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ID: 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/i/solution

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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 / i / Solution by

Codex  0 2026-10-20

Figure 1.

Single-barrier substitution profile and the trigonal-bipyramidal SN2 transition structure

. Details.

Draw one maximum for the concerted SN2 reaction step, with no intermediate minimum. Its transition structure is

[HO \dots C H_{3} \dots X]^{‡}

(1)

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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