What is the removal of a halogen and a hydrogen atom from a molecule called?

In organic chemistry, the removal of a hydrogen atom and a halogen atom (such as chlorine, bromine, or iodine) from a molecule is known as dehydrohalogenation. It is a type of elimination reaction, usually represented as the removal of HX from an alkyl halide. The “dehydro” part means removal of hydrogen, and “halogenation” refers to halogen, so the name literally describes the reaction process.

Dehydrohalogenation typically occurs when an alkyl halide is treated with a strong base such as potassium hydroxide (KOH) or sodium ethoxide (NaOC₂H₅) in an alcoholic medium. The base abstracts a proton (hydrogen atom) from a β-carbon, while the halide ion leaves from the α-carbon, resulting in the formation of a carbon–carbon double bond. The product of this reaction is an alkene.

This reaction follows the β-elimination mechanism, often known as the E2 (bimolecular elimination) or E1 (unimolecular elimination) pathway depending on reaction conditions. In E2 reactions, the removal of hydrogen and halogen occurs simultaneously, whereas in E1, the halogen leaves first, forming a carbocation intermediate.

Dehydrohalogenation is an important method for preparing alkenes from alkyl halides. It also helps in understanding reaction mechanisms, base strength, and stability of carbocations. The reverse of this process is hydrohalogenation, where HX adds to an alkene to form an alkyl halide.