Mechanism of hydrolysis of esters. Chemical properties

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 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 esterification catalyzed by acids also proceeds according to 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 in small amounts - a necessary intermediate in the A AC 1 mechanism. Both B1 mechanisms (of which B AC 1 is unknown) are in fact not catalytic at all, because at the beginning the dissociation of the neutral ether occurs.

Of the eight Ingold mechanisms, only six have been experimentally proven.

The latter method is preferred for saponification of esters of hindered acids, another modification of this method is alkaline hydrolysis of hindered esters in the presence of 18-crown-6-polyester:

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.

Of the eight Ingold mechanisms, only six have been experimentally proven.

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 salts of mono-nitro compounds C (NO 2) 4 are treated with Ag or alkali metal nitrites, or when nitrites act 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. Sec. 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 ground 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, positive 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.

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

Alkaline hydrolysis - ester

Page 1

Alkaline hydrolysis of esters, like acidic, proceeds according to the mechanism of addition - elimination.

Alkaline hydrolysis of esters, sometimes referred to as the specific base catalysis reaction, is actually a substitution reaction (see Sec.

Alkaline hydrolysis of esters by the Bac2 mechanism proceeds through nucleophilic addition at the carbonyl group to form a tetrahedral intermediate (see Sec. This is a general reaction of nucleophiles with the carbonyl group of the ester, and various examples of its application will be discussed later in this section. Interaction with hydride ions leads to reduction, so this reaction will be discussed along with other reduction reactions (see Sec.

Alkaline hydrolysis of esters proceeds with a thermal effect equal to the heat of neutralization of the resulting acid. The reactions of esterification of alcohols with acid chlorides, as well as the first stage of esterification with acid anhydrides, are also exothermic.

Alkaline hydrolysis of esters is an irreversible reaction, since the final product of the reaction (carboxylate anion) does not exhibit the properties of a carbonyl compound due to the complete delocalization of the negative charge.

Alkaline hydrolysis of esters is called saponification. The rate of hydrolysis of esters also increases when heated and when excess water is used.

Alkaline hydrolysis of esters is characteristic of a large number of reactions in which a negatively charged nucleophile attacks the carbonyl carbon of a neutral substrate.

Alkaline hydrolysis of esters is called saponification. The rate of hydrolysis of esters also increases when heated and when excess water is used.

In practice, alkaline hydrolysis of esters is carried out in the presence of caustic alkalis KOH, NaOH, as well as hydroxides of alkaline earth metals Ba (OH) 2, Ca (OH) 2 - The acids formed during hydrolysis are bound in the form of salts of the corresponding metals, so the hydroxides have to be taken at least in equivalent ratio with an ester. Usually an excess of base is used. The separation of acids from their salts is carried out with the help of strong mineral acids.

The reaction of alkaline hydrolysis of esters is called the saponification reaction.

The method of alkaline hydrolysis of esters is included as part of various multi-stage processes of organic synthesis. For example, it is used in the industrial production of fatty acids and alcohols by the oxidation of paraffins (chap.

Pages: 1 2 3 4

4.6. Esters

Esters can be obtained by reacting carboxylic acids with alcohols ( esterification reaction). The catalysts are mineral acids.

Video experience "Obtaining acetic ethyl ether".

The esterification reaction under acid catalysis is reversible.

The reverse process - the splitting of an ester by the action of water to form a carboxylic acid and an alcohol - is called ester hydrolysis. RCOOR' + H2O (H+) RCOOH + R'OH Hydrolysis in the presence of alkali proceeds irreversibly (since the formed negatively charged carboxylate anion RCOO– does not react with the nucleophilic reagent, alcohol).

This reaction is called saponification of esters(by analogy with the alkaline hydrolysis of ester bonds in fats in the production of soap).

Esters of lower carboxylic acids and lower monohydric alcohols have a pleasant smell of flowers, berries and fruits. Esters of higher monobasic acids and higher monohydric alcohols are the basis of natural waxes. For example, beeswax contains an ester of palmitic acid and myricyl alcohol (myricyl palmitate):

CH(CH)–CO–O–(CH)CH

Chemical properties - section Chemistry, GENERAL REGULARITIES OF THE STRUCTURE AND CHEMICAL BEHAVIOR OF OXO COMPOUNDS 1. Hydrolysis of Esters (Acid and Alkaline Catalysis). …

1. Hydrolysis of esters (acid and alkaline catalysis). The ester is a weak acylating agent and can be hydrolyzed in the presence of catalysts (acids or bases).

1.1 Alkaline hydrolysis:

Mechanism of alkaline hydrolysis:

Alkaline hydrolysis has several advantages over acidic:

proceeds at a faster rate, since the hydroxide anion is a stronger and smaller nucleophile in comparison with a water molecule;
in an alkaline environment, the hydrolysis reaction is irreversible, since an acid salt is formed that does not have an acylating ability.

Therefore, in practice, the hydrolysis of esters is often carried out in an alkaline medium.

1.2 Acid hydrolysis:

2. Interesterification reaction. Interaction with alkoxides in a solution of the corresponding alcohol leads to the exchange of alkyl groups of the ester, the reaction is reversible:

3. Ammonolysis reaction:

Esters in nature, their importance in industry. The least reactive derivatives of carboxylic acids, such as esters, amides, and nitriles, are widely used as solvents.

Industrial and preparative value are ethyl acetate, dimethylformamide and acetonitrile. Dimethylformamide is an aprotic solvent for both polar (even salts) and non-polar substances and is currently widely used in industry as a solvent for polyamides, polyimides, polyacrylonitrile, polyurethanes, etc., is used to form fibers and films, prepare adhesives, etc. as well as in laboratory practice.

Esters of lower carboxylic acids ( C1 - C5) and lower alcohols (CH3OH, C2H5OH) have a fruity smell - they are used to perfume soaps and in confectionery. Acetates, butyrates of citronellol, geraniol, linalool, which have a pleasant floral smell, are, for example, part of lavender oil and are used to make soaps and colognes.

Esters of diphenylacetic acid, such as diethylaminoethyl ether (spasmolytin), known as antispasmodics - drugs that relieve spasms of smooth muscles of internal organs and blood vessels. Anestezin - ethyl ether n-aminobenzoic acid, novocaine - diethylaminoethyl ether n-aminobenzoic acid, paralyzing the nerve endings, cause local anesthesia, anesthesia. More powerful than novocaine is xicaine (N- 2,6-dimethylphenylamide N,N'-diethylaminoacetic acid).

Ethyl acetate - colorless liquid, is used as a solvent for dissolving nitrocellulose, cellulose acetate and other polymeric materials, for the manufacture of varnishes, as well as in the food industry and perfumery.

Butyl acetate - colorless liquid with a pleasant odor. Used in the paint and varnish industry as a solvent for nitrocellulose and polyester resins.

Amyl acetates– good solvents for nitrocellulose and other polymeric materials. Isoamyl acetate is used in the food industry (pear essence).

Artificial fruit essences. Many esters have a pleasant smell and are used in the food and perfume industries.

All topics in this section:

GENERAL REGULARITIES OF THE STRUCTURE AND CHEMICAL BEHAVIOR OF OXO COMPOUNDS
Multiple bonds between carbon and oxygen are found in aldehydes, ketones, carboxylic acids, and also in their derivatives. For compounds containing a carbonyl group, the most characteristic

OXO COMPOUNDS
Aldehydes and ketones are derivatives of hydrocarbons that contain a functional group in the molecule called a carbonyl or oxo group. If the carbonyl group is linked to one

Technical methods for obtaining formaldehyde
3.1 Catalytic oxidation of methanol: 3.2 Ka

Specific methods for the aromatic series
11.1 Oxidation of alkylarenes. Partial oxidation of the alkyl group associated with the benzene ring can be carried out by the action of various oxidizing agents. Methyl group - MnO

Nucleophilic addition reactions
1.1 Addition of magnesium alkyls: where

Oxidation reactions of aldehydes and ketones
5.1 Oxidation of aldehydes. Aldehydes oxidize most easily, turning into carboxylic acids with the same number of carbon atoms in the chain:

Reactions of oxidation-reduction (disproportionation)
6.1 The reaction of Cannizzaro (1853) is characteristic of aldehydes that do not contain hydrogen atoms in the α-position, and occurs when they are treated with concentrated p

CARBOXY ACIDS AND THEIR DERIVATIVES
Carboxylic acids are derivatives of hydrocarbons containing a carboxyl functional group (-COOH) in the molecule. This is the most "oxidized" functional group, which is easy to trace,

MONOCARBOXIC ACIDS
Monocarboxylic acids are derivatives of hydrocarbons containing one functional carboxyl group, COOH, in the molecule. Monocarboxylic acids are also called monobasic

isomerism
Structural: · skeletal; · metamerism Spatial: · optical. Synthesis methods. Monocarbon

Reactions of carboxylic acids with nucleophilic reagents
1.1 Formation of salts with metals:

DERIVATIVES OF CARBOXY ACID
Carboxylic acids form a variety of derivatives (esters, anhydrides, amides, etc.), which are involved in many important reactions. General formula for derivatives

How to get
1. Interaction with phosphorus (V) chloride:

Chemical properties
1. Use of anhydrides as acylating agents.

Anhydrides, like acid halides, have high chemical activity, are good acylating agents (often

Methods for obtaining amides
1. Acylation of ammonia:

Chemical properties
1. Hydrolysis of amides 1.1 In an acidic environment:

How to get
1. Esterification reaction: Esterific mechanism

DICARBOXIC ACIDS
The class of dicarboxylic acids includes compounds containing two carboxyl groups. Dicarboxylic acids are subdivided depending on the type of hydrocarbon radical:

General methods for the preparation of dicarboxylic acids
1. Oxidation of diols and cyclic ketones:

isomerism
Structural: · skeletal; position isomerism; metamerism. Spatial: · geometric. Unlimited

Chemical properties of fats
1. Hydrolysis. Among the reactions of fats, hydrolysis, or saponification, which can be carried out with both acids and bases, is of particular importance:

FEATURES OF PHYSICAL PROPERTIES OF HOMO-FUNCTIONAL HYDROCARBON DERIVATIVES
The presence of a functional group associated with a hydrocarbon substituent significantly affects the physical properties of the compounds. Depending on the nature of the functional group (atom), e

HYDROCARBONS
Among the many different functional derivatives of hydrocarbons, there are compounds that are highly toxic and hazardous to the environment, moderately toxic and completely harmless, non-toxic, widely

When esters are heated with alcohols, a double exchange reaction occurs, called interesterification. This reaction is catalyzed by both acids and bases:

To shift the equilibrium in the desired direction, a large excess of alcohol is used.

Methacrylic acid butyl ester (butyl methacrylate) can be obtained in 94% yield by heating methyl methacrylate with n-butanol with continuous removal of methanol as it is formed:

The alcoholysis of esters of carboxylic acids under the influence of alkaline catalysts is of particular preparative importance for the synthesis of esters of thermally unstable carboxylic acids with a long side chain (for example, esters b-ketoacids) and alcohol esters, unstable in acidic media, which cannot be obtained by conventional esterification methods. Sodium alcoholates, sodium hydroxide and potassium carbonate are used as catalysts for such reactions.

Alcoholysis of esters b-keto acids is easily carried out at 90-100°C without a catalyst. For example, acetoacetic acid octyl ester was synthesized from acetoacetic ester using this method:

Thus, it is possible to exchange the primary alcohol with another primary or secondary alcohol with a higher boiling point, but this method is not suitable for obtaining esters from tertiary alcohols. Esters of tertiary alcohols are obtained in a different way - by mutual interesterification of two different esters of carboxylic acids, for example, an ester of formic acid and some other acid:

The reaction is carried out in the presence of catalytic amounts tert-sodium butoxide at 100-120°C.

In this case, the lowest-boiling component of the equilibrium mixture is slowly distilled off, in this case, formic acid methyl ester (methyl formate, bp 34°C).

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Hydrolysis - Ether

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Hydrolysis of ethers in a strongly acidic medium (Sec.

Subsequently, the hydrolysis of ethers became of interest from the point of view of the theory of chemical structure, namely, as a reaction by which one can determine the relative strength of the carbon-oxygen bond depending on the structure of the radical. In the 1930s, there was a practical need to develop a technically acceptable method for the hydrolysis of diethyl ether; this need was dictated by the fact that during the production of synthetic rubber according to the Lebedev method, ether was formed as a by-product, which was expediently converted into alcohol. In this regard, the hydrolysis of diethyl ether was studied in the USSR by Vanscheidt and Lozovskaya and Kagan, Russian and Cherntsov, using aluminum, titanium, thorium, chromium, and manganese oxides as catalysts.

The patent literature describes the hydrolysis of ethers to form alcohols by the action of dilute sulfuric acid at high temperature and pressure; the process was carried out at 272 C and 130 atm for 25 min. This method is used only when excess ethyl ether needs to be disposed of.

The patent literature describes the hydrolysis of ethers with the formation of alcohols under the action of dilute sulfuric acid at high temperature and pressure [ 22J; the process was carried out at 272 C and 130 atm for 25 min. This method is used only when excess ethyl ether needs to be disposed of.

The removal of acetaldehyde from the reaction sphere in the form of an oxime determines the completeness of the hydrolysis of the ether. Do not interfere with the determination of water, alcohols, hydrocarbons.

The hydrolysis of peptides, amides, and esters of phosphoric acid and the hydration of pyridine aldehydes are similarly catalyzed. The hydrolysis of ethers is not catalyzed by metal ions as no chelation occurs and the intermediate cannot be stabilized.

General acid-base catalysis is very common, but there are a few cases in which specific hydrogen or hydroxyl ion catalysis occurs; in this case the rate constant varies linearly with [H3O] and [OH-] and does not depend on the presence of other acidic and basic substances. For example, specific catalysis has been found in the hydrolysis of ethers (see p.

Cleavage of phenol esters with aluminum chloride provides a ready-made method for obtaining difficult-to-synthesize phenol derivatives; some characteristic transformations of phenol esters to the corresponding phenols are listed here. Although the cleavage of alkoxy groups is so easily catalyzed by aluminum chloride, there is no methodological study on the effect of substituents on the hydrolysis of ethers catalyzed by aluminum chloride.

However, for the reaction to be successful, the presence of two, for example, methoxyl groups in the molecule of the azo component or the use of a very active diazo component is necessary. Interestingly, the azo coupling of phenol esters often results in the hydrolysis of the ether group, so that an azo dye is formed, which is a derivative of the phenol itself. Recall that in general the hydrolysis of ethers is very difficult. The mechanism of this reaction has not been studied.

In conclusion, it can be said that saponification under MPA conditions is synthetically advantageous in the case of sterically hindered esters. In this case, the solid potassium hydroxide / toluene system and crown ethers or cryptands should be used as catalysts. In addition, the rate of hydrolysis of ethers of carboxylic acids with concentrated aqueous sodium hydroxide is much higher for hydrophilic carboxylates. Good catalysts are quaternary ammonium salts, especially Bu4NHSO4 and some anionic and nonionic surfactants. This indicates that any of three possible mechanisms can occur: reactions on the surface, micellar catalysis, or a true MFC reaction. Depending on the conditions, each of these mechanisms can be implemented.

We will end up with the following values of CR comA: 311 for HI, 318 for HBr, 329 for HC1, 334 for water, and 334 for ROH. Thus, we can predict that HI will be the most reactive, in full agreement with experience, although concentrated aqueous solutions are used in practice, while our calculations were made for reactions in the gas phase. It is well known that, at room temperature, ethers are practically incapable of reacting with water and alcohols. In addition, it is customary to say that the hydrolysis of ethers is accelerated by hydrogen rather than hydroxyl ions, which is in agreement with the nucleophilic properties established for ethers by our approximate calculations, Addition of hydrogen halide to olefins. First of all, it is necessary to establish whether the rate-determining stage is the electrophilic attack of the hydrogen ion or the nucleophilic attack of the halide ion on the carbon atom of the olefin.

Weak acids and alkalis do not affect them. Alkalis do not contribute to the hydrolysis of ethers. Along with such resistance to hydrolysis, ethers are quite easily oxidized by atmospheric oxygen, especially under the influence of light, forming peroxides (p. Esters, as a rule, are difficult to dissolve in water, but readily dissolve in most organic solvents. Many of the esters have specific pleasant fruity smell, which allows them to be used for the manufacture of artificial fruit essences in confectionery or perfumery, as well as for the identification of certain acids or alcohols by the smell of their esters.

Ethers are neutral liquids that are poorly soluble in water. They do not react with metallic sodium, which makes it possible to remove residual water and alcohol from them using metallic sodium. Ethers are highly durable. Weak acids and alkalis do not affect them. The hydrolysis of ethers proceeds with difficulty when heated with water in the presence of acids. Alkalis do not contribute to the hydrolysis of ethers. Along with such resistance to hydrolysis, ethers are quite easily oxidized by atmospheric oxygen, especially under the influence of light, forming peroxides (p. Esters, as a rule, are difficult to dissolve in water, but readily dissolve in most organic solvents. Many of the esters have specific pleasant fruity smell, which allows them to be used for the manufacture of artificial fruit essences in confectionery or perfumery, as well as for the identification of certain acids or alcohols by the smell of their esters.

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Esters are typical electrophiles. Due to the +M effect of the oxygen atom associated with the hydrocarbon radical, they exhibit a less pronounced electrophilic character compared to acid halides and acid anhydrides:

The electrophilicity of ethers increases if the hydrocarbon radical forms a conjugated system with the oxygen atom, the so-called. activated esters:

Esters enter into nucleophilic substitution reactions.

1. Hydrolysis of esters takes place in both acidic and alkaline environments.

Acid hydrolysis of esters is a sequence of reversible transformations opposite to the esterification reaction:

The mechanism of this reaction involves the protonation of the oxygen atom of the carbonyl group to form a carbocation, which reacts with a water molecule:

Alkaline hydrolysis. Hydrolysis in the presence of aqueous solutions of alkalis is easier than acidic because the hydroxide anion is a more active and less bulky nucleophile than water. Unlike acid hydrolysis, alkaline hydrolysis is irreversible:

Alkali acts not as a catalyst, but as a reactant. Hydrolysis begins with the nucleophilic attack of the hydroxide ion on the carbon atom of the carbonyl group. An intermediate anion is formed, which splits off the alkoxide ion and turns into a carboxylic acid molecule. The alkoxide ion, as a stronger base, abstracts a proton from an acid molecule and turns into an alcohol molecule:

Alkaline hydrolysis is irreversible because the carboxylate anion has a high negative charge delocalization and is not susceptible to attack by the alcohol hydroxyl.

Often the alkaline hydrolysis of esters is called saponification. The term comes from the name of the products of alkaline hydrolysis of fats - soap.

2. The interaction with ammonia (immonolysis) and its derivatives proceeds according to a mechanism similar to alkaline hydrolysis:

3. The reaction of interesterification (alcoholysis of esters) is catalyzed by both mineral acids and shells:

To shift the equilibrium to the right, the more volatile alcohol is distilled off.

4. Claisen ester condensation is typical for esters of carboxylic acids containing hydrogen atoms in the α-position. The reaction proceeds in the presence of strong bases:

The alkoxide ion splits off a proton from the α-carbon atom of the ether molecule. A mesomerically stabilized carbanion (I) is formed, which, acting as a nucleophile, attacks the carbon atom of the carbonyl group of the second ester molecule. The addition product (II) is formed. It splits off the alkoxide ion and turns into the final product (III). Thus, the whole scheme of the reaction mechanism can be divided into three stages:

If two esters containing α-hydrogen atoms react, then a mixture of four possible products is formed. The reaction is used for the industrial production of acetoacetic ester.

5. Recovery of esters:

Primary alcohols are formed by the action of hydrogen gas in the presence of a skeletal nickel catalyst (Raney nickel).

6. The action of organomagnesium compounds followed by hydrolysis leads to the formation of tertiary alcohols.

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18.8.2.A. Mechanisms of ester hydrolysis

18.8.2.A. Mechanisms of ester hydrolysis

Hydrolysis of esters

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Alkaline hydrolysis - ester

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Hydrolysis - Ether

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