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

For the ordinary esters, hydroxide adds to the acyl carbon, producing a tetrahedral oxyanion; collapse expels 2,4-dinitrophenoxide, followed by acid-base equilibration to the benzoate. This is base-promoted acyl-oxygen cleavage, a BAC2 mechanism. Rate-limiting addition naturally gives the observed bimolecular dependence on ester and OH${}^{-}$.

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.

The neutral para-OH ester B lies above the ordinary Hammett line, so an additional accelerating pathway is needed; ordinary resonance donation by OH alone predicts the wrong direction.

The phenoxide B${}^{-}$ lies far below the line: its strong resonance donation reduces acyl electrophilicity, and an anionic ester is less favorable for approach by OH${}^{-}$. A single neutral-substituent $\sigma$ parameter is also a poor description of this charged, strongly conjugated substrate. These deviations should not be forced onto the same straight line.