hydrobromination

Hydrobromination is a chemical reaction in which hydrogen bromide (HBr) is added across a carbon-carbon double or triple bond. This is a type of electrophilic addition reaction, a fundamental process in organic chemistry. The reaction typically occurs in the presence of a solvent.

When hydrobromination of an alkene occurs, a saturated alkyl halide is formed. For example, the reaction of ethene with HBr yields bromoethane. The addition of HBr to unsymmetrical alkenes follows Markovnikov's rule, which states that the hydrogen atom of HBr will attach to the carbon atom of the double bond that already has the greater number of hydrogen atoms. This leads to the formation of the more substituted alkyl halide as the major product.

In the absence of peroxides, the regiochemistry of hydrobromination is governed by Markovnikov's rule due to the carbocation intermediate formed. The positive charge is more stable on the more substituted carbon, leading to the bromide ion attacking that carbon.

However, in the presence of peroxides or under free-radical conditions, the addition of HBr to alkenes follows an anti-Markovnikov pathway. This is known as the peroxide effect or Kharasch effect. In this case, the reaction proceeds via a free radical mechanism, where the bromine radical initiates the addition, and the resulting carbon radical is more stable when the bromine atom is attached to the less substituted carbon. This leads to the formation of the less substituted alkyl halide as the major product.

Hydrobromination of alkynes also occurs, proceeding stepwise. The first addition of HBr to an alkyne forms a vinyl halide, and a second addition can occur to form a geminal dihalide. The regiochemistry of these additions also follows Markovnikov's rule unless peroxides are present.