Alkyl halides geminal dihalides

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

The reactivities of organotin hydrides [24] and chromium (II) complex [25] toward alkyl halides are also in the order of tertiary > secondary > primary alkylhalides. However, this trend is much stronger with the ate complex of 9-BBN than that with these reagents. Consequently, the high selectivity, gentleness, and convenience exhibited by the reagent has the practical synthetic application (Eq. 25.20) Benzylic halides are also reduced easily (entries 11-13, Table 25.10), whereas aryl and vinylhalides are inert (entries 19,20). Benzylic geminal dihalides are reduced stepwise (entries 16, 17). The reaction of 1-phenylallyl-chloride [26] which also contains 33% of cinnamyl chloride with an equiv of ate complex of 9-BBN affords mixture of allylbenzene (36%) and 3-metliylstyrene (66%). 

Electrophilic addition to terminal alkynes (unsymmetrical) is regioselective and follows Markovnikov s rule. Hydrogen halides can be added to alkynes just like alkenes, to form first the vinyl halide, and then the geminal alkyl dihalide. The addition of HX to an alkyne can be stopped after the first... 

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