Question

a. Rank the following esters from most reactive to least reactive in the first slow step of a nucleophilic acyl substitution reaction (formation of the tetrahedral intermediate):

b. Rank the same esters from most reactive to least reactive in the second slow step of a nucleophilic acyl substitution reaction (collapse of the tetrahedral intermediate).

Short answer

a) The decreasing reactivity order of ester (formation of the tetrahedral intermediate)

b) The decreasing reactivity order of ester (collapse of the tetrahedral intermediate)

Step-by-step solution

The decreasing reactivity order of ester (formation of the tetrahedral intermediate) (a)

Decreasing reactivity order of ester (a)

Halogens attached to the benzene withdraw electrons from the ring inductively. Weaker the base attached to the acyl group stronger its electron withdrawing ability and basicity of OR group. Hence, it is easier to form tetrahedral intermediate. Weakly activating substituent like alkyl group is slightly more electron donating than electron withdrawing. Hence, they have less reactivity compared to first compound and less easy to form tetrahedral intermediate.

The decreasing reactivity order of ester (formation of the tetrahedral collapse) (b)

Decreasing reactivity order of ester (b)

The tetrahedral intermediate collapses by eliminating the OR group. The lower the basicity of OR group it is easier to eliminate it. Thus, rate of formation of tetrahedral intermediate and tetrahedral collapse is decreased by increase in basicity of OR group.