Regioselectivity dehydrohalogenation of alkyl

In addition to being regioselective dehydrohalogenation of alkyl halides is stereo selective and favors formation of the more stable stereoisomer Usually as m the case of 5 bromononane the trans (or E) alkene is formed m greater amounts than its cis (or Z) stereoisomer... 

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates m the direction that leads to the more highly substi tuted alkene... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

With a regioselectivity opposite to that of the Zaitsev rule, the Hofmann elimination is sometimes used in synthesis to prepare alkenes not accessible by dehydrohalogenation of alkyl halides. This application decreased in importance once the Wittig reaction (Section 17.12) became established as a synthetic method. Similarly, most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods. 

As is the case for the dehydrohalogenation of alkyl halides, the reaction is regioselective. We will discuss this feature of dehydration reactions in Section 9.18. 

As previously mentioned (cf. Sections 6.07.4.2.2 and 6.07.5.3), 2-( 1 -methylvinyl)tetrazoles 201 have been obtained by regioselective alkylation of tetrazole 2 and 5R-tetrazoles 24 with l-halopropan-2-ols and 3-bromopropene followed by dehydrohalogenation of the intermediate products 266 (Scheme 30) <1997MC41>. Similarly, dehydrobromina-tion of 2-bromoethyl-5-nitrotetrazole 457 afforded in good yield 2-vinyl-5-nitrotetrazole 458 (Equation 89) <1997RJO 771>. 

For the two reactions shown above, only a single alkene can result from the dehydrohalogenation of the alkyl halide. In alkyl halides having two or three adjacent carbon atoms with hydrogen atoms that could lead to different ehmination products, a regioselectivity is observed. We will discuss this feature of dehydrohalogenation reactions in Section 9.17. 

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