Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 67 / e

Triethylamine attacks the benzyl carbon and bromide leaves, giving the quaternary benzyltriethylammonium salt. A useful substitution transition structure iswith developing N-C bonding and breaking C-Br bonding. The C-H/D bonds remain intact, so the small effects are secondary.

For X=NO${}_2$, a relatively associative SN2 reaction transition state can give a small inverse effect, $0.988$, through altered bending constraints. The unsubstituted case, $0.993$, is close to cancellation of opposing contributions. Electron-donating OMe stabilizes benzylic positive charge, favoring a looser, more dissociative transition state and a slightly normal effect, $1.014$; it also explains the increased relative rate, $7.4$ versus $1$ for H and $0.52$ for NO${}_2$.

Thus the data indicate increasing dissociative character with electron donation. A contribution from a separate ionization pathway is possible for the strongest donor, but these small isotope effects and three relative rates do not uniquely require a sharp SN2-to-SN1 mechanism switch.