1,1-dihaloalkane alkanal

Alkenes undergo addition reactions with hydrogen to form alkanes, with halogens to form dihaloalkanes, with hydrogen halides to form monohaloalkanes and with water to form alcohols. For example ... 

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. 

If a dihaloalkane is heated in alcoholic KOH solution, the two halogen atoms, bonded to the neighbour carbons, are eliminated and the alkane forms a triple bond and becomes an allqme. This reaction is called an elimination reaction. 

Typically, attempts to synthesize 1,2-di-Grignard reagents from 1,2-dihaloalkanes fail, because metal halide elimination after insertion of the first magnesium atom is faster than formation of a second magnesium-carbon bond. The only l,2-di(halomagnesio)alkane [16] synthesized in the classic fashion is a l,2-di(bromomagnesio)cyclopropane 22 [Eq. 

The reaction of l-metallo-l,l-bis(seleno)alkanes with electrophiles is another interesting approach to selenoacetals (Schemes 76 and 77), which inter alia allows the synthesis of l,l-bis(seleno)cyclo-propanes (Scheme 26, a, b and d Scheme 77, c) and also permits the synthesis of functionalized selenoacetals by concomitant C—C bond formation (Scheme 79). Other methods listed above are of limited use. The substitution of dihaloalkanes by selenolates is the method of choice for the synthesis of bis(seleno)methanes (Scheme 72, and can be applied to the synthesis of the relatively unhin-... 

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