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ID: past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-4/66/c/ii/solution

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

Codex  0 2026-10-20

The three regions represent competing pathways whose leaving-group sensitivities differ.

Region 1, $β_{lg} = - 1.33$ : good leaving groups permit E1cB mechanism. OH $^{-}$ first deprotonates the para phenol. The phenoxide lone pair then pushes through the ring, with loss of ArO $^{-}$ from the acyl group, giving a quinoid ketene, schematically $O = C_{6} H_{4} = C = O$ . Water/hydroxide traps the ketene and proton transfers restore the aromatic hydroxybenzoate. Rate-limiting fragmentation has extensive C–OAr cleavage, hence strong dependence on leaving-group basicity.
Region 2, $β_{lg} = - 0.35$ : as ArO $^{-}$ becomes a worse leaving group, elimination slows steeply and ordinary hydroxide addition to neutral C becomes competitive/dominant. The tetrahedral acyl-substitution pathway is much less sensitive to leaving-group p $K_{a}$ .
Region 3, $β_{lg} = - 0.20$ : hydroxide attacks the deprotonated phenoxide ester C $^{-}$ . Its low electrophilicity and anion-anion interaction make addition slow. The leaving group is little involved in that addition transition state. Combined with prior deprotonation, this is the third-order term.

The bend in the upper plot is a crossover of mechanisms, not a discontinuous change in the identity of the leaving group. The lower plot is the separate hydroxide-on-phenoxide pathway.

^[ref]

Figure 1.
Parallel acyl-substitution and conjugate-base elimination pathways
. **Details**.

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