= Solution
\b[A is <pentane-2,3-dione>, $\mathrm{CH_3C(=O)C(=O)CH_2CH_3}$.] Its index of <hydrogen> deficiency is $(2\times5+2-8)/2=2$, accounted for by two <ketone> carbonyls. The two resonances near 200 ppm, together with the three saturated <carbon> signals and the <proton> integrals, identify an unsymmetrical <adjacent diketone> rather than an <ester> or an alkene-containing structure.
The assignments are the carbonyl-adjacent methyl at $\delta_C=23.7$, the methylene at $29.2$, and the terminal ethyl methyl at $7.0$ ppm. The two <carbonyl> carbons give $197.5$ and $199.8$ ppm. \b[Their individual ordering is not uniquely established by the supplied one-dimensional spectra]; either ordering between the methyl-side and ethyl-side carbonyls should be kept tentative without a correlation or another independent assignment. In the displayed <attached proton test> phase convention, the two methyls have one sign, while the methylene and the nonprotonated carbonyls have the opposite sign. The solvent signal near 77 ppm is not a sixth <carbon> of A.
\Image[/past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2023/paper-1-diketone-carbons.png]
{title=Original structural assignment of the five carbon environments in A; the two carbonyl shifts are an unordered pair}
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