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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-1/1/b/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 / 1 / b / Solution by

Codex  0 2026-10-20

Carbon-13 NMR spectra: 1 = C, 2 = A, 3 = B. Approximate shifts, read from the printed axes, are shown in the annotated structures.

Figure 1.
Derived carbon assignments for the three isomers, excluding the solvent signal near 77 ppm
. **Details**.

C: $HC \equiv C - CH (OC H_{2} C H_{3})_{2}$ has five carbon environments: acetal CH near $91$ ; substituted/terminal alkyne carbons near $79$ and $74$ ; OCH $_{2}$ near $61$ ; and methyl near $15 ppm$ . One alkyne signal is close to the CDCl $_{3}$ solvent region, which must not be counted as a new molecular environment.
A: $EtOCO - CH = C (C H_{3})_{2}$ has ester carbonyl near $167$ , substituted alkene C near $157$ , vinyl CH near $116$ , OCH $_{2}$ near $60$ , two nonequivalent alkene methyls near $28$ and $21$ , and ethoxy methyl near $14 ppm$ .
B: $C H_{2} = CH - C H_{2} - OCO - C H_{2} - C H_{2} - C H_{3}$ has carbonyl near $173$ , vinyl CH near $133$ , terminal vinyl CH $_{2}$ near $118$ , OCH $_{2}$ near $65$ , acyl-alpha CH $_{2}$ near $36$ , the next CH $_{2}$ near $19$ , and terminal methyl near $14 ppm$ .

The two methyls in A are chemically distinct because one is cis and the other trans to each substituent at the other alkene carbon. Their two measured shifts are secure; assigning an individual methyl to a particular side of the drawn double bond requires an additional stereospecific shift argument or experiment, so their spatial interchange is left explicit.

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