E1 is the fourth corner of this chapter's square: unimolecular like SN1, eliminating like E2. It shares SN1's first step exactly, which means the two always occur together and always give a mixture — a fact that makes E1 less useful synthetically than the other three, and more important to recognize when it is happening.
E1 starts exactly like SN1
E1 begins with the same rate-determining ionization as SN1: the leaving group departs unassisted, giving a flat, sp²-hybridized carbocation. Everything said about carbocation stability in the SN1 section applies here unchanged — 3° > 2° > 1°, with allylic and benzylic cations stabilized by resonance.
What differs is the second step. Instead of a nucleophile attacking the cationic carbon, a weak base removes a hydrogen from a beta carbon, and the electrons from that C–H bond flow in to form a new pi bond with the cationic carbon. Two arrows in that step: base's lone pair to the hydrogen, and the C–H bond into the C–C bond.
The base is usually something weak that happens to be present — the solvent, or the departed leaving group. E1 does not need a strong base, because the cation is so electron-poor that even water will remove a beta proton from it.
Zaitsev's rule
When several beta hydrogens are available, removing different ones gives different alkenes, and Zaitsev's rule applies cleanly here: the more substituted alkene is the major product.
The reason is alkene stability, and it is the same hyperconjugation argument that stabilizes carbocations — more alkyl groups on the double bond means more C–H bonds able to donate into the pi system. A tetrasubstituted alkene is roughly 6 kcal/mol more stable than a monosubstituted one.
Unlike E2, E1 gives Zaitsev products essentially regardless of the base, because the base is weak and small by definition — there is no bulky-base option that would push toward the less substituted alkene instead. Where E2's product can be steered, E1's cannot.
Step 1 (slow): bromide ionizes off, giving the tertiary cation (CH₃)₂C⁺–CH₂CH₃.
Step 2 (fast): ethanol removes a beta hydrogen. Two choices are available.
Taking one from the ethyl CH₂ gives 2-methyl-2-butene, trisubstituted — the Zaitsev major product, about 70%.
Taking one from a methyl gives 2-methyl-1-butene, disubstituted — the minor product.
And running alongside both, ethanol can attack the cation directly, giving the SN1 ether. All three products come out of the same flask.
Rate law and the SN1 connection
E1 is unimolecular: rate = k[substrate]. The base's concentration does not appear, because the base is not involved until after the rate-determining step.
Because E1 and SN1 share the identical carbocation intermediate, they occur under identical conditions — weak nucleophile and weak base, polar protic solvent, a substrate that ionizes readily — and they essentially always compete. Any reaction you would call SN1 is producing some E1 product too, and vice versa. This is why neither is a reaction you would choose for a synthesis where yield matters.
Two levers shift the balance. Higher temperature favours elimination, since it makes two molecules from one and so gains entropy, and the TΔS term grows with T. More substitution around the cation also favours elimination, both because the cationic carbon is crowded and because the resulting alkene is more substituted and more stable.
Where E1 actually shows up
Although it is rarely chosen deliberately with an alkyl halide, E1 is the mechanism of one genuinely important transformation: acid-catalyzed dehydration of alcohols. Strong acid protonates the OH, converting a hopeless leaving group into water; water leaves to give a carbocation; and a weak base removes a beta proton to give the alkene. Every step is E1. Running the reaction hot and distilling off the alkene as it forms drives the equilibrium and makes it preparatively useful — a reaction covered properly in Module 8.
The chapter's square, completed
| Substitution | Elimination | |
|---|---|---|
| Concerted | SN2 | E2 |
| Via cation | SN1 | E1 |
The rows share a mechanism type and therefore share conditions: SN2 and E2 both want strong reagents and give no rearrangement; SN1 and E1 both want weak reagents and ionizing solvents, and both rearrange. The columns share an outcome. Reading the table this way is the fastest route into the decision framework in the next section.
What carries forward
E1 is the mechanism of alcohol dehydration in Module 8, and its reverse — protonating an alkene to give a carbocation — is the mechanism of Markovnikov addition in Module 7. Recognizing that the same carbocation sits at the centre of SN1, E1, alkene addition and alcohol dehydration is what turns four separate reactions into one idea.