Substrate structure may allow El

The first elimination of the chapter. (f-BuBr plus hydroxide) illustrates something very important the starting material is a tertiary alkyl halide (and would therefore substitute only by 1) it can dim- 

Polar solvents also favour El reactions because they stabilize the intermediate carbocation. El eliminations from alcohols in aqueous or alcohol solution are particularly common, and very useful. 

An acid catalyst is used to promote loss of water, and in dilute H2SO4 or HCl the absence of good nucleophiles ensures that substitution does not compete. Under these conditions, the secondary alcohol cyclohexanol gives cyclohexene. 

But the best El eliminations of all are with tertiary alcohols. The alcohols can be made using the methods of Chapter 9 nucleophilic attack by an organometallic on a carbonyl compound. Nucleophilic addition, followed by El elimination, is the best way of making this substituted cyclohexene, for example. Note that the the proton required in the first step is recovered in the last—the reaction requires only catalytic amounts of acid. 

In El mechanisms, once the leaving group has departed almost anything wilt serve as a base to remove a proton from the intermediate carbocation. Weakly basic solvent molecules (water or alcohols), for example, are quite sufficient, and you will often see the proton just falling off in reaction mechanisms. We showed the loss of a proton like this in the last example, and in the chart on this page. The superacid solutions we described in Chapter 17 were designed with this in mind the counterions BF4 and SbFe are not only nonnucleophilic but also i nonbasic. 

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