Chapter 12, Problem 12.9P

Interpretation Introduction

Interpretation:

The complete mechanism that accounts for the formation of racemic $\text{trans-1-bromo-2-chlorocyclohexane}$ is to be drawn when cyclohexene is treated with molecular bromine in aqueous $\text{NaCl}$ solution instead of pure water.

Concept introduction:

When alkenes are treated with molecular bromine in the presence of pure water as a solvent, a mixture of stereoisomers is produced. The final product is a halohydrin. If the same reaction is carried out in the presence of a solvent other than pure water, say aqueous solution of sodium chloride, a racemic mixture of dihalo compounds is formed along with the racemic mixture of halohydrins. This is because the chloride ions in sodium chloride would serve as a nucleophile and open the cyclic bromonium ion to form dihalo compounds.

When ${\text{Br}}_{\text{2}}$ adds to cyclohexene, step one is electrophilic addition that produces the cyclic bromonium ion intermediate, in which ${\text{Br}}_{\text{2}}$ acts as an electrophile, and the carbon-carbon double bond acts as a nucleophile. Step two is the attack of ${\text{Cl}}^{\text{-}}$ to produce a dihalo compound. This reaction will produce a racemic mixture of bromohydrin as well because water is the solvent for aqueous sodium chloride solution and would compete with the chloride ions.

In order to account for trans stereochemistry, the reaction must follow the mechanism that includes the formation of cyclic bromonium ion as an intermediate.