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:
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:
2. Preparation of Esters by the 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.
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.