 Solution

The kink indicates that two pathways with different leaving-group sensitivities compete: one dominates for good leaving groups and another for poorer ones. A single straight-line free-energy relationship is insufficient. The additional hydroxide-order information can be accommodated by rapid phenol deprotonation,Two rates are then possible. A route with one hydroxide equivalent in its overall kinetic dependence contributes $k_2[\mathrm{C}][\mathrm{O}{\mathrm{H}}^{-}]$. Attack of a further hydroxide on phenoxide ester contributesThus $k_3=k_{\mathrm{add}}{K}_{\mathrm{OH}}$. The third-order term need not involve a simultaneous collision of three molecules: sequential acid-base equilibration followed by bimolecular attack produces it.