Hydrochlorination

Hydrochlorination is a chemical reaction that involves the addition of hydrogen chloride (HCl) to an unsaturated organic compound, typically an alkene or alkyne. In this process, the double or triple bond in the organic molecule breaks, and a hydrogen atom and a chlorine atom from the HCl molecule are added to the carbon atoms that were previously involved in the multiple bond. The reaction is generally electrophilic in nature, with the hydrogen atom acting as the electrophile.

The regioselectivity of hydrochlorination, meaning which carbon atom receives the hydrogen and which receives the chlorine, is often governed by Markovnikov's rule. This rule states that in the addition of a protic acid (like HCl) to an alkene or alkyne, the hydrogen atom will attach to the carbon atom that already has the greater number of hydrogen atoms attached. Conversely, the halide atom (chlorine in this case) will attach to the more substituted carbon atom. This results in the formation of a more stable carbocation intermediate during the reaction mechanism.

Hydrochlorination is a fundamental reaction in organic synthesis and is used in the production of various chemicals, including vinyl chloride, a precursor to polyvinyl chloride (PVC) plastic. The reaction can be carried out under various conditions, often employing catalysts to increase the reaction rate and selectivity. The overall reaction is exothermic.