An
elimination reaction is a type of
organic reaction in which two
substituents are removed from a molecule in either a one or two-step mechanism. The one-step mechanism is known as the
E2 reaction, and the two-step mechanism is known as the
E1 reaction. The numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively. In rare cases, for molecules possessing particularly poor leaving groups, a third type of reaction,
E1CB exists. In most organic elimination reactions, at least one hydrogen is lost to form the double bond: the
unsaturation of the molecule increases. It is also possible that a molecule undergoes
reductive elimination, by which the valence of an atom in the molecule decreases by two; though this is more common in inorganic chemistry. An important class of elimination reactions is those involving
alkyl halides, with good
leaving groups, reacting with a
Lewis base to form an
alkene. Elimination may be considered the reverse of an
addition reaction. When the substrate is asymmetric,
regioselectivity is determined by
Zaitsev's rule or through
Hofmann elimination if the Carbon with the most substituted Hydrogen is inaccessible.