Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2021 / Paper 1 / 5 / c

Draw one maximum for the concerted SN2 reaction step, with no intermediate minimum. Its transition structure iswith O-C forming and C-X breaking, both shown as partial bonds. O-C-X is approximately linear, and the three C-H bonds lie in the equatorial plane. The overall charge is $-1$, distributed over the entering and leaving groups.

The sketch illustrates an exergonic case; the elementary-step description alone does not supply a numerical reaction energy or determine the product-reactant energy difference for every possible X.

A transition structure is a saddle point on the potential-energy surface: motion along the reaction coordinate lowers energy in either direction. It has one unstable vibrational direction and no intermediate residence basin.

An intermediate is a local minimum, separated from neighboring states by barriers, with a finite lifetime. It may be short-lived, but that does not make it a transition structure. The elementary SN2 reaction step has a transition structure and no distinct bonded intermediate.