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).
Observe the structures of the esters carefully.
a)
The order of reactivity of esters in the first step of nucleophilic substitution reactions is determined by the comparison of the electron density present on the carbonyl carbon as first step includes the formation of tetrahedral intermediate on the carbonyl carbon of acyl halide which is favoured by the attack of oxygen atom present on the carbonyl carbon of ester, thereby leads to the formation of carbocation. Therefore, more is the electron density on the carbonyl group; more will be the nucleophilicity of the carbonyl group which leads to the increase in the reactivity of the ester towards nucleophilic acyl substitution reactions.
The ester C given in the figure contains an electron releasing group at the para position which donates electron and leads to the increase in the nucleophilicity of carbonyl group. Therefore, the ester C is most reactive towards nucleophilic acyl substitution reaction and the ester C has one electron withdrawing chlorine group at para position which decreases electron density on the carbonyl group as compared to the ester A which does not has any substituent on benzene ring and the ester B having cyclohexane group attaches to the ester group has least reactivity due to absence of resonance stabilization. Thus, the order of reactivity of the given esters in the first step towards nucleophilic acyl substitution reactions is C > A > D > B.
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Solved in 3 steps
