See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 - Identify compound (A): Compound (A) is 2-methyl-2-(2-oxoethyl)-1,3-dioxolane. It contains a five-membered cyclic acetal (1,3-dioxolane) ring with a methyl group and a -CH2CHO side chain at C2 of the ring. The molecular formula can be determined from the structure. Step 2 - Reaction with H+: Under acid catalysis, acetals hydrolyze. The 1,3-dioxolane (cyclic acetal) in compound (A) undergoes acid-catalyzed ring opening/hydrolysis. However, since (B) is an isomer of (A) (not a hydrolysis product requiring water), the reaction is an intramolecular process - specifically, the aldehyde group (-CHO) in the side chain can form a new acetal with the diol that would be generated, or alternatively, an intramolecular transacetalization occurs. Step 3 - Mechanism: Under H+ catalysis, the cyclic acetal opens to reveal a hemiacetal or oxocarbenium ion intermediate. The aldehyde (-CHO) present in the side chain can then undergo intramolecular cyclization. The key transformation is that the aldehyde (-CHO) at the end of the chain reacts intramolecularly with the diol released from the acetal to form a new 1,3-dioxolane ring, but now the acetal carbon bears an H (from CHO) and an acetyl (CH3CO-) group results - this is an intramolecular acetal rearrangement. Step 4 - Product (B): The product is formed by intramolecular transacetalization where the aldehyde carbon becomes the new acetal center. The aldehyde -CH2-CHO provides the carbon (with H), and the two oxygens from the original dioxolane close onto this carbon to give a new 1,3-dioxolane ring with H at C2. The methyl group that was on the original acetal carbon is now a pendant -C(=O)-CH3 (acetyl) group. This gives 1-(1,3-dioxolan-2-yl)ethan-1-one, which matches option (b): a 1,3-dioxolane ring with H at C2 and an acetyl group (-C(=O)CH3) attached. Step 5 - Verification that (A) and (B) are isomers: Both have the same molecular formula (same connectivity elements rearranged); (A) has the acetal at the quaternary carbon with methyl and -CH2CHO chain, while (B) has the acetal at the former aldehyde carbon with H and acetyl group - constitutional isomers, consistent with the problem statement. Step 6 - Why other options fail: (a) and (d) involve an exo-methylene and OH group, suggesting elimination rather than transacetalization - not the expected acid-catalyzed acetal rearrangement product. (c) retains an aldehyde and OH with the dioxolane intact, which does not represent a simple isomerization product of the acetal rearrangement. Therefore, the correct answer is B.