Question

a. What species other than an acid can be used to increase the rate of the transesterification reaction that converts methyl acetate to propylacetate?

b. Explain why the rate of aminolysis of an ester cannot be increased by H⁺, HO^-, or RO-.

Short answer

1. The transesterification reaction rate is increased by using the conjugate base of propanal (${\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{O}}^{\text{-}}$).
2. When hydrogen ion $\left({\text{H}}^{\text{+}}\right)$ is used as a catalyst, the amine will not be able to act as a nucleophile; and when hydroxide ion $\left({\text{OH}}^{\text{-}}\right)$is used as a catalyst, the product formed will be a carboxylate ion rather than the amide; when alkoxide ion $\left({\text{RO}}^{\text{-}}\right)$is used as a catalyst, the product formed will be an ester rather than the amide.

Step-by-step solution

Step-by-step-solutionStep1: Rate of transesterification Reaction

The chemical reaction taking place between “ester and alcohol which is catalyzed by acid” is known as transesterification reaction.

Conversion of methyl acetate to propyl acetate is done by reacting ester methyl acetate with alcohol propanol; this transesterification reaction rate is increased by using the reactant propanol conjugate base that is ${\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{O}}^{\text{-}}$.

The conjugate base is a nucleophile that will readily attack the carbonyl carbon of methyl acetate (ester), increasing the transesterification reaction rate.

Rate of aminolysis Reaction

The hydrogen ion $\left({\text{H}}^{\text{+}}\right)$, hydroxide ion $\left({\text{OH}}^{\text{-}}\right)$, and alkoxide ion $\left({\text{RO}}^{\text{-}}\right)$are not used to increase the rate of the aminolysis Reaction due to following reasons:

• If hydrogen ion (${\text{H}}^{\text{+}}$) is used as a catalyst, theamine will be protonated in the acidic solutionand, therefore, will not be able to react as a nucleophile (lone pairs are used).

• If hydroxide ion (${\text{OH}}^{\text{-}}$) is used as a catalyst, it will be thebest nucleophilein the solution. Hydroxide ions add to the ester, and the product of the aminolysis reaction will be a carboxylate ion rather than the amide.

• If alkoxide ion ($\text{RO}{}^{\text{-}}$) is used as a catalyst, it will be thebest nucleophilein the solution. Alkoxide ions add to the ester, and the product of the aminolysis reaction will be an ester rather than an amide.