish between alkaline and acid hydrolysis of esters. In both cases, an alcohol and an acid are formed:
a) acid hydrolysis
b) alkaline hydrolysis
Examples of problem solving
EXAMPLE 1
| Exercise | Determine the mass of acetic acid that can be obtained during the saponification of ethyl acetate with a mass of 180 g. |
| Solution | We write the reaction equation for the hydrolysis of ethyl ester of acetic acid using the gross formula: C 4 H 8 O 2 + H 2 O ↔ CH 3 COOH + C 2 H 5 OH. Calculate the amount of ethyl acetate substance (molar mass - 88 g / mol), using the mass value from the conditions of the problem: υ (C 4 H 8 O 2) \u003d m (C 4 H 8 O 2) / M (C 4 H 8 O 2) \u003d 180/88 \u003d 2 mol. According to the reaction equation, the number of moles of ethyl acetate and acetic acid are: υ (C 4 H 8 O 2) \u003d υ (CH 3 COOH) \u003d 2 mol. Then, you can determine the mass of acetic acid (molar mass - 60 g / mol): m (CH 3 COOH) \u003d υ (CH 3 COOH) × M (CH 3 COOH) \u003d 2 × 60 \u003d 120g. |
| Answer | The mass of acetic acid is 120 grams. |
The hydrolysis of esters and all other acid derivatives requires acidic or alkaline catalysis. With acid hydrolysis, carboxylic acids and alcohols are obtained (reverse esterification reaction), with alkaline hydrolysis, salts of carboxylic acids and alcohols are formed.
Acid hydrolysis of esters:
S N mechanism, nucleophile - H 2 O, the alkoxy group is replaced by hydroxyl.
Alkaline hydrolysis of esters: the reaction proceeds in two stages with 2 moles of base, the resulting acid is converted into a salt.
S N mechanism, Nu = -OH
Formation of salt compounds Amides are neutral substances, since the basic properties of ammonia are weakened by the substitution of a hydrogen atom in it with an acidic residue. Therefore, the NH 2 group in amides, unlike amines, forms an onium cation only with difficulty. However, with strong acids, amides give salts, such as Cl, which are easily decomposed by water. On the other hand, the hydrogen of the NH 2 group in amides is more easily replaced by metals than in ammonia and in amines. Acetamide, for example, easily dissolves mercury oxide, forming the compound (CH 3 CONH) 2 Hg.
It is possible, however, that during the formation of metal derivatives, amide isomerization occurs and the resulting compound has an isomeric (tautomeric) structure of an imidic acid salt
i.e., there is an analogy with hydrocyanic acid salts.
2. Action of nitrous acid Amides react with nitrous acid, like primary amines, to form carboxylic acids and release nitrogen:
3. Saponification When boiled with mineral acids and alkalis, amides add water, forming carboxylic acid and ammonia:
4. Action of halide alkyls. Under the action of alkyl halides on amides or their metal derivatives, N-substituted amides are obtained:
5. Action of phosphorus pentachloride. Under the action of phosphorus pentachloride on amides, chloramides
easily decomposed into hydrochloric acid and imide chlorides
The latter with ammonia can give salts amidines;
6. Conversion to amines. By vigorous reduction of amides, primary amines with the same number of carbon atoms can be obtained:
7. Hoffmann's reaction. Under the action of hypohalogenite or bromine and alkali on amides, amines are formed, and the carbon atom of the carbonyl group is cleaved off in the form of CO 2 (A. Hoffman). The course of the reaction can be represented as follows:
In educational manuals, another interpretation of the mechanism of this reaction is still often found:
However, this course of the reaction is less plausible, since the formation of a fragment
with a nitrogen atom carrying two free electron pairs is unlikely.
This mechanism is opposed, in particular, by the fact that if the radical R is optically active, then it does not racemize as a result of the reaction. Meanwhile, even the fleeting existence of the free radical R - : would lead to the loss of optical activity.
Chemical properties. The nitro group is one of the most strong electron-withdrawing groups and is able to effectively delocalize negative. charge. In the aromatic conn. as a result of induction and especially mesomeric effects, it affects the distribution of electron density: the nucleus acquires a partial positive. charge, to-ry localized Ch. arr. in ortho and para positions; Hammett constants for the NO 2 group s m 0.71, s n 0.778, s + n 0.740, s - n 1.25. So arr., the introduction of the NO 2 group dramatically increases the reaction. ability org. conn. in relation to nukleof.reagents and complicates p-tion with elektrof. reagents. This determines the widespread use of nitro compounds in org. synthesis: the NO 2 group is introduced into the desired position of the org molecule. Comm., carry out decomp. p-tion associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In the aromatic In a row, a shorter scheme is often used: nitration-transformation of the NO 2 group.
The formation of nitrone to-t in a series of aromatic nitro compounds is associated with the isomerization of the benzene ring into the quinoid form; for example, nitrobenzene forms with conc. H 2 SO 4 colored salt product f-ly I, o-nitrotoluene exhibits photochromism as a result vnutrimol. proton transfer to form a bright blue O-derivative:
Under the action of bases on primary and secondary nitro compounds, salts of nitro compounds are formed; ambident anions of salts in p-tions with electrophiles are able to give both O- and C-derivatives. So, alkylation of salts of nitro compounds with alkyl halides, trialkylchlorosilanes or R 3 O + BF - 4 gives O-alkylation products. Recent m.b. also obtained by the action of diazomethane or N,O-bis-(trimethylsilyl)acetamide on nitroalkanes with pK a< 3 или нитроновые к-ты, напр.:
Acyclic alkyl esters of nitrone to-t are thermally unstable and decompose according to intramol. mechanism:
R-ts and and with r and ry v o m s vyaz z and C-N. Primary and secondary nitro compounds at loading. with a miner. to-tami in the presence. alcohol or aqueous solution of alkali form carbonyl Comm. (see Neph reaction). R-tion passes through the interval. the formation of nitrone to-t:
As a source Comm. silyl nitrone ethers can be used. The action of strong to-t on aliphatic nitro compounds can lead to hydroxamic to-there, for example:
There are many methods for the reduction of nitro compounds to amines. Widely used iron filings, Sn and Zn in the presence. to-t; with catalytic hydrogenation as catalysts use Ni-Raney, Pd / C or Pd / PbCO 3, etc. Aliphatic nitro compounds are easily reduced to amines LiAlH 4 and NaBH 4 in the presence. Pd, Na and Al amalgams, when heated. with hydrazine over Pd/C; for aromatic nitro compounds, TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. polynitro compounds are selectively reduced to nitramines with Na hydrosulfide in CH 3 OH. There are ways to choose. recovery of the NO 2 group in polyfunctional nitro compounds without affecting other f-tions.
Under the action of P(III) on aromatic nitro compounds, a succession occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. R-tion is used for the synthesis of condenser. heterocycles, for example:
R-ts and with the preservation of the NO 2 group. Aliphatic nitro compounds containing an a-H-atom are easily alkylated and acylated to form, as a rule, O-derivatives. However, mutually mod. dilithium salts of primary nitro compounds with alkyl halides, anhydrides or carboxylic acid halides leads to products of C-alkylation or C-acylation, for example:
Known examples vnutrimol. C-alkylations, e.g.:
Primary and secondary nitro compounds react with aliphatic. amines and CH 2 O with the formation of p-amino derivatives (p-tion Mannich); in the district, you can use pre-obtained methylol derivatives of nitro compounds or amino compounds:
Nitromethane and nitroethane can condense with two molecules of methylolamine, and higher nitroalkanes with only one. At certain ratios of reagents p-tion can lead to heterocyclic. connection, for example: with interaction. primary nitroalkane with two equivalents of a primary amine and an excess of formaldehyde form Comm. f-ly V, if the reagents are taken in a ratio of 1:1:3-comm. forms VI.
Aromatic nitro compounds easily enter into p-tion nucleof. substitution and much more difficult, in the district of the electroph. substitution; in this case, the nucleophile is directed to the ortho and pore positions, and the electrophile is directed to the meta position to the NO 2 group. Velocity constant nitration of nitrobenzene is 5-7 orders of magnitude less than that of benzene; this produces m-dinitrobenzene.
During the carboxylation of primary nitroalkanes by the action of CH 3 OMgOCOOCH 3 a-nitrocarboxylic acids or their esters are formed.
When treating salts of mono-nitro compounds C (NO 2) 4 ., with Ag or alkali metal nitrites, or under the action of nitrites on a-halo-nitroalkanes in an alkaline medium (Ter Meer district), gem-dinitro compounds are formed. Electrolysis of a-halo-nitroalkanes in aprotic p-solvents, as well as the treatment of Cl 2 nitro compounds in an alkaline medium or the electrooxidation of salts of nitro compounds lead to vic-dinitro compounds:
The nitro group does not render beings. influence on free-radical alkylation or aromatic arylation. conn.; p-tion leads to the main. to ortho- and para-substituted products.
To restore nitro compounds without affecting the NO 2 group, NaBH 4, LiAlH 4 are used at low temperatures or diborane solution in THF, for example:
Aromatic di- and tri-nitro compounds, in particular 1,3,5-trinitrobenzene, form stable brightly colored crystals. they say complexes with aromatic Comm.-donors of electrons (amines, phenols, etc.). Complexes with picric to-one is used to isolate and purify aromatic. hydrocarbons. Intermod. di- and trinitrobenzenes with strong bases (HO - , RO - , N - 3 , RSO - 2 , CN - , aliphatic amines) leads to the formation of Meisen-heimer complexes, which are isolated as colored alkali metal salts.
Suitable oxidizing agents for these reactions are chromic or nitric acid, chromium mixture, manganese dioxide or selenium dioxide.
During oxidation with chromic acid, alcohol nucleophilically adds to chromic acid, while water is split off and an ester of chromic acid is formed (this is the first stage of the reaction, it is similar to the formation of esters of carboxylic acids, cf. Section E, 7.1.5.1). In the second stage, which probably goes through a cyclic transition state, the a-hydrogen of the alcohol passes to the chromate residue, and the metal passes from the hexavalent state to the tetravalent state:
| n-CH3O> P-tert-C 4 H 9 > P-CH 3 > P-Cl> P-NO 2 | (G.6.20) |
When primary alcohols are oxidized, the resulting aldehyde must be protected from further oxidation to carboxylic acid. It is possible, for example, to constantly distill off the aldehyde from the reaction mixture: this is quite feasible, since the boiling point of the aldehyde is usually lower than the boiling point of the corresponding alcohol. Nevertheless, the yield of aldehydes during oxidation with dichromate rarely exceeds 60%. It is noteworthy that when the reaction is carried out properly, multiple carbon-carbon bonds are almost not affected.
Aldehydes are also formed by heating alcohols with an aqueous neutral dichromate solution, but only benzyl alcohols give good yields.
Higher yields of aldehydes can be obtained by oxidizing primary alcohols tert-butyl chromate (in petroleum ether, benzene or carbon tetrachloride) or manganese dioxide (in acetone, petroleum ether, carbon tetrachloride or dilute sulfuric acid). These reagents also make it possible to obtain unsaturated and aromatic aldehydes in good yields.
The oxidation of secondary alcohols to ketones is even easier than the oxidation of primary alcohols. The yields here are higher, since, firstly, the reactivity of secondary alcohols is higher than that of primary ones, and secondly, the resulting ketones are much more resistant to oxidation compared to aldehydes. In a series of steroids and terpenes, the oxidation of secondary alcohols with a complex of chromic acid with pyridine, as well as chromic anhydride in dimethylformamide, has proven itself well. A good oxidizing agent is also chromic anhydride in acetone; it can be used to oxidize unsaturated secondary alcohols without affecting the multiple carbon-carbon bond.
A new method, also suitable for hindered alcohols, is oxidation with dimethyl sulfoxide in acetic anhydride.
According to the method below, the reaction is carried out in a two-phase system. The formed ketones are extracted with an organic solvent and thus protected from further oxidation.
disaccharides- carbohydrates, the molecules of which consist of two monosaccharide residues, which are connected to each other due to the interaction of two hydroxyl groups.
In the process of formation of a disaccharide molecule, one molecule of water is split off:
or for sucrose:
Therefore, the molecular formula of disaccharides is C 12 H 22 O 11.
The formation of sucrose occurs in plant cells under the influence of enzymes. But chemists have found a way to implement many of the reactions that are part of the processes that occur in wildlife. In 1953, the French chemist R. Lemieux for the first time carried out the synthesis of sucrose, which was called by his contemporaries "the conquest of the Everest of organic chemistry."
In industry, sucrose is obtained from sugar cane juice (content 14-16%), sugar beet (16-21%), as well as some other plants, such as Canadian maple or pear.
Everyone knows that sucrose is a crystalline substance that has a sweet taste and is highly soluble in water.
Sugar cane juice contains the carbohydrate sucrose, commonly referred to as sugar.
The name of the German chemist and metallurgist A. Marggraf is closely associated with the production of sugar from beets. He was one of the first researchers to use a microscope in his chemical studies, with which he discovered sugar crystals in beet juice in 1747.
Lactose - crystalline milk sugar, was obtained from the milk of mammals as early as the 17th century. Lactose is a less sweet disaccharide than sucrose.
Now let's get acquainted with carbohydrates that have a more complex structure - polysaccharides.
Polysaccharides- high-molecular carbohydrates, the molecules of which consist of many monosaccharides.
In a simplified form, the general scheme can be represented as follows:
Now let's compare the structure and properties of starch and cellulose - the most important representatives of polysaccharides.
The structural unit of the polymer chains of these polysaccharides, the formula of which is (C 6 H 10 O 5) n, are glucose residues. In order to write down the composition of the structural unit (C 6 H 10 O 5), you need to subtract a water molecule from the glucose formula.
Cellulose and starch are of vegetable origin. They are formed from glucose molecules as a result of polycondensation.
The equation for the polycondensation reaction, as well as the inverse process of hydrolysis for polysaccharides, can be conditionally written as follows:
Starch molecules can have both a linear and branched type of structure, cellulose molecules can only have a linear one.
When interacting with iodine, starch, unlike cellulose, gives a blue color.
These polysaccharides also have various functions in the plant cell. Starch serves as a reserve nutrient, cellulose performs a structural, building function. Plant cell walls are made up of cellulose.
CANNICEROREACTION, oxidizing-reducing disproportionation of aldehydes under the action of alkali with the formation of primary alcohols and carboxylic acids, for example:
The aldehyde is treated with conc. aqueous or water-alcohol solution of alkali during cooling or slight heating. Catalysts - decomp. metals (eg Ag, Ni, Co, Cu) and their oxides. Aldehydes that do not contain atomH in the a-position to the carbonyl group enter the p-tion. Otherwise, it is not the Cannizzaro reaction that is preferable, but the aaldol condensation. Electron-withdrawing substituents in the aromatic ring. aldehydes speed up the process, while electron donors slow it down. Benzaldehydes with substituents in the ortho positions do not react in Cannizzaro; o- and p-hydroxybenzaldehydes react only in the presence. Ag. R-tion with the use of two razl.aldehydes (the so-called cross Cannizzaro reaction) is used by Ch. arr. to obtain a high yield of primary alcohols from aromatic. aldehydes. In this case, formaldehyde usually acts as a reducing agent:
ArCHO + CH 2 O: ArCH 2 OH + HCOOH
In the synthesis of polyhydroxymethylated Comm. formaldehyde participates in the first stage in the aldol condensation, and then as a reducing agent in the cross Cannizzaro reaction:
The proposed mechanism of the Cannizzaro reaction in Homog. environment includes the stage of hydride transfer
For aromatic aldehydes, the possibility of participation in the Cannizzaro reaction of radical anions formed as a result of one-electron transfer cannot be ruled out. R-tion, similar to the Cannizzaro reaction, is carried out with intramol. disproportionation of a-ketoaldehydes in the presence. alkalis (Cannizzaro rearrangement):
Cannizzaro reaction is used for prom. synthesis of pentaerythritol, preparative production of alcohols, carboxylic acids, etc. R-tion was discovered by S. Cannizzaro in 1853.
Pyrrole, furan and thiophene are five-membered heterocyclic compounds with one heteroatom.
The numbering of atoms in a heterocycle begins with a heteroatom and proceeds counterclockwise. Positions 2- and 5-are called a-positions, 3- and 4- are called b-positions.
According to formal features, these compounds are aromatic, since they are conjugated cyclic p-systems, which include 6p electrons - 4 electrons of the diene system - and a pair of electrons of the heteroatom. The cycle is practically planar, which means that the hybridization state of the heteroatom is close to sp 2 .
Resonance structures are presented below, illustrating the delocalization of electrons of a heteroatom along a heterocyclic ring using furan as an example.
The above resonance structures show that the heteroatom (in this case, the oxygen atom), as a result of mesomeric interaction with the diene π-system, transfers the electron density to the ring, as a result of which a certain negative charge arises on the carbon atoms in the heterocycle, and on the oxygen atom, respectively, a positive charge charge. The oxygen atom, of course, in addition to the positive mesomeric effect, also exhibits a negative inductive effect. However, its manifestation in the properties of the compounds under consideration is less pronounced, and therefore five-membered heterocycles with one heteroatom are referred to p-excess aromatic heterocyclic compounds. The resonance leads to some evenness of the bond lengths in the heterocycle, which also indicates a certain aromaticity of the system.
