Question

We saw that acid anhydrides react with alcohols, water, and amines. In which of these reactions can the tetrahedral intermediate eliminate the carboxylate ion even if it does not lose a proton before the elimination step? Explain.

Short answer

Acetic anhydride on reaction with amines does not lose a proton before it eliminates the carboxylate ion because the carboxylate ion is a weaker base.

Step-by-step solution

Reaction of acidic anhydride with water

Acidic anhydride on reaction with water leads to the formation of two equivalents of carboxylic acids reaction occurs by the incoming nucleophile and the formation of tetrahedral intermediate followed by losing proton behave has a strong base than the by-product which is carboxylate ion.

Hence, here before the elimination of carboxylate ion, the loss of proton occurs as follows:

Reaction of acidic anhydride with alcohol

Acetic anhydride on reaction with alcohol leads to the formation of ester and a carboxylic acid.

The alcoholic oxygen reacts with the carbonyl carbon of acidic anhydride and the tetrahedral intermediate is formed followed by the loss of proton and break of the C-O group formed by the ester and acid as followed:

Reaction of acidic anhydride with amine

Acetic anhydride on reaction with amine leads to the formation of amide and a carboxylate ion.

As the amine nitrogen has a lone pair attack on the carbonyl of acetic anhydride and the proton produced in the reaction because amine is a stronger base than the carboxylate ion which is produced as a product so that the amine protonation occurs by the acid itself generated in the reaction and the carboxylate ion is a better leaving group as shown below:

Hence, no loss of proton occurs in the reaction with an amine.