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 of 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 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 at the beginning the dissociation of the neutral ether occurs.
Of the eight Ingold mechanisms, only six have been experimentally proven.
The structural formula of esters in general terms:
where R and R' are hydrocarbon radicals.
Hydrolysis of esters
One of the most characteristic abilities of 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 an ester carbonyl group, 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 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 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 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.
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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 highly toxic and environmentally hazardous compounds, 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
Page 1
Hydrolysis of ethers in a strongly acidic medium (Sec.
Subsequently, the hydrolysis of ethers began to be 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 successful completion of the reaction, 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 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.
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. 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 are easily soluble in most organic solvents. 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 are easily soluble in most organic solvents. 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 called functional derivatives of carboxylic acids of the general formula RC(O)OR" .
Esters of carboxylic acids (as well as sulfonic acids) are named similarly to salts, only instead of the name of the cation, the name of the corresponding alkyl or aryl is used, which is placed before the name of the anion and is written together with it. The presence of the -COOR ester group can also be expressed in a descriptive way, for example, "R-ester of (such and such) acid" (this method is less preferred due to its cumbersomeness):
Esters of lower alcohols and carboxylic acids are volatile liquids, with a pleasant smell, poorly soluble in water and well - in most organic solvents. The odors of esters are reminiscent of the smells of various fruits, which is why in the food industry, essences are prepared from them that mimic fruit odors. The increased volatility of esters is used for analytical purposes.
Hydrolysis. The most important of the acylation reactions is the hydrolysis of esters with the formation of an alcohol and a carboxylic acid:
The reaction is carried out in both acidic and alkaline environments. acid catalyzed ester hydrolysis - the reverse reaction of esterification, proceeds according to the same mechanism A AC 2:
The nucleophile in this reaction is water. The equilibrium shift towards the formation of alcohol and acid is provided by the addition of excess water.
Alkaline hydrolysis is irreversible; during the reaction, a mole of alkali is consumed per mole of ether, i.e., alkali in this reaction acts as a consumable reagent, and not a catalyst:
Hydrolysis of esters in alkaline environment proceeds via the bimolecular acyl mechanism B AC 2 through the formation of a tetrahedral intermediate (I). The irreversibility of alkaline hydrolysis is provided by the practically irreversible acid-base interaction of the carboxylic acid (II) and the alkoxide ion (III). The resulting anion of carboxylic acid (IV) is itself a fairly strong nucleophile and therefore is not subjected to nucleophilic attack.
Ammonolysis of esters. Amides are obtained by ammonolysis of esters. For example, under the action of aqueous ammonia on diethyl fumarate, full fumaric acid amide is formed:
In the ammonolysis of esters with amines with low nucleophilicity, the latter are first converted into amides of alkali or alkaline earth metals:
Amides of carboxylic acids: nomenclature; the structure of the amide group; acid-base properties; acid and alkaline hydrolysis; splitting by hypobromites and nitrous acid; dehydration to nitriles; chemical identification.
Amides called functional derivatives of carboxylic acids of the general formula R-C (O) -NH 2- n R "n, where n = 0-2. In unsubstituted amides, the acyl residue is connected to an unsubstituted amino group, in N-substituted amides one of the hydrogen atoms is replaced by one alkyl or aryl radical, in N,N-substituted - by two.
Compounds containing one, two, or three acyl groups attached to the nitrogen atom are generically called amides (primary, secondary, and tertiary, respectively). The names of primary amides with an unsubstituted group - NH 2 are derived from the names of the corresponding acyl radicals by replacing the suffix -oil (or -yl) with -amide. Amides formed from acids with the suffix -carboxylic acid receive the suffix -carboxamide. Amides of sulfonic acids are also named after their respective acids, using the suffix -sulfonamide.
The names of the radicals RCO-NH- (as well as RSO 2 -NH-) are formed from the names of amides, changing the suffix -amide to -amido-. They are used if there is an older group in the rest of the molecule or the substitution occurs in a more complex structure than the radical R:
In the names of N-substituted primary amides RCO-NHR" and RCO-NR"R" (as well as similar sulfonamides), the names of the radicals R" and R" are indicated before the name of the amide with the symbol N-:
Amides of this type are often referred to as secondary and tertiary amides, which is not recommended by IUPAC.
N-Phenyl-substituted amides are given the suffix -anilide in their names. The position of the substituents in the aniline residue is indicated by numbers with strokes:
In addition, semi-systematic names have been preserved in which the suffix -amide is connected to the base of the Latin name of the carboxylic acid (formamide, acetamide), as well as some trivial names such as "anilides" (acylated anilines) or "toluidides" (acylated toluidines).
Amides are crystalline substances with relatively high and distinct melting points, which allows some of them to be used as derivatives for the identification of carboxylic acids. In rare cases, they are liquids, for example, formic acid amides - formamide and N,N-dimethylformamide - known dipolar aprotic solvents. The lower amides are highly soluble in water.
Amides are one of the most resistant to hydrolysis functional derivatives of carboxylic acids, due to which they are widely distributed in nature. Many amides are used as medicines. For about a century, paracetamol and phenacetin, which are substituted amides of acetic acid, have been used in medical practice.
The structure of amides. The electronic structure of the amide group is largely similar to the structure of the carboxyl group. The amide group is a p,π-conjugated system in which the lone pair of electrons of the nitrogen atom is conjugated with the electrons of the C=O π bond. Delocalization of the electron density in the amide group can be represented by two resonance structures:
Due to conjugation, the C-N bond in amides has partially doubly linked character, its length is significantly less than the length of a single bond in amines, while the C=O bond is somewhat longer than the C=O bond in aldehydes and ketones. Amide group due to conjugation has a flat design . Below are the geometric parameters of the N-substituted amide molecule, determined using X-ray diffraction analysis:
An important consequence of the partially doubly bonded nature of the C-N bond is a rather high energy barrier to rotation around this bond, for example, for dimethylformamide it is 88 kJ/mol. For this reason, amides having different substituents on the nitrogen atom can exist as π-diastereomers. N-substituted amides exist predominantly as Z-isomers:
In the case of N,N-disubstituted amides, the ratio of E- and Z-isomers depends on the volume of radicals connected to the nitrogen atom. Stereoisomers of amides are configurationally unstable, their existence has been proved mainly by physicochemical methods, and they have been isolated individually only in a few cases. This is due to the fact that the rotation barrier for amides is still not as high as for alkenes, for which it is 165 kJ/mol.
Acid-base properties. Amides have weak both acidic and basic properties . The basicity of the amides lies within the range of Pk BH + from -0.3 to -3.5. The reason for the reduced basicity of the amino group in amides is the conjugation of the lone pair of electrons of the nitrogen atom with the carbonyl group. When interacting with strong acids, amides are protonated at the oxygen atom in both dilute and concentrated acid solutions. This kind of interaction underlies acid catalysis in amide hydrolysis reactions:
Unsubstituted and N-substituted amides exhibit weak NH-acid properties , comparable to the acidity of alcohols and remove a proton only in reactions with strong bases.
Acid-base interaction underlies the formation of amides intermolecular associates , the existence of which explains the high melting and boiling points of amides. The existence of two types of associates is possible: linear polymers and cyclic dimers. The predominance of one type or another is determined by the structure of the amide. For example, N-methylacetamide, for which the Z-configuration is preferred, forms a linear associate, and lactams, which have a rigidly fixed E-configuration, form dimers:
N, N-Disubstituted amides form dimers due to the dipole-dipole interaction of 2 polar molecules:
Acylation reactions. Due to the presence of a strong electron-donating amino group in the conjugated system of amides, the electrophilicity of the carbonyl carbon atom, and hence the reactivity of amides in acylation reactions, is very low. Low acylating ability of amides is also explained by the fact that the amide ion NH 2 - is a bad leaving group. Of the acylation reactions, hydrolysis of amides is important, which can be carried out in acidic and alkaline media. Amides are much more difficult to hydrolyze than other functional derivatives of carboxylic acids. The hydrolysis of amides is carried out under more severe conditions compared to the hydrolysis of esters.
Acid hydrolysis amides - irreversible reaction leading to the formation of a carboxylic acid and an ammonium salt:
In most cases, the acid hydrolysis of amides proceeds according to the mechanism bimolecular acid acylation A AC 2 , i.e. similar to the mechanism of acid hydrolysis of esters. The irreversibility of the reaction is due to the fact that ammonia or amine in an acidic environment is converted into an ammonium ion that does not have nucleophilic properties:
Alkaline hydrolysis Same irreversible reaction; as a result of it, a salt of a carboxylic acid and ammonia or an amine are formed:
Alkaline hydrolysis of amides, like the hydrolysis of esters, proceeds via tetrahedral mechanism IN AC 2 . The reaction begins with the addition of a hydroxide ion (nucleophile) to the electrophilic carbon atom of the amide group. The resulting anion (I) is protonated at the nitrogen atom, and then a good leaving group, an ammonia or amine molecule, is formed in the bipolar ion (II). It is believed that the slow stage is the decay of the tetrahedral intermediate (II).
For anilides and other amides with electron-withdrawing substituents at the nitrogen atom, the decomposition of the tetrahedral intermediate (I) can proceed through the formation of the dianion (II):
Cleavage with nitrous acid. When interacting with nitrous acid and other nitrosating agents, amides are converted into the corresponding carboxylic acids with yields up to 90%:
Dehydration. Unsubstituted amides under the action of phosphorus (V) oxide and some other reagents (POC1 3, PC1 5, SOCl 2) are converted into nitriles:
47. Carboxylic acids: halogenation according to Gell-Volhard-Zelinsky, using the reaction for synthesis a -hydroxy and a -amino acids.
Halogenation of aliphatic carboxylic acids.
Aliphatic carboxylic acids are halogenated at the α-position with chlorine or bromine in the presence of catalytic amounts red phosphorus or phosphorus halides (Gell-Volhard-Zelinsky reaction ). For example, when hexanoic acid is brominated in the presence of red phosphorus or phosphorus(III) chloride, 2-bromohexanoic acid is formed in high yield, for example:
It is not the carboxylic acid itself that undergoes bromination, but the acid chloride formed from it in situ. The acid chloride has stronger CH-acid properties than the carboxylic acid and more easily forms the enol form.
Enol (I) adds bromine to form a halogen derivative (II), which further abstracts a hydrogen halide and turns into an α-halogen-substituted acid halide (III). At the last stage, the unsubstituted carboxylic acid halide is regenerated.
Other heterofunctional acids are synthesized from the resulting α-halo-substituted acids using nucleophilic substitution reactions.
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.
