E1 elimination

Where E2 is a single all-at-once event, E1 takes its time, in two acts. First, the leaving group departs entirely on its own, taking the bonding electrons and leaving behind a carbocation — a carbon short of an electron pair and bearing a positive charge. Then, in a separate second step, a weak base (often just the solvent) plucks a proton from a carbon next to the positive centre, and the electrons of that C-H bond swing in to form the pi bond. The E is for elimination, the 1 for unimolecular.

The 1 tells you the kinetics: only one species — the substrate — matters in the slow, rate-determining first step, so rate = k[substrate] and the base concentration does not appear in the rate law. That first ionization is the bottleneck, which means E1 needs a substrate that can form a reasonably stable carbocation: tertiary works well, secondary sometimes, primary essentially never. An ionizing, polar protic solvent (water, alcohols) that stabilizes ions, plus heat, plus a weak base, are the classic E1 conditions.

Crucially, E1 shares its very first step — the formation of the carbocation — with SN1. The two reactions branch only afterward: if a nucleophile bonds to the cation you get substitution (SN1); if a base removes a beta proton you get elimination (E1). They run side by side and usually give a mixture, with heat favouring elimination. And because a free carbocation forms, the skeleton can rearrange (hydride or alkyl shifts) to a more stable cation before the alkene forms, sometimes giving products with a shifted double bond that students do not expect.