See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: Reactivity of alcohols toward HBr follows the order: tertiary > secondary > primary, because the reaction proceeds via carbocation formation (SN1) or direct substitution, and more substituted carbocations are more stable, lowering the activation energy. Step 1: Analyze each option to determine whether the SECOND alcohol is more reactive than the FIRST. Option (a): - First alcohol: 1-methylcyclopentan-1-ol → tertiary alcohol (C1 of cyclopentane bears OH, CH3, and two ring carbons) - Second alcohol: cyclohexanol (cyclohexan-1-ol with H shown) → secondary alcohol - Tertiary > Secondary, so the FIRST is more reactive than the second. This does NOT satisfy the condition. Option (b): - First alcohol: 1-ethylcyclopentan-1-ol → tertiary alcohol (C1 of cyclopentane bears OH, ethyl group, and two ring carbons) - Second alcohol: 1-cyclopentylethanol (cyclopentyl-CH(OH)-CH3) → secondary alcohol - Tertiary > Secondary, so the FIRST is more reactive. This does NOT satisfy the condition. Option (c): - First alcohol: CH3-CH(OH)-CH2-CH3 = butan-2-ol → secondary alcohol - Second alcohol: CH3-CH2-CH(CH3)-CH2-OH = 2-methylbutan-1-ol → primary alcohol - Secondary > Primary, so the FIRST is more reactive than the second. This does NOT satisfy the condition. Option (d): - First alcohol: CH3-CH(OH)-CH2-CH3 = butan-2-ol → secondary alcohol - Second alcohol: (CH3)2C(OH)-CH2-CH3 = 2-methylbutan-2-ol → tertiary alcohol - Tertiary > Secondary, so the SECOND alcohol is more reactive than the first. This SATISFIES the condition. Step 2: Confirm option (d). The second alcohol, 2-methylbutan-2-ol, is a tertiary alcohol because the carbon bearing the OH group is attached to three other carbon groups [(CH3), (CH3), and (CH2CH3)]. Tertiary alcohols react fastest with HBr because they form the most stable tertiary carbocations. The first alcohol, butan-2-ol, is secondary and thus less reactive. Therefore, the second alcohol in pair (d) is indeed more reactive than the first. Why other options fail: (a) and (b) have tertiary alcohols first and secondary second; (c) has secondary first and primary second — in all cases the first is more reactive, not the second. Therefore, the correct answer is D.