Elimination reactions are important as a method for the preparation of alkenes.
The term "elimination" describes the fact that a small molecule is lost during the process.
A 1,2-elimination indicates that the atoms that are lost come from adjacent C atoms.
The two most important methods are:
- Dehydration (- H2O) of alcohols, and
- Dehydrohalogenation (- HX) of alkyl halides.
There are three fundamental events in these elimination reactions:
- Removal of a proton
- Formation of the CC π bond
- Breaking of the bond to the leaving group
Depending on the relative timing of these events, different mechanisms are possible:
- Loss of the LG to form a carbocation then removal of H+ with formation of C=C bond (two steps): E1 reaction
- Simultaneous H+ removal, C=C bond formation and loss of the LG (one step): E2 reaction
- Removal of H+ to form a carbanion then formation of C=C bond with loss of the LG (two steps): E1cb reaction (less common)
In many cases the elimination reaction may proceed to alkenes that are constitutional isomers with one formed in excess of the other. This is described as regioselectivity.
Zaitsev's rule, based on the dehydration of alcohols, describes 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. Similarly, eliminations often favor the more stable trans-product over the cis-product (stereoselectivity)