Orientation in E2 elimination as a function of base strength

The direction of elimination is also affected by steric effects. Highly hindered bases shift the orientation in dehydrohalogenations toward more Hofmann rule elimination. This effect can be reasonably attributed to the fact that the internal hydrogen that must be removed for Saytzeff rule elimination becomes inaccessible to very bulky bases, and abstraction of less-hindered protons is favored  

Two elements of stereochemistry enter into determining the ratio of isomeric olefins formed in E2 reactions. First, elimination may proceed in a syn or anti fashion  

Second, in many cases, the product olefin may be a mixture of the cis- and trans-isomers. The product ratio therefore depends on these details of 

Although there is usually a preference for anti elimination in acyclic systems, recent studies have shown that syn elimination is also competitive in some elimina-tions. Table 6.6 summarizes some of the available data on syn versus anti elimination in acyclic systems. 

The general trend revealed by these and other data is that anti stereochemistry is normally preferred for reactions involving good leaving groups such as bromide and tosylate. With poorer leaving groups (e.g., fluoride, trimethylamine) syn 

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Table 6.S. Orientation in E2 Elimination as a Function of Base Strength...

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