Esters, carboxylic acids and alcohols


carboxylic acid
 
ester
Esters are referred to as carboxylic acid derivatives because of the structural similarity highlighted in the images at the right.

Carboxylic acids and their derivatives are interconverted by reactions that are classed as substitutions because part of the reagent and the organic molecule are exchanged.

Carboxylic acids react with alcohols in the absence of water to form esters.
 
 + CH3OH   + HO–H

Esters react with water to form carboxylic acids and alcohols.
Both this reaction and the reaction above are faster in the presence of a strong acid catalyst.
 
+ HO–H       + CH3OH
 
The reaction of an ester to form a carboxylic acid can also be described as a hydrolysis. Hydrolyses are reactions with water.
 
Hydrolyses of esters are both faster and give better yields if they are done in aqueous alkali. This is because the carboxylic acid product reacts further to form the carboxylate anion which does not react by the substitution reaction to form the ester. Once reaction is complete the carboxylic acid and alcohol are isolated by acidification of the reaction mixture.
Dr Sheila Woodgate
© The University of Auckland 2002