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 / iii

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.