= Solution
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2021/paper-1-q5-substitution-profile.png]
{title=Single-barrier substitution profile and the trigonal-bipyramidal SN2 transition structure}
Draw one maximum for the concerted <SN2 reaction> step, with no intermediate minimum. Its transition structure is
$$
\boxed{\left[\mathrm{HO\cdots CH_3\cdots X}\right]^\ddagger}
$$
with 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.
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