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. It is an example of an addition reaction, which means that the atoms of the halogen molecule add across the carbon-carbon double bond of the alkene. The double bond breaks, and a halogen atom forms a single bond with each of the carbon atoms, resulting in a dihalogenated compound.

For instance, 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, and the change in colour from the brown bromine to a colourless solution provides a test for the presence of the carbon-carbon double bond.

The mechanism of this reaction is electrophilic addition. The high electron density of the double bond induces a dipole in the halogen molecule, making one of the halogen atoms slightly positive. This atom is then attracted to the electron-rich double bond, leading to the formation of a cyclic halonium ion. This ion is highly unstable and reacts with a halide ion in the solution to form the final dihalogenated product.

This reaction is important in organic chemistry as it allows for the functionalisation of alkenes, making them more reactive and useful in further chemical reactions. It's also a key topic in GCSE Chemistry, so understanding it well can really help with your studies!