The reaction of 3-methylcyclohexene (1) with HBr yields trans 2a and cis 2b brom

Question

The reaction of 3-methylcyclohexene (1) with HBr yields trans 2a and cis 2b bromides (among other products). On the other hand, the analogous reaction of 3-bromocyclohexene (3) gives only trans-1,2-dibromocyclohexane (4a). No cis dibromide 4b is observed. Which of the following statements accounts for the unexpected behavior of 3-bromocyclohexene (3)?

Select one:

a. The secondary carbocation can be trapped by Br– approaching only from the axial direction.

b. A bromonium ion is the intermediate responsible for the observed stereochemistry.

c. The cis isomer does not form because it is less stable.

d. The products have to be chiral, and thus, the cis isomer cannot form because it is meso.

Explanation / Answer

b. A bromonium ion is the intermediate responsible for the observed stereochemistry.

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