Addition of Halogens to Olefins

The addition of chlorine and bromine to olefins is a very general reaction. 

Substantial amounts of syn addition have been observed for cis-1-phenylpropene (27-80% syn addition), trans-l-phenylpropene (17-29% syn addition), and ds-stilbene (up to 90% syn addition in polar solvents). A common feature 

The intermediate has carbonium character at the phenyl-substituted carbon. The effect of this diminished bridging is to increase the rate of rotation around the central C-C bond. If such rotation takes place, anti stereospecificity is, of course, lost. 

Chlorination shows relatively less tendency to give anti addition products than does bromination. Although chlorination of aliphatic olefins gives largely anti addition, syn addition is often dominant for phenyl-substituted olefins  

Chlorination can be accompanied by reactions characteristic of carbonium-ion intermediates. Branched olefins can give products that are the result of elimination of a proton from a cationic intermediate  

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Mare, P. B. D. de la Kinetics of Thermal Addition of Halogens to Olefinic... 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

Addition of halogens to olefins in a nitrile solvent containing silver perchlorate leads to nitrilium ions, which may be trapped with azide ion to yield 1,5-disubstituted tetrazoles, e.g., 138 [Eq. (17)].341 343 The... 

An electron spin resonance study of intermediates in radical addition of halogens to olefins suggests an intermediate radical having a preference for the conformation (641) at low temperatures. ... 

The addition of halogens to olefins in polar media proceeds as a heterolytic process. In acetic acid and other polar solvents at a low content of X2, the rate... 

Bromine readily adds to a double bond at -20 to 20 C to give dibromides in high yield, e.g., allyl bromide gives 98% 1,2,3-tribromo-propane. Chloroform, carbon tetrachloride, acetic acid, or ether are recommended solvents for the addition of halogen to olefins. Heat or sunlight favors dehydrohalogenation reactions. 

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