Ei mechanism

The E_i 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.

Compounds that undergo elimination through cyclic transition states upon heating, with no other reagents present, are given the designation as E_i reactions. Depending on the compound, elimination takes place through a four, five, or six-membered transition state.

The elimination must be syn and the atoms coplanar for four and five-membered transition states, but coplanarity is not required for six-membered transition states.

There is a substantial amount of evidence to support the existence of the E_i mechanism such as: 1) the kinetics of the reactions were found to be first order, 2) the use of free-radical inhibitors did not affect the rate of the reactions, indicating no free-radical mechanisms are involved 3) isotope studies for the Cope elimination indicate the C-H and C-N bonds are partially broken in the transition state, this is also supported by computations that show bond lengthening in the transition state and 4) without the intervention of other mechanisms, the E_i mechanism gives exclusively syn elimination products.

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