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Cyclopropane undergoes ring-opening reactions with HBr and with Br2. Identify the products [A] and [B] formed respectively.

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 1,3-dibromopropane
4. [A] is 1-bromopropane and [B] is 1,2-dibromopropane

Correct answer: [A] is 1-bromopropane and [B] is 1,2-dibromopropane

Solution

Cyclopropane's ring strain makes it susceptible to electrophilic addition. With HBr, ring opening gives 1-bromopropane [A]. With Br2, electrophilic addition across the C-C bond gives 1,2-dibromopropane [B] (anti addition).

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