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Reactions of Carboxylic Acids Involving -OH

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Formation of Acid Chlorides

Carboxylic acids react with PCl5, PCl3 or SOCl2 (thionyl chloride) to give acid chlorides

Reaction of carboxylic acids with thyonyl chloride yields acid chloride
Reaction of carboxylic acids with PCl5 yields acid chloride
Reaction of carboxylic acids with PCl3 yields acid chloride

Can you do the following conversion? You may have to use the reactions that we studied in previous tutorials.

Convert benzoic acid to benzaldehyde.

Esterification

When acids are warmed with alcohols in the presence of conc. H2SO4 (or dry HCl), pleasant fruity smelling esters are formed.

Formation of esters from carboxylic acids : esterification

Important points regarding esterification

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 amides from carboxylic acids

Formation of Anhydrides

When two molecules of carboxylic acids are heated with a dehydrating agent such as phosphorus pentoxide, acid anhydrides are formed.

Formation of acid anhydrides from carboxylic acids

Questions

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.

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