How do alkenes react with diatomic halogens?

Alkenes react with diatomic halogens in a process called halogenation, forming a dihalogenated alkane.

In more detail, this reaction is a type of addition reaction, which is characteristic of alkenes. Alkenes are hydrocarbons with a carbon-carbon double bond. When they react with diatomic halogens such as chlorine (Cl2) or bromine (Br2), the double bond breaks and the halogen atoms add across the carbon atoms, forming a dihalogenated alkane.

The reaction mechanism involves the formation of a cyclic halonium ion intermediate. The double bond of the alkene polarises the halogen molecule, causing one of the halogen atoms to become slightly positive and the other slightly negative. The slightly positive halogen atom is then attracted to the electron-rich double bond, leading to the formation of the halonium ion. The remaining negatively charged halogen ion then attacks the halonium ion, resulting in the formation of the dihalogenated alkane.

This reaction is commonly used in organic chemistry to test for the presence of alkenes. When bromine water is added to an alkene, it decolourises, indicating the presence of a carbon-carbon double bond. This is because the bromine atoms have added across the double bond, forming a colourless dibromo compound.

In summary, the reaction of alkenes with diatomic halogens is a key aspect of their chemical behaviour. It is an addition reaction that results in the formation of a dihalogenated alkane, and it is also used as a test for the presence of alkenes in a sample.