Dehydrohalogenation Zaitsev rule

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... 

PROBLEM 5.17 Write the structures of all the alkenes that can be formed by dehydrohalogenation of each of the following alkyl halides. Apply the Zaitsev rule to predict the alkene formed in greatest amount in each case. 

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 Similarly, most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods. 

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... 

If there is possibility of formation of more than one alkene due to the availability of more than one 3-hydrogen atoms, usually one alkene is formed as the major produet. These form part of a pattern first observed by Russian ehemlst, Alexander Zaitsev (also pronounced as Saytzeff) who In 1875 formulated a rule which can be summarised as in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. Thus, 2-bromopentane gives pent-2-ene as the major product. 

If an alkyl halide contains more than two carbons in its chain, and the carbon atoms adjacent to the carbon atom bonded to the halogen each have hydrogen atoms bonded to them, two products will form. The major product is predicted by Zaitsev s Rule, which states that the more highly branched alkene will be the major product. For example, in the dehydrohalogenation reaction between 2-chlorobutane and sodium methoxide, the major product is 2-butene. 

Alkyl dihalides can undergo two consecutive dehydrohalogenations, giving products that contain two double bonds. In the following example, Zaitsev s rule predicts the most stable product of the first dehydrohalogenation, but not the most stable product of the second. The reason Zaitsev s rule fails in the second reaction is that a conjugated diene is more stable than an isolated diene. 

List the alkenes that would be formed when each of the following alkyl halides is subjected to dehydrohalogenation with potassium ethoxide in ethanol and use Zaitsev s rule to predict the major product of each reaction (a) 2 bromo-3-methylbutane and (b)2 -bromo-2,3-dimethylbutane. 

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