Carboxylic Acid Derivatives

CARBOXYLIC ACID DERIVATIVES

There are four carboxylic acid derivatives. These are generally represented as$R-\stackrel{\stackrel{O}{||}}{C}-Z$

where Z is halogen (usually Cl), OCOR', OR' or NH₂ (or NHR¢ or NR₂').

(a) When Z is halogen (usually Cl), the derivatives are called as acid chlorides.

(b) When Z is $-$OR', the derivatives are called as esters.

(c) When Z is, the derivatives are called carboxylic anhydrides.

(d) Where Z is $-$ NH₂, the derivatives are called amides. When Z is $-$NHR' or $-$NR₂' they are called N – substituted amides.

Synthesis of acid derivatives

Carboxylic acid derivatives are exclusively prepared from carboxylic acids. The preparation methods of carboxylic acid derivatives are already discussed under the chemical reactions of carboxylic acids.

Illustration 1. (a) $PhLi\underset{2.H^{+}}{\overset{1.ExcessC{O}_2}{\to }}\left(A\right)$

(b) ${\left(C{H}_3\right)}_{3}COOH\underset{H^{+}}{\overset{2eq.C{H}_{3}Li}{\to }}\left(B\right)$

${\left(C{H}_3\right)}_{3}COOH\underset{B{r}_2,\Delta }{\overset{A{g}_{2}O}{\to }}\left(C\right)$

Solution:

(a)

(b)

${\left(C{H}_3\right)}_{3}CCOOH\to {\left(C{H}_3\right)}_{3}C-\stackrel{\stackrel{O}{||}}{C}-OC{H}_3\left(B\right)$

${\left(C{H}_3\right)}_{3}CCOOH\underset{B{r}_2,\Delta }{\overset{A{g}_{2}O}{\to }}{\left(C{H}_3\right)}_{3}CBr+C{O}_2$

CHEMICAL REACTIONS OF ACID DERIVATIVES

(i) Acyl chloride

We have already seen that acyl chlorides are the most reactive of all acid derivatives. As a result, acyl chlorides are often selected as the starting material for the preparation any other acid derivative. Let us see how this is done.

Reaction of acetyl chloride with olefins

Acetyl chlorides add on to the double bond of an olefin in the presence of a catalyst (AlCl₃ or ZnCl₂) to form a chloro ketone which on heating, eliminates a molecule of hydrogen chloride to form an unsaturated ketone.

Conversion into acids: Hydrolysis.

Illustration 2. Acetyl chloride reacts with water more readily than methyl chloride. Explain.

Solution: Alkyl halides are much less reactive than acyl halides in nucleophilic substitution because nucleophilic attack on the tetrahedral carbon of RX involves a hindered transition state. Also, to permit the attachment of the nucleophile a bond must be partly broken. In CH₃COCl, the nucleophile, attack on > C = O involves a relatively unhindered transition of acyl halides occurs in two steps. The first step is similar to addition to carbonyl compound and the second involves the loss of chlorine in this case.

(ii) Carboxylic acid anhydrides

Carboxylic acid anhydrides can be used to prepare esters and amides.

Anhydrides can be hydrolysed to get back acids.

Illustration 3.

The compound (X) is

(A) CH₃COOH (B) BrCH₂ – COOH

(C) (CH₃CO)₂O (D) CHO – COOH

Solution: (C)

(iii) Esters

Ester hydrolysis

Acid catalysed esterification is an essentially reversible reaction. If you follow the backward course of reactions of esterification it gives you the mechanism for ester hydrolysis.

Base promoted hydrolysis of esters: Saponification

Esters undergo base promoted hydrolysis also. This reaction is known as saponification, because it is the way most of the soaps are manufactured. Refluxing an ester with aqueous NaOH produces an alcohol and the sodium salt of the acid.

This reaction is essentially irreversible because carboxylate ion is inert towards nucleophilic substitution.

Mechanism

If an ester is hydrolysed in a known amount of base (taken in excess), the amount of base used up can be measured and used to calculate the saponification equivalent; the equivalent weight of the ester, which is similar to the neutralization equivalent of an acid.

Transesterification

Esters can also be prepared by transesterification (an alcohol displacing another from an ester)

The mechanism of this reactions is quite similar to esterification.

Transesterification is an equilibrium reaction. To shift the equilibrium to right, it is necessary to use a large excess of the alcohol whose ester we wish to make or else to remove one of the products from the reaction mixture.

Reduction of esters

(i) Catalytic hydrogenation

$C{H}_{3}C{H}_{2}C{H}_2\stackrel{\stackrel{O}{||}}{C}-O-C{H}_3\underset{{150}^{0}Chighpressure}{\overset{H_2,CuO,Hg/H^{+}}{\to }}C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}OH+C{H}_{3}OH$.

(ii) Chemical reduction

$C{H}_{3}C{H}_{2}C{H}_2\stackrel{\stackrel{O}{||}}{C}-O-C{H}_3\underset{\left(or\right)Na/C_2{H}_{5}OH}{\overset{LiAl{H}_4/H^{+}}{\to }}C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}OH+C{H}_{3}OH$

Reaction of esters with Grignard reagents

The reaction of carboxylic esters with Grignard's reagent is a good method for the preparation of 3° alcohols.

Initially ketones are formed. However, as we know, ketones themselves readily react with Grignard reagent to yield teriary alcohols.

Claisen Condensation

When ethyl acetate reacts with sodium ethoxide, it undergoes a condensation reaction. After acidification, the product is a $\beta$ - keto ester, ethyl aceto acetate (commonly known as – aceto acetic ester)

Condensation of this type is known as Claisen Condensation. For esters, it is the exact counterpart of the Aldol Condensation. Like the Aldol Condensation, the Claisen Condensation involves nucleophilic attack by a carbanion on an electron – deficient carbonyl compound. In the Aldol Condensation, nuclephilic attack leads to addition (the typical reaction of aldehydes and ketones). In the Claisen Condensation, nucleophilic attack leads to substitution (the typical reaction of acyl compounds)

Mechanism

This step is highly favourable and draws the overall equilibrium toward the product formation

When planning a claisen condensation with an ester it is important to use alkoxide ion that has the same alkyl group as the alkoxyl group of the ester. This is to avoid the possibility of transesterification.

An intramolecular claisen condensation is called Dieckmann condensation.

In general, the Dieckmann condensation is useful only for the preparation of five and six membered rings.

Amides

Preparation: In the laboratory amides are prepared by the reaction of ammonia with acid chlorides or, acid anhydrides.

Basic Character of Amides:

Amides are very feebly basic and form unstable salts with strong inorganic acids. e.g. RCONH₂HCl. The structure of these salts may be I or II

Acidic Character of Amides:

Amides are also feebly acidic e.g. they dissolve mercuric oxide to form covalent mercury compound in which the mercury is probably linked to the nitrogen.

2RCONH₂ + HgO $\to$ (RCONH)₂Hg + H₂O

(a) Hydrolysis of amides

Amides undergo hydrolysis when they are heated with aqueous acid or aqueous base.

Acidic hydrolysis

Illustration 4. Acetamide reacts with NaOBr in alkaline medium to form

(A) NH₃­ (B) CH₃NH₂

(C) CH₃CN (D) C₂H₅NH₂

Solution: (B)