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ID: past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3/39/d/solution

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 39 / d / Solution by

Codex  0 2026-10-20

Figure 1.
Cyclohexene assignments with explicit ambiguity for the overlapping allylic methylenes
. **Details**.

Number ring positions

r_{1}

through

r_{6}

around the ring, with

r_{1} = r_{2}

the double bond. These auxiliary ring-position labels are distinct from the printed carbon-signal numbers.

$r_{1}$ : quaternary alkene carbon 4, attached to the OCH $_{2}$ side chain.
$r_{2}$ : alkene CH 8, proton E.
$r_{3}$ : allylic CH $_{2}$ , one of carbon signals 14/15; its protons are K and one of G/H/I.
$r_{4}$ : methine 12, proton L, attached to the tertiary-alcohol carbon 10.
$r_{5}$ : CH $_{2}$ 17, protons J and M.
$r_{6}$ : the other allylic CH $_{2}$ 15/14, carrying the remaining two of G/H/I.

The HSQC pairs J and M with 17 and places L on 12. It also places K and one unresolved member of G/H/I on one of the closely spaced allylic carbons, with the other two G/H/I protons on the other. COSY connects the vinylic E to the adjacent allylic part and connects the methine L to its neighboring methylenes; HMBC links F to the substituted alkene end and the gem-dimethyl group to the methine end.

The printed overlap does not justify a unique assignment of every G/H/I letter or a forced 14-versus-15 assignment to those two ring positions. The drawing retains those alternatives explicitly. In the half-chair, L is pseudoaxial; the resolved large/small couplings are consistent with pseudoaxial/pseudoequatorial methylene partners, but exact axial letter labels within the overlapped group remain uncertain.

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