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Ei mechanism
The Ei mechanism (Elimination Internal/Intramolecular), also known as a thermal syn elimination or a pericyclic syn elimination, in organic chemistry is a special type of elimination reaction in which two vicinal substituents on an alkane framework leave simultaneously via a cyclic transition state to form an alkene in a ''syn'' elimination. This type of elimination is unique because it is thermally activated and does not require additional reagents unlike regular eliminations which require an acid or base, or would in many cases involve charged intermediates. This reaction mechanism is often found in pyrolysis.
==General Features==
Compounds that undergo elimination through cyclic transition states upon heating, with no other reagents present, are given the designation as Ei reactions. Depending on the compound, elimination takes place through a four, five, or six-membered transition state.〔〔Carey, F. A.; Sunburg, R. J. ''Advanced Organic Chemistry: Reaction and Synthesis'', 5th Ed.; Part B; Springer: New York, 2010〕
The elimination must be ''syn'' and the atoms coplanar for four and five-memebered transition states,〔Branko, J. (1997). "Theoretical studies of thermal syn elimination reaction of organic amine oxide, sulfoxide and phosphoxide by ab initio and density functional methods". ''Theo. Chem.'' 389 257-263.〕 but coplanarity is not required for six-membered transition states.〔
There is a substantial amount of evidence to support the existence of the Ei mechanism such as: 1) the kinetics of the reactions were found to be first order,〔O’Connor, G.L.; Nace, H. R. (1953). "Further Studies on the Chugaev Reaction and Related Reactions". ''J. Am. Chem. Soc.'' 75 2118-.〕 2) the use of free-radical inhibitors did not affect the rate of the reactions, indicating no free-radical mechanisms are involved
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