Limonene (1-methyl-4-(prop-1-en-2-yl)cyclohexene, C10H16) is subjected to ozonolysis with reductive work-up (Zn/H2O). The total number of carbonyl…

Limonene has two C=C bonds: the trisubstituted ring double bond (C1=C2) and the exocyclic isopropenyl double bond. Each cleaved C=C gives two C=O groups (the carbonyl oxygen is attached to each carbon of the former double bond). Ring cleavage: C1 becomes a methyl ketone ($\mathrm{CH_3C{=}O}$) and C2 becomes an aldehyde ($\mathrm{CHO}$); the ring is opened but both ends stay on the same chain, so this product is a keto-aldehyde with 2 C=O. Isopropenyl cleavage: the carbon bearing $\mathrm{CH_3}$ becomes an acetyl group on the chain (1 C=O, in the same keto-aldehyde molecule) and the $\mathrm{=CH_2}$ carbon becomes $\mathrm{HCHO}$ (1 C=O). Total C=O groups = 4 (3 in the chain-type keto-aldehyde, 1 in formaldehyde). Answer: 4.