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

Hydroxide attack at the aromatic carbon bearing the leaving-group oxygen, followed by SNAr reaction, would put solvent-derived

^{18}

O into the liberated dinitrophenol. Its absence rules out that aryl C-O cleavage route, conventionally called BAr2, as the significant pathway.

Acyl C-O cleavage instead releases the original aryloxy oxygen with the leaving group, so unlabeled dinitrophenol is expected for both tetrahedral acyl substitution and the elimination route below.

Solution

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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**.

Solution

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The positive

Δ S^{‡} = + 40 J K^{- 1} mo l^{- 1}

is compatible with fragmentation/desolvation in the elimination route. A simple rate-limiting association of hydroxide with a neutral ester would usually have a less favorable activation entropy.

The measured activation parameters belong to the overall second-order rate constant: the acid-base pre-equilibrium and solvent reorganization contribute as well. Consequently positive activation entropy supports the fragmentation interpretation together with the leaving-group slope, but does not by itself prove a particular elementary transition structure.

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 0  0

Solution

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For B,

Δ V^{‡} = + 12 c m^{3} mo l^{- 1}

is consistent with bond fission and release/reorganization of electrostricted solvent on reaching a fragmentation transition state. The ordinary methoxy ester has

Δ V^{‡} = - 13 c m^{3} mo l^{- 1}

, consistent with associative hydroxide addition and the compact, strongly solvated developing tetrahedral species.

The opposite signs corroborate distinct mechanisms. Since both values include solvation and any pre-equilibria, they should not be interpreted merely as geometric volumes of isolated molecules.

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

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