= 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-q1-proton-assignments.png]
{title=Approximate proton shifts and coupling patterns derived from the printed spectra}
\b[A, spectrum 4:] ethoxy CH$_3$ at about $1.3$ is a <NMR triplet>, and OCH$_2$ at about $4.15$ is a <NMR quartet>. The two <alkene> methyls at about $1.9$ and $2.2$ each integrate to three <protons>; they are apparent <NMR singlets> on the overview but have small allylic <NMR doublet> splitting. The vinyl CH at about $5.7$ is correspondingly a narrow <NMR multiplet>, formally a <quartet of quartets> when its two methyl couplings are unequal, or approximately a <NMR septet> if they coincide. A broadened overview must not be used to declare all those small couplings exactly zero.
\b[C, spectrum 5:] the <acetal> CH at about $5.25$ and terminal C≡CH at about $2.55$ each integrate to one <proton> and show their small mutual long-range <NMR doublet> splitting. The ethoxy methyls give a six-proton <NMR triplet> near $1.25$. The OCH$_2$ region has two sets near $3.6$ and $3.75$, each integrating to two <protons>. These are an <ABX3 spin system>: <geminal coupling> and coupling to CH$_3$ produce approximately one <doublet of quartets> for each <proton> type, with second-order distortion in the expanded trace. Treating the whole region as one ideal four-proton <NMR quartet> misses the printed structure.
\b[B, spectrum 6:] the internal <alkene> CH near $5.95$ is a <doublet of doublets of triplets> from trans/cis coupling to CH$_2$ and <vicinal coupling> to OCH$_2$. The two terminal <alkene> <protons> near $5.2$–$5.35$ are distinct <doublet of doublets>-type signals, with possible additional small allylic splitting. OCH$_2$ near $4.6$ is principally a <NMR doublet>. The acyl-alpha CH$_2$ near $2.3$ is a <NMR triplet>, the middle CH$_2$ near $1.65$ an approximate <NMR sextet> or overlapping <NMR triplet> of quartets, and the terminal methyl near $0.95$ a <NMR triplet>.
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