The E1 and E1cB Reactions

Core Concepts

E1: ionization first, deprotonation second

E1 proceeds in two steps. Step 1 (slow, rate-determining): the leaving group departs with its electron pair, forming a carbocation. Step 2 (fast): a base — the solvent or a weak base such as ethanol — removes a β-hydrogen and the C–H electrons form the alkene. The rate law is first order because only the substrate appears in the slow step. Conditions mirror S_N1: good leaving group, cation-stabilizing substrate (tertiary > secondary), polar protic solvent, weak base, and usually heat.

Because a carbocation forms, rearrangements are possible: a hydride or alkyl shift from an adjacent carbon can convert a less stable cation into a more stable one before deprotonation, so products reflect the rearranged cation. E1 still obeys Zaitsev's rule, but the cation that deprotonates may differ from the initial one.

E1 versus S_N1: the same intermediate, different fates

E1 and S_N1 share the carbocation. Whether it eliminates or is trapped depends on conditions: more base or higher temperature favors E1; a more nucleophilic, less basic solvent favors S_N1; more β-hydrogens favor elimination.

E1cB: deprotonation first, leaving group second

E1cB also has two steps, in reverse order. Step 1 (usually rate-determining): a base removes a β-hydrogen to form a carbanion — in carbonyl compounds, an enolate — stabilized by the electron-withdrawing group. Step 2: the electron pair reforms a π bond as the leaving group departs.

E1cB is favored when the β-hydrogen is unusually acidic and the leaving group is poor. Electron-withdrawing groups (carbonyl, nitro, cyano, sulfonyl) lower the β-H pKa into a range a base can reach; poor leaving groups (OH, OR, NR₃⁺, sometimes F) depart slowly, so deprotonation comes first. In arrow-pushing terms: the base's electron pair forms the H–base bond; the C–H pair moves onto the α-carbon (or into the carbonyl π system) to give the carbanion/enolate, whose lone pair then forms the C=C bond as the leaving group takes the C–X pair.

Telling the mechanisms apart

• Kinetics: E2 and E1cB are second order overall; E1 is first order.
• Isotope effect: E2 and (usually) E1cB show a primary KIE on the β-H; E1 shows only a small secondary KIE.
• Intermediates: E1 has a carbocation; E1cB has a carbanion/enolate (no rearrangements).
• Substrate preference: E1 wants a tertiary cation; E1cB wants an acidic β-H; E2 wants a strong base and good LG in one step.

How It Works / Step-by-Step Process

1. Survey the substrate: what β-H, what leaving group, what base?
2. Good LG plus stable cation (3°) → E1/S_N1: write the cation, check for shifts, then deprotonate.
3. Acidic β-H or poor LG → E1cB: deprotonate first, then eject the LG; strong base plus strong LG → E2.

Worked Example: tert-Butyl Chloride in Ethanol

Heating tert-butyl chloride, (CH₃)₃CCl, in ethanol (weak, polar protic) favors E1/S_N1 chemistry. Step 1: chloride leaves (slow), giving the tertiary cation (CH₃)₃C⁺. Step 2: ethanol (weak base) removes a β-H from a methyl group, forming the double bond:

(CH₃)₃CCl ^{EtOH, Δ}⟶ (CH₃)₂C=CH_{22-methylpropene} + HCl

The rate law is first order: rate = k[(CH₃)₃CCl] — base concentration never enters. Expect also a substitution product (tert-butyl ethyl ether) from trapping of the cation; all nine methyl hydrogens are equivalent.

Worked Example: E1 with a Hydride Shift

Consider 2-bromo-3-methylbutane, CH₃CH(Br)CH(CH₃)CH₃, under E1 conditions. Step 1: bromide leaves, giving a secondary carbocation at C2. Step 2 (fast): a hydride shifts from C3 to C2, producing a tertiary carbocation. Step 3: a β-H is removed, giving 2-methyl-2-butene as major:

CH₃CH(Br)CH(CH₃)₂ ^{weak base, Δ}⟶ CH₃CH=C(CH₃)₂ + HBr

Lesson: predict the rearranged cation before the alkene — deprotonating the initial secondary cation would predict different, less substituted alkenes and miss the major product.

Worked Example: E1cB with 4-Chloro-2-butanone

4-Chloro-2-butanone, ClCH₂CH₂C(=O)CH₃, has an acidic α-hydrogen (pKa ≈ 19–20) and a modest leaving group (β-chloride). With a base, step 1 deprotonates the α-carbon, giving an enolate stabilized by resonance:

ClCH₂CH₂C(=O)CH₃ ^B-⟶ [ClCH₂CH^- C(=O)CH₃ ↔ ClCH₂CH=C(O^-)CH₃]

Step 2: the enolate's electron pair forms the C=C bond as chloride departs, giving methyl vinyl ketone (but-3-en-2-one):

[enolate] ⟶ CH₂=CHC(=O)CH₃ + Cl^-

The overall rate is second order, with a primary KIE on the α-H because deprotonation is slow. A strong, non-selective base could instead give E2 or substitution — E1cB wins with an acidic β-H and a mediocre leaving group.

Common Confusions