<P> 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, E1, exists . </P> <P> 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 . </P>

What does e1 stand for in organic chemistry
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