See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Concept: The reactivity of alcohols towards HBr depends on the stability of the carbocation intermediate formed after protonation and departure of water. The order of reactivity is: tertiary > secondary > primary alcohols. Ring expansion or ring contraction (Wagner-Meerwein rearrangements) and neopentyl-type rearrangements can also enhance reactivity. Step 1: Analyze option (d). First alcohol: CH3-CH(OH)-CH2-CH3 — this is 2-butanol, a secondary alcohol. Second alcohol: (CH3)2C(OH)-CH2-CH3 — this is 2-methyl-2-butanol (or 2-methyl-2-butanol), a tertiary alcohol. Step 2: Compare reactivities in option (d). The first alcohol (2-butanol) is secondary, forming a secondary carbocation upon protonation and loss of water. The second alcohol ((CH3)2C(OH)-CH2-CH3) is tertiary, forming a more stable tertiary carbocation upon protonation and loss of water. Tertiary alcohols react faster with HBr than secondary alcohols because tertiary carbocations are more stable. Therefore, in option (d), the second alcohol IS more reactive than the first. Step 3: Check other options. (a) First is 1-methylcyclopentanol (tertiary), second is 1-cyclohexanol (secondary). Here the first (tertiary) is MORE reactive than the second (secondary), so this pair does NOT satisfy the condition. (b) First is 1-ethylcyclopentanol (tertiary), second is a secondary alcohol. Again the first is tertiary and more reactive, so the second is LESS reactive. Does not satisfy the condition. (c) First: CH3-CH(OH)-CH2-CH3 is secondary (2-butanol). Second: CH3-CH2-CH(CH3)-CH2-OH is primary (with a methyl branch). A primary alcohol is LESS reactive than a secondary alcohol towards HBr under normal SN1 conditions, so the second is less reactive. Does not satisfy the condition. Step 4: Conclusion. Only in option (d) is the second alcohol (tertiary) more reactive than the first alcohol (secondary) towards HBr. Therefore, the correct answer is D.