Preparation of Esters

Ester is usually prepared by their reaction with alcohols (or phenols) from carboxylic acids or their functional derivatives. Some of the common methods for the . preparation of esters are given below. 

1. Preparation of ester by Direct esterification:

 Esters are usually prepared by refluxing a mixture of a carboxylic acid and an alcohol in the presence of a little mineral acid, usually cone. H2S04 or dry HCl that acts as catalyst. This reaction which is known as esterification, is reversible, and at equilibrium, appreciable quantities of both the reactants and the products are present.

direct-esterification-formation-of-an-ester-and-water-from-carboxylic-acid-and-alcohol

 However, the equilibrium can be shifted to the right side by, remove water from the mixture of the reaction as it is formed, thereby  increasing the yield of the ester.
The mechanism of the esterification reaction is that of a typical acid catalyzed nucleophilic substitution reaction at an acyl carbon atom, as shown below:
mechanism-of-the-esterification-reaction-in-ester-preparation

 If we follow this pathway backward, it will provide the mechanism for the acid catalyzed hydrolysis of an ester.Whether we push the reaction forward (esterification) or backward (hydrolysis of ester) depends on the conditions we choose For esterification, we use acid or alcohol (which ever is cheaper)‘ in excess, and, if possible, remove the water as it is formed ; for ester hydrolysis, we use a large excess of water, i.e we reflux the ester with dilute aqueous HCl .

2: Steric hindrance:

The presence of large groups near the reaction site, whether in alcohol or in acid, slows down both the forward (esterification) and the reverse (ester hydrolysis) reactions. due to steric hindrance, Tertiary alcohols, for example, react so slowly in esterification that instead they usually undergo elimination. The preparation of esters of tertiary alcohols or acids such as mesitoic acid (2, 4, -trimethylbenzoic acid) requires special methods.
Since phenols are not susceptible to dehydration, phenolic esters can be prepared by the usual method.

preparation-of-phenolic-ester

If both a carboxyl and a hydroxyl groups are present in the same molecule and are suitably situated with respect to each other (1, 4or 1,5-positions) the molecule undergoes intramolecular esterification to form a cyclic ester, commonly known as a lactone. Thus,Gymma -and sigma-hydroxy carboxylic acids on heating yield 7 and 5 lactones, respectively, under the usual esterification conditions. The most common lactones are those with five-or sis membered rings.
 Lactones are hydrolysed by an aqueous base to yield open-chain salts of hydroxy carboxylic acids.

2. Preparation of Esters by the use of diazomethane:.

 Methyl esters can be prepared by the reaction of carboxylic acids with diazomethane 1n ethereal solution.

use-of-diazomethane

The reaction occurs almost instantaneously, but is limited by the high price and the hazardous nature of diazomethane.

3. Preparation of Esters from acid chlorides:.

Esters can also be prepared by the reaction of acid chlorides with alcohols. Since acid chlorides are much more reactive than carboxylic acids toward nucleophilic substitution. This reaction occurs rapidly and does not require a catalyst. However, pyridine is usually used to react with HCl that is formed in reaction.

preparation-of-Esters-from-acid-chlorides

Thus, the problems encountered in the direct esterification of carboxylic acids, i.e. reversibility of the reaction and difficulty with the sterically hindered reactants, can’t easily avoided if the acid is first converted to acid chloride which is then allowed to East With an ester forming alcohol (or phenol). All classes of alcohols can be used When esters are prepared by this method.

This reaction is not reversible because the ester is much more stable than the acid chloride, and also because one of the products, i.e., HCl is a gas that can be easily removed from the reaction mixture, thus preventing the reverse reaction from taking place.


4. Preparation of Esters from acid Anhydrides.

Esters can also be prepared from acid anhydrides by their reaction With alcohols. In this case also no catalyst is required.

Esters-from-acid-Anhydrides

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