Hydrohalogenaion

Hydrohalogenaion refers to the process of adding a hydrogen halide molecule across a double or triple bond, typically in an organic compound. This reaction is a fundamental transformation in organic chemistry and is often used to synthesize alkyl halides. The hydrogen halide, such as hydrogen chloride (HCl), hydrogen bromide (HBr), or hydrogen iodide (HI), dissociates into a proton (H+) and a halide ion (X-). The double or triple bond, being electron-rich, acts as a nucleophile and attacks the proton, forming a carbocation intermediate. The halide ion then acts as a nucleophile and attacks the carbocation, completing the addition and forming the alkyl halide product.

The regiochemistry of hydrohalogenaion, meaning where the hydrogen and halide atoms add to the unsaturated carbon atoms, is governed by Markovnikov's rule. This rule states that in the addition of a protic acid (like a hydrogen halide) to an unsymmetrical alkene or alkyne, the hydrogen atom will attach to the carbon atom with the greater number of hydrogen atoms already attached. Conversely, the halide atom will attach to the carbon atom with fewer hydrogen atoms. This preference arises from the stability of the carbocation intermediate formed during the reaction. The more substituted carbocations are more stable due to hyperconjugation and inductive effects, thus leading to the preferred product.

Hydrohalogenaion can also occur in the presence of peroxides, particularly with HBr. In the presence of peroxides, the addition of HBr follows an anti-Markovnikov pathway, meaning the bromine atom adds to the less substituted carbon and the hydrogen atom adds to the more substituted carbon. This radical mechanism is distinct from the polar mechanism that dictates Markovnikov addition and is a crucial exception to the general rule.