Chapter 8.SE, Problem 69AP

Interpretation Introduction

Interpretation:

Reaction of HBr with 3-methylcyclohexene yields four products: and trans-1-bromo-2-methylcyclohexene. But an analogous reaction of HBr with 3-bromocyclohexene gives trans-1,2-dibromocyclohexane as the sole product. The structures of possible intermediates are to be drawn and further why only a single product is formed in the case of 3-bromocyclohexene is to be explained.

Concept introduction:

The reaction of HBr with cycloalkenes occurs in two steps. In the first step the π electrons of the double bond in the cycloalkenes attack the positively polarized hydrogen of HBr to yield a carbocation. In the second step the carbocation being planar can be attacked by the bromide ion from both sides to give different products.

To draw:

The structures of the possible intermediates in the reactions of HBr with 3-methylcyclohexene and 3-bromocyclohexene and to explain why the former gives cis- and trans-1-bromo-2-methylcyclohexenes while 3-bromocyclohexene gives trans-1,2-dibromocyclohexane as the sole product.