Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 1 / b / i / Solution

B is 2-Methylpropanoyl chloride, $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CHC}(=\mathrm{O})\mathrm{Cl}$. The single index of index of hydrogen deficiency is supplied by the carbonyl. Its relatively high-frequency infrared spectroscopy band identifies an acyl chloride, while two equivalent methyl groups explain why four carbons produce only three carbon-13 NMR resonances.

Assign ${\delta }_C=178.1$ to the acyl chloride carbonyl, $46.3$ to the methine and $18.9$ ppm to both methyl groups. Their chemical equivalence in NMR follows from the symmetry of the isopropyl fragment in this achiral environment.