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Chapter 5:
Structure and Preparation of Alkenes.
Elimination Reactions
Ch 5 contents
Selectivity

In many cases elimination reactions may proceed to give alkenes that are isomeric but with one formed in excess of the other (i.e. a major product).

Regioselectivity (products are constitutional isomers):

dehydration of 2-methylcychexanol gives mainly tri-substituted alkene
dehydrohalogenation of 2-bromo-2-methylbutane gives mainly tri-substituted alkene

Zaitsev's rule, based on experiment observations of the dehydration of alcohols, expresses the 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-LG bonds can dictate that the anti-Zaitsev products dominate.

Stereoselectivity (products are stereoisomers)

dehydration of 3-pentanol gives mainly trans-2-pentene
dehydrohalogenation of 2-bromopentane gives mainly trans-2-pentene
Similarly, eliminations often favour the more stable trans-product over the cis-product.
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Dr. Ian Hunt, Department of Chemistry, University of Calgary