Ethyl p-nitrobenzoate has been found to undergo saponification faster than ethyl p-methoxybenzoate.

(a) Consider the mechanism of saponification, and explain the reasons for this rate enhancement.

(b) Would you expect ethyl p-methoxybenzoate to undergo saponification faster or slower than ethyl benzoate?

The rate of reaction depends on its activation energy that is, the difference in energy between starting material and the transition state.

The transition state in saponification is similar in structure and therefore in energy, to the tetrahedral intermediate.

Tetrahedral carbon has no resonance overlap with a benzene ring, so any resonance effect of a substituent on the ring will have very little influence on the energy of the transition state.

For example, anything that stabilizes the starting material will therefore increase the activation energy, slowing the reaction, anything that destabilizes the starting material will decrease the activation energy, speeding the reaction.

Tetrahedral intermediate

One of the resonance forms of ethyl p-nitrobenzoate has a positive charge on the benzene carbon adjacent to the carbonyl carbon. This resonance form destabilizes the starting material, lowering the activation energy, and speeding the reaction.

So, methyl p-nitrobenzoate undergoes saponification faster than methyl benzoate.

Destabilized resonance contributor

(b) One of the resonance forms of ethyl p-methoxybenzoate has all atoms with full octets and a negative charge on the most electronegative atom. This resonance form stabilizes the starting material, increasing the activation energy, and slowing the reaction.

The stabilized resonance contributor is represented as follows:

Stabilized resonance contributor