E1cB elimination reaction example

In an E1cB (Elimination Unimolecular conjugate Base) elimination reaction, 2 substituents are removed (“elimination”) from a single molecule (“unimolecular,” as the rate-determining step involves one molecular entity) via a carbanion intermediate (the “conjugate base” of the starting material). An E1cB elimination reaction occurs under basic conditions, where a particularly poor leaving group (such as -OH or -OR) and an acidic hydrogen are removed to form an additional bond. E1cB is a two-step process. First, a base abstracts the most acidic proton to generate a stabilized anion. The lone pair of electrons on the anion then moves to the neighboring atom, expelling the leaving group and usually forming a double bond. (Courtesy of Wikimedia Commons. The Creative Commons Attribution-ShareAlike 3.0 Unported License.)