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Thursday, October 25, 2012
Saturday, October 6, 2012
Addition of Halogens to Alkenes
Addition of Halogens to Alkenes
Alkenes readily react with bromine and chlorine to form 1,2-dihaloalkanes (vicinal dihalides). In this reaction the π bond of the alkene reacts with the halogen to produce two new carbon-halogen σ bonds. The reaction is commonly carried out in an inert solvent such as CH2Cl2, CHCl3, or CCl4.
The mechanism for the addition of both bromine and chlorine is an electrophilic addition pathway involving an ionic intermediate. For both acyclic and cyclic non-conjugated alkenes this intermediate is a bromonium or chloronium ion. Nucleophilic attack of halide ion on one of the two electrophilic carbons of the bromonium (chloronium) ion gives the product.
The mechanism of the reaction of 2-methylpropene with bromine is shown below.
In the second step of the reaction the incoming bromide ion reacts with carbon on the side opposite the carbon-bromine bond that is being broken. The consequence of this reaction stereochemistry is seen most easily in cycloalkenes where only the trans dihalocycloalkane product is found. The addition of bromine to cyclopentene is a case in point:
In fact, it was the observation that the halogenation of alkenes involves this anti addition (the two halogen atoms add to opposite sides of the double bond) that led early investigators to postulate the existence of a halonium ion intermediate.