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Cyclopropane undergoes the following two reactions: (i) Cyclopropane + HBr gives product [A] (ii) Cyclopropane + Br2 gives product [B] Identify [A] and [B] correctly.

1. [A] is 1-bromopropane and [B] is 1,3-dibromopropane
2. [A] is 2-bromopropane and [B] is 1,2-dibromopropane
3. [A] is bromocyclopropane and [B] is 1,2-dibromocyclopropane
4. [A] is propyl bromide and [B] is dibromopropane (unspecified)

Correct answer: [A] is 1-bromopropane and [B] is 1,3-dibromopropane

Solution

Cyclopropane undergoes ring-opening addition reactions. With HBr, the C-C bond breaks and HBr adds across it to give 1-bromopropane (all carbons of cyclopropane are equivalent, so only one mono-bromo product is possible). With Br2, ring opening gives 1,3-dibromopropane via a carbocation intermediate or concerted mechanism.

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