Final answer:
The addition of a hydrogen halide to an alkene results in the formation of a monosubstituted alkyl halide through an electrophilic addition reaction.
Step-by-step explanation:
When an alkene reacts with a hydrogen halide such as HF, HCl, HBr, or HI, the product is a monosubstituted alkyl halide.
This type of reaction is an example of an electrophilic addition reaction, where the hydrogen halide adds across the double bond of the alkene, resulting in the formation of a molecule that has a halogen atom attached to one of the previously double-bonded carbons, while the other carbon gains a hydrogen atom.
For instance, when ethene (an alkene) reacts with HCl, chloroethane is produced.
The reaction proceeds with the π-bond electrons of the alkene attacking the electrophilic hydrogen of the HCl, leading to the formation of a carbocation intermediate which is then quickly attacked by the chloride ion, completing the formation of the chloroethane.