Stereochemistry halogen addition

A dihaloalkene is an intermediate and is the isolated product when the alkyne and the halogen are present m equimolar amounts The stereochemistry of addition is anti... 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Addition of halogens often occurs without ring opening. Early reports described a ciy-addition 2) though the stereochemistry of addition of chlorine to (229) is cis-160), addition of bromine is cis- in non-polar solvents in the presence of sunlight but trans-in relatively polar ones 161), and addition of bromine to l,2-dichloro-3,3-dimethylcyclopropene leads to a mixture of (E)- and (Z)-dibromodichlorides83). 

Stereochemistry of Halohydrin Formation Because the mechanism involves a halonium ion, the stereochemistry of addition is anti, as in halogenation. For example, the addition of bromine water to cyclopentene gives fran.v-2-bromocyclopentanol, the product of anti addition across the double bond. 

Bromine and chlorine add to alkynes just as they add to alkenes. If 1 mole of halogen adds to 1 mole of an alkyne, the product is a dihaloalkene. The stereochemistry of addition may be either syn or anti, and the products are often mixtures of cis and trans isomers. 

Stereochemistry of Alkene Hydrogenation 230 6.15 Stereochemistry of Halogen Addition 250... 

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