Stereoselectivity in E2 Reactions

The El mechanism, which occurs in the dehydrohalogenaiion of tertiary alkyl halides, is a two-step process. The first step is formation of a carbocation by a heterolytic cleavage of the C—X bond. This 

As in the E2 reaction, the strength of the carbon—halogen bond affects the rate of the reaction. In fact, the differences in reactivity are larger in El reactions because only the carbon—halogen bond breaks in the rate-determining step. Alkyl iodides have the weakest carbon—halogen bond, and hence react at the fastest rate. We recall that the formation of a carbocation is also the first step for the Sj 1 reaction. We will see in Chapter 10 that the El mechanism competes with an Sj l mechanism. 

Because the rate-determining step of an El reaction involves only the substrate, the formation of the carbocation is a unimolecular reaction. In the second, more rapid deprotonation step, a base such as an alkoxide ion or a solvent such as an alcohol removes a proton from a p-carbon atom adjacent to the cationic center. The overall result is loss of HX and formation of a 71 bond. These possibilities are outlined below. 

How many products can result from the dehydrobromination of 3-bromo-2,3-dunethylpentane Predict the major alkene product formed. Predict the alkene formed in the smallest amount. 

The product of the dehydrobromination of rnr/M-l-bromo-2-methylcyclohexane is not 1-methylcyclo-hexene, the Zaitsev product, but rather 3-methylcyclohexene. Explain why. (Hint Remember that the ring-flipping process gives a mixture of two conformations.) 

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