Reaction with Halogens

Alkenes react with halogens at room temperature by electrophilic addition. The two halogen atoms add to the two carbons of the C=C, giving a dihalogenoalkane with the halogens on adjacent carbons:CH₂=CH₂ + Br₂ → CH₂BrCH₂Br   (1,2-dibromoethane)Br₂ is a non-polar molecule, so how can it be an electrophile? As a Br₂ molecule approaches the C=C, the high electron density of the double bond repels the electron pair in the Br–Br bond. This induces a dipole: the Br atom nearer the C=C gains a partial positive charge (δ+) and the further one becomes δ–.Mechanism:
1. A curly arrow goes from the C=C to the δ+ Br, and a curly arrow goes from the Br–Br bond to the δ– Br. The Br–Br bond breaks heterolytically, giving a carbocation and a Br^– ion.
2. A lone pair on Br^– attacks the positive carbon of the carbocation.Other alkenes and other halogens react in the same way. Propene and bromine give 1,2-dibromopropane, CH₃CHBrCH₂Br. Chlorine gives dichloro compounds, for example ethene and Cl₂ give 1,2-dichloroethane.Worked example: what forms from but-2-ene, CH₃CH=CHCH₃, and chlorine? The C=C is between carbons 2 and 3, so one Cl adds to each: CH₃CHClCHClCH₃, 2,3-dichlorobutane.