1,2-Dihaloalkanes dehydrohalogenation

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... 

A variety of sulfur reagents have been used for the dehydrohalogenation of vic-dihaloalkanes to alkenes. Aqueous sodium trithiocarbonate in the presence of a phase transfer catalyst was reported to give high yields under very mild conditions [217]. A radical mechanism involving a one-electron transfer was proposed as the first step in this reductive elimination. 

Hydrogen halide eliminations take place from 1,1- and 1,2-dihaloalkanes (equations 5 and 6), as w ell as from vinyl halides (equations 7 and 8). Dehydrohalogenation... 

Amines are generally too weak to effect a double dehydrohalogenation from dihaloalkanes. If, however, the hydrogens are made highly acidic by electronegative groups, elimination does take place (e.g. equation 65) . Recently it has been found... 

Dehydrohalogenation of 1,2- or 1 J-Dihaloalkanes Elimination of Acids Other Than Hydrogen Halides... 

Scheme 30 illustrates the general reaction for the dehydrohalogenation of 1,2- and 1,1-dihaloalkanes. 

To prepare an alkyne from an alkene, the alkene is first treated with 1 mole of either bromine (Brj) or chlorine (CI2) to give a dihaloalkane (Section 6.3D). Treating the dihaloalkane with 2 moles of a strong base such as sodium amide (NaNH2) in liquid ammonia [NHj(/)] brings about two successive dehydrohalogenations. 

Synthesis of an Aikyne from an Aikene (Section 7.5B) Treating an alkene with Bij or CI2 gives a dihaloalkane. Treating the dihaloalkane with NaNHj or another strong base results in two successive dehydrohalogenations to give an alkyne. 

Because vicinal dihaloalkanes are readily available from alkenes by halogenation, this sequence, called halr enation-double dehydrohalogenation, is a ready means of converting alkenes into the corresponding alkynes. 

Dehydrohalogenation of dihaloalkanes proceeds through the intermediacy of haloalkenes, also called alkenyl halides. Althongh mixtures of E- and Z-haloalkenes are in principle possible, with diastereomerically pnre vicinal dihaloalkanes only one product is formed because elimination proceeds stereospeciflcally anti (Section 11-6). 

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