Question

Addition of $B{r}_2$ to cis-$2$-hexene gives racemic mixture, even though attack of the bromide ion on unsymmetrical bromonium ion intermediate is not equally likely at both carbons. The reason for the racemic product are:

• A. the reaction is reversible, after a while both enantiomers are produced in equal amount
• B. the bromonium ion formed has a plane of symmetry
• C. two enantiomeric bromonium ions are formed in equal amounts
• D. four different stereo isomers are formed, but all in equal amount

Answer 1

Answer: (C)

two enantiomeric bromonium ions are formed in equal amounts

Enantiomersare chiral molecules that are mirror images of each other. Furthermore, the molecules are non-superimposable on one another. This means that the molecules cannot be placed on top of one another and give the same molecule.

Even though attack of the bromide ion on unsymmetrical bromonium ion intermediate is not equally likely at both carbons, but racemic products are formed as two enantiomeric bromonium ions are formed in equal amounts, as shown in above mechanism.