The typical reaction is that of an alkene with bromine, either in the gas phase or with bromine dissolved in some inert organic solvent such as tetrachloromethane; both at room temperature.

The mechanism:

Points:

• Bromine is not polar, but becomes polarised, and hence electrophilic, by the electron density in the C=C bond.
• Draw the + charge of the carbocation out of the way of the curly arrow showing the incoming electron pair from the nucleophilic bromide ion.
• The mechanism involves a bridged 'bromonium' ion in reality, but this is not addressed at A level.

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Dr Rod Beavon   17 Dean's Yard   London    SW1P 3PB

e-mail:  rod.beavon@westminster.org.uk