How do alkenes react with halogens?

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

In more detail, alkenes are hydrocarbons that contain a carbon-carbon double bond. This double bond is the site of high electron density, making it an area of interest for electrophiles, which are electron-seeking species. Halogens, such as chlorine (Cl2) or bromine (Br2), are examples of such electrophiles.

The reaction between alkenes and halogens is known as halogenation. This is an addition reaction, where the halogen molecule adds across the carbon-carbon double bond. The double bond breaks, and a halogen atom attaches to each of the carbon atoms that were previously double-bonded. This results in a dihalogenated compound. For example, if ethene (C2H4) reacts with bromine (Br2), the product is 1,2-dibromoethane (C2H4Br2).

The reaction is usually carried out in a solvent such as tetrachloromethane or hexane. The halogenation of alkenes is a useful reaction in organic chemistry because it allows for the introduction of halogen atoms into a molecule, which can then be used for further chemical transformations.

The reaction is also used as a test for 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 reacts with the alkene to form a colourless dibromo compound, removing the original brown colour of the bromine water.

In summary, alkenes react with halogens in a halogenation reaction to form dihalogenated compounds. This reaction is an important tool in organic chemistry and also serves as a test for the presence of alkenes.