= Solution
\b[<Carbon-13 NMR> spectra: 1 = C, 2 = A, 3 = B.] Approximate shifts, read from the printed axes, are shown in the annotated structures.
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2021/paper-1-q1-carbon-assignments.png]
{title=Derived carbon assignments for the three isomers, excluding the solvent signal near 77 ppm}
* \b[C:] $\mathrm{HC{\equiv}C-CH(OCH_2CH_3)_2}$ has five <carbon> environments: <acetal> CH near $91$; substituted/terminal <alkyne> <carbons> near $79$ and $74$; OCH$_2$ near $61$; and methyl near $15\,\mathrm{ppm}$. One <alkyne> signal is close to the CDCl$_3$ <solvent> region, which must not be counted as a new molecular environment.
* \b[A:] $\mathrm{EtOCO-CH=C(CH_3)_2}$ 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 $14\,\mathrm{ppm}$.
* \b[B:] $\mathrm{CH_2=CH-CH_2-OCO-CH_2-CH_2-CH_3}$ 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 $14\,\mathrm{ppm}$.
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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