Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 66 / a / ii / Solution

The Hammett equation is $\log (k_X/k_H)=\rho {\sigma }_X$. The positive slope, $\rho =+1.97$, means electron-withdrawing para substituents accelerate hydrolysis. They increase carbonyl electrophilicity and stabilize the additional negative charge associated with the addition transition state/tetrahedral intermediate. Strong electron donation has the opposite effect. Hydroxide addition to the acyl carbon, followed by tetrahedral-intermediate collapse and aryloxide loss, is consistent with both this trend and the bimolecular rate law.