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 / iii / Solution

The positive $\Delta S^{‡}=+40\mathrm{J}{\mathrm{K}}^{-1}\mathrm{mo}{\mathrm{l}}^{-1}$ 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.