Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section B / 10 / ii / b

For Reaction 1, doubling substrate concentration doubles the rate, while doubling ethanol concentration leaves it unchanged. ThusRate-limiting ionization of the tertiary propargylic chloride gives a carbocation stabilized by substitution and propargylic/allenylic resonance. The crowded reaction center disfavors backside attack, while neutral ethanol is a weaker nucleophile than ethoxide. Ethanol capture and deprotonation occur after the rate-determining step.

For Reaction 2, independently doubling either reactant doubles the rate, and doubling both gives a fourfold increase:The benzylic carbon is primary and accessible, and the strong nucleophile can displace chloride in a concerted step. The reacting bond is benzylic C–Cl, not the ring C–Cl bond. These kinetic orders support the proposed mechanisms in the stated model; the rate table alone would not identify every intermediate in a more complicated reaction network.