Mechanism of Halogen Addition to Alkenes Halonium Ions

The mass 82 isotope of bromine PBr) is radioactive and is used as a tracer to identify the origin and destination of individual atoms in chemical reactions and biological transformations. A sample of 1,1,2-tribromocyclohexane was prepared by adding 82Br 82Br to ordinary (nonradioactive) 1-bromocyclohexene. How many of the bromine atoms in the 1,1,2-tribromocyclohexane produced are radioactive Which ones are they  

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine  

Electrons flow from the ir system of ethylene to Br2, causing the weak bromine-bromine bond to break. By analogy to the customary mechanisms for electrophilic addition, we might represent this as the formation of a carbocation in a bimolecular elementary step. 

Until it was banned in the United States in 1984, 1,2-dibromoethane (ethylene dibromide, or EDB) was produced on a large scale for use as a pesticide and soil fumigant. 

Such a carbocation, however, is not formed but is bypassed in favor of a cyclic bromonium ion, a more stable structure in which the positive charge resides on bromine, not carbon. 

16 MECHANISM OF HALOGEN ADDITION TO ALKENES HALONIUM IONS 

Neither bromine nor ethylene is a polar molecule, but both are polarizable, and an induced-dipole/induced-dipole force causes them to be mutually attracted to each other. This induced-dipole/induced-dipole attraction sets the stage for Br2 to act as an electrophile. Electrons flow from the tt system of ethylene to Br2, causing the weak bromine-bromine bond to break. By analogy to the customary mechanisms for electrophilic addition, we might represent this as the formation of a carbocation in a bimo-lecular elementary step. 

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Mechanism of Halogen Addition to Alkenes Halonium Ions... 

Addition of bromine and chlorine to an alkene is the addition of two halogens across the carbon-carbon double bond.The mechanism involves a bridged halonium ion as an intermediate and is an anti stereoselective. 

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