Chapter 5 : Selectivity (original) (raw)

Selectivity

In many cases elimination reactions may proceed to alkenes that are isomeric but with one formed in excess of the other.

Rgioselectivity (products are constitutional isomers):

Zaitsev's rule, based on experiment observations of the dehydration of alcohols, expressesthe preference for eliminations to give the highly substituted (more stable) alkene, which may also be described as the Zaitsev product.
The rule is not always obeyed, some reactions give the anti-Zaitsev product which is sometimes described as the Hoffman product. (Hoffman studied the elimination of ammonium salts)
Care is needed with E2 eliminations of cyclic systems since the antiperiplanar alignment of the C-H and C-LGbonds can dictate that the anti-Zaitsev products dominate.

Stereoselectivity (products are stereoisomers)

Similarly, eliminations often favour the more stable _trans_-product over the _cis_-product.