Stereochemistry of E2 eliminations for some acyclic substrates

CHAPTER 6 POLAR ADDITION Substrate Base, solvent (%) anti (%) syn Ref. 

The factors that determine whether syn or anti elimination predominates are still subject to investigation. One factor that is believed to be important is whether the base is free or present as an ion pair. The evidence is that an ion pair 

There is a very strong preference for anti stereochemistry when the elimination is promoted by a weakly basic anion, such as halide or thiophenoxide. It has been proposed that these bases promote elimination via an E2 transition state characterized by association of the base with both the jS-hydrogen and the carbon atom carrying the leaving group, but this explanation has been criticized. In any case, 

The dehydration of alcohols is an important elimination reaction that takes place under acidic rather than basic conditions. It involves an El mechanism. The function of the acidic reagent is to convert the hydroxyl group to a better leaving group by protonation  

Since a carbonium ion or closely related species is the intermediate, orientation would be expected to follow the Saytzeff rule favoring the most stable olefin. The carbonium ion mechanism also explains the general trend in relative reactivity exhibited in this reaction. Tertiary alcohols are the most reactive, and reactivity decreases on going to secondary and primary alcohols. Also in accord with the cationic mechanism is the fact that rearranged products are found in cases where a carbonium ion intermediate could be expected to rearrange.  

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Table 6.8. Stereochemistry of E2 Eliminations for Some Acyclic Substrates...

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