eliminaatioreaktio

Eliminaatioreaktio, or elimination reaction, is a class of organic transformations in which a molecule loses two substituents, typically a hydrogen atom and a leaving group, to form a multiple bond, most commonly a carbon–carbon double bond. This type of reaction often competes with nucleophilic substitution, where a leaving group is replaced by a nucleophile.

The mechanisms most commonly encountered are E1, E2, and E1cB. E1 is a two-step process in which the leaving group departs to form a carbocation, followed by deprotonation to yield the alkene. It is favored by stable carbocations and weak bases, usually with tertiary substrates. E2 is a concerted, one-step process in which a base abstracts a proton while the leaving group leaves, typically requiring an anti-periplanar arrangement. E2 rates depend on substrate and base strength, and strong bases promote this pathway, especially for secondary and primary substrates under appropriate conditions. E1cB proceeds via formation of a carbanion intermediate and is invoked when the leaving group is poor but a stabilized carbanion forms.

Substrates and conditions vary: tertiary and some secondary alkyl halides or tosylates commonly undergo E1 or E2, while alcohols can be dehydrated under acidic conditions to form alkenes. Regioselectivity is often governed by Zaitsev’s rule, giving the more substituted alkene, though Hofmann products can predominate with bulky bases. Stereochemistry matters for E2, which often requires an anti-periplanar alignment of hydrogen and leaving group.

Applications include the synthesis of alkenes for further transformations and the strategic construction of carbon–carbon double bonds in complex molecules.