Chapter 16.11, Problem 30P

(a)

Interpretation Introduction

Interpretation: The species which are used for the transesterification of esters other than acids has to be predicted.

Concept introduction: Transesterification is the process of formation of a new ester molecule from the reaction of alcohol and an ester.  This is like hydrolysis of ester but here nucleophile is alcohol molecule instead of ${\text{H}}_{\text{2}}\text{O}$.  As alkoxide groups have same basicity, therefore, an excess amount of alcohol is used.  The addition of an acid also increases the rate of reaction by protonating the carbonyl oxygen atom. A general transesterification is  written as,

If a better nucleophile is used in place of alcohol molecule that will also increase the rate of transesterification reaction.  The conjugate base of an alcohol is an alkoxide which is a better nucleophile and can attack the carbonyl carbon even its oxygen is not protonated.

(b)

Interpretation Introduction

Interpretation: The rate of aminolysis reaction of esters does not increase in presence of ${\text{H}}^{\text{+}}$, ${\text{OH}}^{-}$ and ${\text{OR}}^{-}$ has to be explained.

Concept introduction: The reaction of a carboxylic acid derivative with ammonia, primary and secondary amines gives an amide this reaction is known as aminolysis reaction.  This reaction involves two equivalent of ammonia or amine molecule.  An amine is better nucleophile as the electronegativity of nitrogen is less as compared to an oxygen atom and hence it can donate its lone pair easily.