Dehydrohalogenation derivs. from halides

Dehydrohalogenation (s. a. Ethylene derivatives from halides) CC Hal Dehydrohalogenation-rearrangement 16, 938 Deoxymercuration, stereospecific 16, 970 Dequaternization s. Amines, tert., from ammonium salts, quaternary Derivatives s. a. subentry derivatives... 

C(R)=NR group with a nitrilium salt RCssNR .222 The acylation of the enamine can take place by the same mechanism as alkylation, but another mechanism is also possible, if the acyl halide has an a hydrogen and if a tertiary amine is present, as it often is (it is added to neutralize the HX given off). In this mechanism, the acyl halide is dehydrohalogenated by the tertiary amine, producing a ketene (7-14) which adds to the enamine to give a cyclobutanone (5-49). This compound can be cleaved in the solution to form the same acylated imine salt (27) that would form by the more direct mechanism, or it can be isolated (in the case of enamines derived from aldehydes), or it may cleave in other ways.223... 

Dehydrohalogenation of (bromomethyl)cyclohexane with potassium ferf-butoxide, a sterically hindered and strong base, would give the desired alkene as the major product. The substitution derived from SN2 displacement of the halide by the alkoxide might also be obtained as a minor product. 

Lithium or sodium alkynylides derived from 1-alkynes react with primary alkyl halides in the presence of HMPA to give disubstituted alkynes. This method is limited to primary alkyl halides that are not branched at the (3-position. Secondary and tertiary alkyl halides tend to undergo dehydrohalogenation (E2-elimination). 

Condensation [274] of the bromide (412) with the unsaturated derivative (378) using silver triflate gave an a p yield ratio of 42 21% and condensation of (412) with (377) gave 26 8%. From these results the authors concluded that the 3-P-hy-droxyl group on the neuraminic acid halide prevents dehydrohalogenation and assists glycosidation. 

The dehydrohalogenation of a yQ-chloroalkylbenzene is readily accomplished by refluxing with excess aqueous methanolic potassium hydroxide. Substituted a-alkylstyrenes which are difficult to obtain by other methods are prepared in this way by a two-step process involving catalytic condensation of aromatic compounds with aliphatic chlorohydrins followed by removal of hydrogen halide from the resulting haloalkylated derivatives. ... 

The majority of grafting from syntheses have been accomplished by converting polymeric halides into macro-initiators. Allyl halides provide suitably stable carbocation derivatives and consequently the partial dehydrohalogenation of poly(vinyl chloride) has been found to enhance the rate of styrene grafting in the presence of aluminium chloride. Similar sites available in chloroprene were activated by silver hexafluorophosphate to induce grafting of IBVE. Nitrosyl and... 

Unsaturated hydrocarbons are often prepared commercially from compounds derived originally from alkanes. Alkenes, for example, are produced from the dehydration of alcohols (Section 21.10) or the dehydrohalogenation of an alkyl halide. These two impressive terms describe very similar processes that are quite simple, at least in principle. Dehydration is the removal of a water molecule dehydrohalogena-tion is the removal of a hydrogen atom and a halogen atom. For example, a water molecule may be separated from propyl alcohol, C3H7OH, to make propylene ... 

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