Replacement of hydrogen by halogen in olefins

Replacement of hydrogen by halogen is, as we have shown, of relatively little importance for saturated hydrocarbons the reaction is considerably more interesting for hydrocarbons containing C=C bonds. 

Admittedly haloolefins in which the halogen is attached to doubly bonded carbon are prepared both in the laboratory and industrially by removing halogen (X2) or hydrogen halide (HX) from polyhalogenated hydrocarbons. 

Halogen (X2) can be removed thermally or by use of metals (Fe, Mg, Zn, Al). E.g., 1,2-dichloroethylene is obtained industrially from 1,1,2,2-tetrachloroethane by means of iron turnings and steam,140 and l,2-dichloro-l,2-difluoroethylene from l,2-difluoro-l,l,2,2-tetra-chloroethane by means of Zn dust in ethanol.342,343 

Removal of hydrogen halide (HX) in the preparation of haloolefins is governed by Saytzeff s rule that the proton is removed preferentially from the carbon carrying the smallest number of hydrogen atoms.344 Removal of HX thermally or by aqueous-alcoholic alkali is important for laboratory work but particularly so on an industrial scale.10 Solid NaOH or KOH is used in the preparation of 2,3-dibromo-l-propene from 1,2,3-tribromopropane as described 

Removal of HBr from bromosteroids under mild conditions is effected by leaving them in pyridine containing 5-10% of AgN03 at room temperature for 30-48 hours.349,350 

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