The compound that forms a stable hemiacetal when treated with \(\mathrm{CH}_{3} \mathrm{OH}\) and dry \(\mathrm{HCl}\) is (a) benzaldehyde (b) cyclohexanone (c) cyclopropanone (d) propanone

First, let's identify the carbonyl compounds given in the exercise: (a) benzaldehyde \(\mathrm{(C}_{6}\mathrm{H}_{5}\mathrm{CHO)}\) (b) cyclohexanone \(\mathrm{(C}_{6}\mathrm{H}_{10}\mathrm{O)}\) (c) cyclopropanone \(\mathrm{(C}_{3}\mathrm{H}_{4}\mathrm{O)}\) (d) propanone \(\mathrm{(CH}_{3}\mathrm{COCH}_{3})\)

Hemiacetals are formed by nucleophilic addition of an alcohol to a carbonyl group, usually an aldehyde or ketone. The general reaction is as follows: Carbonyl compound + Alcohol \(\xrightarrow[]{\mathrm{Acidic}\ \mathrm{conditions}} \) Hemiacetal In our case, the alcohol is methanol (CH3OH) and the acid is dry HCl. The carbonyl compound will be one of the given options.

Hemiacetals are generally not very stable and tend to revert back to the original carbonyl compound and alcohol. However, there are some structural features that make hemiacetals more stable. One such feature is the presence of a 5 or 6-membered ring in the molecule, which confers additional stability due to the formation of a cyclic hemiacetal.

Based on the stability factor of cyclic hemiacetals, we can predict which carbonyl compound will form a stable hemiacetal when treated with methanol and dry HCl: (a) Benzaldehyde: This compound cannot form a cyclic hemiacetal since there is no nearby OH group to form a five or six membered ring. (b) Cyclohexanone: This compound can form a cyclic hemiacetal when reacted with methanol as it can form a stable 6-membered ring. (c) Cyclopropanone: Even though it could form a ring, a 3-membered ring is too small and would lead to a highly strained cyclic hemiacetal. (d) Propanone: This compound cannot form a cyclic hemiacetal since there are no nearby OH groups to form a five or six-membered ring. Based on our analysis, the correct option is (b) cyclohexanone, as it forms a stable cyclic hemiacetal when treated with methanol and dry HCl.