When Br2 adds to a cis alkene that has different...

When $\mathrm{Br}_{2}$ adds to a cis alkene that has different substituents attached to each of the two $s p^{2}$ carbons, such as cis-2-heptene, identical amounts of the two threo enantiomers are obtained even though $\mathrm{Br}^{-}$ is more likely to add to the less sterically hindered carbon of the bromonium ion. Explain why identical amounts of the two enantiomers are obtained.

Key Concepts

Bromonium Ion Intermediate

In the electrophilic addition of a dihalogen to an alkene, the first key step is the formation of a cyclic bromonium ion intermediate. This intermediate is formed when the alkene’s ? electrons attack a bromine molecule, leading to a three-membered ring with a positively charged bromine. The formation of this bridged, cyclic ion is central to understanding how the reaction proceeds, as it sets up the conditions for the subsequent anti-addition of the nucleophilic bromide ion.

Anti-Addition Mechanism

The bromonium ion intermediate undergoes ring opening via attack by the bromide ion from the backside relative to the bridging bromine. This anti-attack results in the formation of vicinal dibromides with the substituents added on opposite faces of the former double bond. The anti-addition is stereospecific and is an essential feature in determining the stereochemical outcome of the reaction.

Regioselective Nucleophilic Attack

Although the bromide ion is more inclined to attack the less sterically hindered carbon of the bromonium ion, the intermediate’s structure allows for attack at either carbon. This means that the nucleophilic attack can occur at two different positions, each leading to a product that is the non-superimposable mirror image of the other. This regioselectivity, despite its bias, still results in the formation of two enantiomeric products.

Formation of Enantiomeric Mixture

The combination of a symmetric or near-symmetric intermediate and the anti-addition mechanism leads to the production of equal amounts of two enantiomers. Even if one pathway is slightly favored due to steric factors, the overall stereochemical outcome remains a racemic mixture because the reaction furnishes enantiomers through two competing, mirror-image pathways. This symmetry in the mechanism ensures that the final products are obtained in identical amounts.

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