See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the reactant: The starting material is cyclobutane with an isopropenyl group attached: cyclobutyl-C(CH3)=CH2. More precisely, one carbon of the cyclobutane ring bears a substituent -C(CH3)=CH2. Step 2 - HBr addition follows Markovnikov's rule: H+ adds to the terminal =CH2 (less substituted carbon), generating a carbocation at the internal carbon: cyclobutyl-C+(CH3). This tertiary carbocation is adjacent to the cyclobutane ring. Step 3 - Ring expansion via 1,2-shift: The carbocation formed [cyclobutyl-C+(CH3)] has a tertiary carbon cation adjacent to the strained cyclobutane ring. A 1,2-carbon shift (ring expansion) occurs: one C-C bond of the cyclobutane migrates to the adjacent carbocation, expanding the 4-membered ring to a 5-membered ring (cyclopentyl cation). This gives a tertiary cyclopentyl carbocation: specifically 1-methyl-cyclopentyl cation (the methyl and the ring carbons rearrange to give a tertiary carbocation on the cyclopentane ring). Step 4 - The rearranged carbocation: After ring expansion, we get a cyclopentane ring with a tertiary carbocation bearing a methyl group (1-methylcyclopent-1-yl cation). Step 5 - Bromide attacks the carbocation: Br- attacks the tertiary carbocation to give 1-bromo-1-methylcyclopentane... but looking at option (a), it shows a cyclopentane with Br at one carbon and a methyl at an adjacent carbon. Re-examining: the cyclobutane ring expansion from cyclobutyl-C+(CH3) gives a cyclopentane ring where the cationic center retains the methyl group, yielding 1-bromo-1-methylcyclopentane as the ring-expanded product. Option (a) depicts a cyclopentane ring with Br and methyl substituents consistent with this ring expansion product, making it the major product due to the relief of cyclobutane ring strain and formation of a more stable tertiary carbocation. Step 6 - Why other options fail: (b) would require a different substitution pattern; (c) and (d) are cyclohexane-based products requiring a 5->6 ring expansion which is not favored here since we start with cyclobutane (4-membered ring expanding to 5-membered is favorable); no further expansion to 6-membered is the major pathway. Therefore, the correct answer is A.