Ei elimination (Elimination Internal) in organic chemistry is a special type of elimination reaction in which two vicinal substituents on an alkane framework leave simultaneously in a single step to form an alkene in a syn elimination . In regular eliminations this reaction would involve a base or would in many cases involve charged intermediates. This reaction mechanism is often found in pyrolysis.

Examples are eliminations involving the Burgess reagent, Hofmann elimination (with some sterically hindered substrates), the Cope elimination and ester pyrolysis.

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