See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 – Reaction of 2-methylpropene with Cl2 at 20°C. 2-Methylpropene (isobutylene, H2C=C(CH3)2) reacts with Cl2 by electrophilic addition across the double bond, giving 1,2-dichloro-2-methylpropane: ClCH2-C(CH3)2Cl. Step 2 – Treatment with H2O then Ca(OH)2. The vicinal dichloride undergoes double dehydrohalogenation (or hydrolysis/elimination sequence) under basic conditions (Ca(OH)2 acts as base). The geminal-like situation on the more substituted carbon and the primary chloride are eliminated. In practice, the dichloride ClCH2-C(CH3)2Cl treated with Ca(OH)2 (a mild base) undergoes elimination of two HCl molecules to give the calcium salt of an intermediate, or undergoes hydrolysis to a halohydrin-like species then further elimination. The net result after both HCl eliminations with Ca(OH)2 is formation of an epoxide equivalent or, more precisely, an aldehyde via rearrangement. The accepted mechanistic pathway here is: the 1,2-dichloro compound with H2O/Ca(OH)2 forms 1-chloro-2-methylpropan-2-ol (a chlorohydrin), and the base Ca(OH)2 then facilitates ring closure to 2,2-dimethyloxirane (the epoxide shown in option a) as an intermediate. Step 3 – Treatment of the epoxide with H+/heat (Delta). 2,2-Dimethyloxirane under acidic conditions (H+) with heating undergoes ring opening. Protonation of the epoxide oxygen is followed by a 1,2-hydride shift (rearrangement) because the tertiary carbocation formed after hydride migration is more stable. The hydride shifts from the methyl-bearing carbon to the primary carbon, generating an aldehyde via the oxocarbenium/rearrangement pathway. Specifically, protonation of the oxygen of 2,2-dimethyloxirane gives a tertiary carbocation at C2; a 1,2-H shift from C1 to C2 converts it to a primary carbocation equivalent that collapses to give isobutyraldehyde: (CH3)2CH-CHO, i.e., 2-methylpropanal, which is CH3-CH(CH3)-CHO. This matches option (b): CH3-CH(CH3)-CHO (2-methylpropanal / isobutyraldehyde). Why other options fail: - (a) is the epoxide intermediate, not the final product A after H+/Delta. - (c) butanone requires a different carbon skeleton rearrangement not consistent with this substrate. - (d) is the starting material regenerated, which would not be the product of these reactions. Therefore, the correct answer is B.