Reactions of Carboxylic Acids Involving -OH
Formation of Acid Chlorides
Carboxylic acids react with PCl5, PCl3 or SOCl2 (thionyl chloride) to give acid chlorides
Can you do the following conversion? You may have to use the reactions that we studied in previous tutorials.
Convert benzoic acid to benzaldehyde.
When acids are warmed with alcohols in the presence of conc. H2SO4 (or dry HCl), pleasant fruity smelling esters are formed.
Important points regarding esterification
- conc. H2SO4 acts as dehydrating agent.
- Since the reaction is an equilibrium reaction, ester is distilled off to shift the equilibrium in forward reaction.
Let's do the conversion. Remember, some reactions involved might have been discussed in previous tutorials.
Convert benzene to methyl benzoate.
Formation of amide
Carboxylic acids are treated with ammonia to form ammonium salts which upon heating lose a molecule of water to form amides.
Formation of Anhydrides
When two molecules of carboxylic acids are heated with a dehydrating agent such as phosphorus pentoxide, acid anhydrides are formed.
Question : During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed. Why?
Answer : The reaction of a carboxylic acid and an alcohol in the presence of an acid catalyst is a reversible reaction (the reaction is given above under the heading esterification). Hence, to shift the equilibrium in the forward direction, either the water or the ester should be removed immediately after it is formed.