E1cB is an elimination reaction which looks similar to E2, only the leaving group can be a hydroxide, which cannot be the case with regards to E2 leaving groups. Negative charges are stabilized by conjugation with carbonyl groups. The proton which is removed using a strong base is adjacent to a carbonyl group, which makes the proton rather acidic, and can therefore be removed by the base without the leaving group departing at the same time. The resulting anion is stable enough to exist due to delocalization on to the carbonyl group. Although the anion is stabilized by the carbonyl group, it still prefers to lose a leaving group and become an alkene, which forms the rate-determining step for the elimination. Below is an example of an E1cB reaction which shows the formation of an acrolein.

Click the structures and reaction arrows in sequence to view the 3D models and animations respectively

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