See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: Carbocation rearrangements (hydride or methyl shifts) occur to give a more stable carbocation or when the carbon skeleton changes. A change in carbon skeleton specifically means a methyl shift (1,2-methyl shift) or alkyl shift rather than just a hydride shift. Step 1: Analyze each carbocation for possible rearrangement with change in carbon skeleton. Option (a): CH3-C(CH3)2-CH2(+) is a neopentyl-type primary carbocation. The carbon bearing the positive charge (CH2+) is primary. Adjacent to it is a quaternary carbon bearing three methyl groups. A 1,2-methyl shift from the quaternary carbon to the primary carbocation center would move a methyl group, changing the carbon skeleton from neopentyl (2,2-dimethylpropyl) arrangement to a tertiary carbocation with a different connectivity. This is a classic methyl shift that changes the carbon skeleton, giving a tertiary carbocation: (CH3)2C(+)-CH2-CH3 (t-amyl/2-methylbut-2-yl cation). The skeleton changes from a 2,2-dimethyl arrangement to a 2-methyl arrangement on a longer chain. Step 2: Check other options. Option (b): CH3-CH2-CH2(+) is a primary n-propyl carbocation. A 1,2-hydride shift gives CH3-CH(+)-CH3 (secondary isopropyl cation). This is a hydride shift, NOT a methyl/alkyl shift, so the carbon skeleton (3 carbons in a chain) does NOT change. Option (c): CH3-CH(CH3)-CH2-CH2(+) is a primary carbocation. A 1,2-hydride shift gives CH3-CH(CH3)-CH(+)-CH3, which is a secondary carbocation. This is again a hydride shift; the carbon skeleton (4 carbons with one methyl branch) remains the same. Option (d): CH3-CH(+)-CH3 is already a secondary (isopropyl) carbocation. It is relatively stable and has no driving force for rearrangement; also no adjacent group that could cause a skeleton change. Step 3: Conclusion. Only option (a) involves a methyl (alkyl) shift that genuinely changes the carbon skeleton from a neopentyl framework to a tert-amyl framework. Therefore, the correct answer is A.