1.1- dihalo-1-alkene alkanal

The method of greatest applicability is the reaction of alkali metal phosphides with dihalo-alkanes, -alkenes, etc. (equation 4). Generally this reaction proceeds well and in high yield excess dihaloalkane should be avoided as this leads to the production of phosphonium salts which can contaminate the product. 

Analogous sulfur compounds are also prepared. 1,2-Dihalo compounds lead to the formation of alkenes (see above), as do l-halo-2-X-substituted alkanes (X = NRjOR, SR, etc.). In the aromatic series 2-heterosubstituted reagents can be stable , for elimination would lead to highly strained arynes. Similarly, the elimination reaction may be difficult when allenes ... 

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