Alkyl halides Zaitsev rule

Elimination Reactions of Alkyl Halides Zaitsevs Rule... 

I Elimination Raactlont of Alkyl Halides Zaitsev s 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... 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

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... 

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods... 

Here s an example how might we prove that E2 elimination of an alkyl halide gives the more highly substituted alkene (Zaitsev s rule, Section 11.7) Does reaction of 1-chloro-l-methylcyclohexane with strong base lead predominantly to 1-methyl cyclohexene or to methylenecyclohexane ... 

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. 

In Chapter 7, we saw that eliminations of alkyl halides usually follow Zaitsev s rule that is, the most substituted product predominates. This rule applies because the most-substituted alkene is usually the most stable. In the Hofmann elimination, however, the product is commonly the /east-substituted alkene. We often classify an elimination as giving mostly the Zaitsev product (the most-substituted alkene) or the Hofmann product (the least-substituted alkene). 

The alkyl halide has two different p C atoms (labeled Pi and P2), so two different alkenes are possible one formed by removal of HCI across the a and Pi carbons, and one formed by removal of HCI across the a and P2 carbons. Using the Zaitsev rule, the major product should be A, because it has the more substituted double bond. 

Problem 8.12 What alkenes are formed from each alkyl halide by an E2 reaction Use the Zaitsev rule to predict the major product. 

Figure 25.11 contrasts the products formed by E2 elimination reactions using an alkyl halide and an amine as starting materials. Treatment of the alkyl halide (2-bromopentane) with base forms the more substituted alkene as the major product, following the Zaitsev rule. In contrast, the three-step Hofmann sequence of an amine (2-pentanamine) forms the less substituted alkene as major product. 

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