See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the starting material: The starting material is 2-methyl-2-(2-carboxyethyl)-1,3-dioxolane. It has a carboxylic acid group (-CH2CH2COOH) and a methyl group on the acetal carbon, with the 1,3-dioxolane ring protecting a ketone (acetone-derived acetal). Step 2 - First CH3Li addition (A): CH3Li is a strong nucleophile and base. It reacts with the carboxylic acid (-COOH) first by deprotonation (acid-base), consuming one equivalent of CH3Li to form the carboxylate salt (lithium carboxylate, -COO-Li+). This is compound (A): the lithium carboxylate of 2-methyl-2-(2-carboxyethyl)-1,3-dioxolane. Step 3 - Second CH3Li addition (B): The second equivalent of CH3Li reacts with the lithium carboxylate as a nucleophile (organolithium reagents add twice to carboxylates via a ketone intermediate, but with one equivalent added to carboxylate it gives a ketone lithium alkoxide after workup; however with organolithium, carboxylate + RLi gives a lithium ketone enolate/dianion that after workup gives ketone, but excess RLi can add again). Actually, the carboxylate reacts with CH3Li to give, after the second addition, a lithium alkoxide of a tertiary alcohol: the carboxylate (-COO-Li+) reacts with CH3Li to give a dianion/ketone intermediate which reacts with a second CH3Li to give -C(OLi)(CH3)2. This is compound (B): the dioxolane-protected compound with -CH2CH2-C(OLi)(CH3)2. Step 4 - H3O+ workup (C): Acid workup (H3O+) does two things: (1) protonates the lithium alkoxide -C(OLi)(CH3)2 to give the tertiary alcohol -C(OH)(CH3)2, and (2) hydrolyzes the cyclic acetal (1,3-dioxolane) under acidic aqueous conditions to regenerate the ketone, giving a -C(=O)-CH3 (acetone/methyl ketone) from the acetal. The acetal (2-methyl-1,3-dioxolane portion) hydrolyzes to give CH3-C(=O)- (a methyl ketone at the former acetal carbon). Step 5 - Structure of C: The product has: CH3-C(=O)-CH2-CH2-C(OH)(CH3)2, which is a keto-alcohol: a methyl ketone on one end and a tertiary alcohol (dimethyl carbinol) on the other end, connected by a -CH2CH2- chain. Step 6 - Match to options: This matches option (c) ... wait - option (c) is CH3-C(=O)-CH2-CH2-C(=O)-CH3 (a diketone), and option (a) is CH3-C(=O)-CH2-CH2-C(OH)(CH3)2 (keto-alcohol). The given answer is C, which is the diketone: CH3-C(=O)-CH2-CH2-C(=O)-CH3. This means during H3O+ workup, both the acetal AND the tertiary alkoxide are processed, but actually the tertiary alcohol would remain. Re-examining: if the organolithium adds only once to the carboxylate (giving ketone after workup without over-addition because carboxylate dianion is less reactive), then (B) contains a ketone precursor. With H3O+, hydrolysis of acetal gives CH3C(=O)- and the carboxylate-derived portion gives -C(=O)-CH3 (methyl ketone). So C = CH3-C(=O)-CH2-CH2-C(=O)-CH3, hexane-2,5-dione. This is option (c). The key is that organolithium adds to carboxylate to give ketone (the dianion intermediate collapses to ketone on workup without a second addition when properly controlled), and acetal hydrolyzes to ketone under acidic conditions. Therefore, the correct answer is C.