E1 vs E2 Elimination: Mechanisms Compared Simply

Elimination removes a small molecule (usually HX or H2O) and creates a double bond. E1 is a two-step process through a carbocation; E2 is a single concerted step. The choice decides the rate law, the stereochemistry, and whether rearrangements occur.

E1 loves stable carbocations, weak bases, and polar protic solvents; E2 loves strong bases and anti-periplanar geometry. Wrong conditions give substitution instead of elimination, or the wrong alkene.

Key Concepts

• E1 mechanism: the leaving group leaves first, forming a carbocation (slow step); then a weak base removes a beta hydrogen (fast). Rate depends only on the substrate.
• E2 mechanism: a strong base removes a beta hydrogen as the leaving group departs, in one concerted step. No carbocation forms, so no rearrangement is possible.
• Rate laws: E1 is first order, rate = k[RX]; E2 is second order, rate = k[RX][base]. Concentrations are in mol/L; k has units s-1 for E1 and L·mol-1·s-1 for E2.
• Saytzeff vs Hofmann: both usually give the more substituted alkene, but a bulky base like tert-butoxide gives the less substituted Hofmann product instead.
• Carbocation rearrangements: only E1 can rearrange — a hydride or methyl shift can move the positive charge to a more stable carbon before the base acts, changing the product skeleton.
• Stereochemistry: E2 requires the departing H and X to be anti-periplanar (180° apart, opposite sides), so the geometry of the starting material controls which alkene stereoisomer forms.

Formulas

• E1 rate law: rate = k[(CH3)3CBr]. Doubling the alkyl halide concentration doubles the rate; changing the base concentration does nothing to the rate.
• E2 rate law: rate = k[CH3CH2Br][OH-]. Doubling either the substrate or the base doubles the rate; doubling both quadruples it.
• E2 example: CH3CH2CHBrCH3 + KOH(alc) → CH3CH=CHCH3 (major, Saytzeff) + CH2=CHCH2CH3 (minor) + KBr + H2O.

Worked Example

A reaction can follow E1 or E2. The substrate concentration is doubled and the base concentration is tripled. By what factor does the rate increase under each mechanism?

1. E1 rate law: rate = k[RX]. Only [RX] matters.
2. Doubling [RX] multiplies the E1 rate by 2. Tripling the base has no effect on an E1 rate.
3. So under E1, the rate becomes 2 times the original.
4. E2 rate law: rate = k[RX][base]. Both concentrations matter.
5. Doubling [RX] gives a factor of 2, tripling [base] gives a factor of 3; combined factor = 2 × 3 = 6.
6. So under E2, the rate becomes 6 times the original. This 2-versus-6 difference is exactly how kinetic experiments distinguish the two mechanisms in the laboratory.

Common Misconceptions

• E1 is called "1" because it has one step. Wrong — the name counts the molecularity of the rate-determining step (unimolecular: only the substrate). E1 actually has two steps; E2 has one step but is bimolecular.
• A strong base always gives the Saytzeff product. Wrong. A bulky strong base like (CH3)3CO- cannot reach hindered hydrogens and gives the Hofmann (less substituted) alkene instead.
• Rearrangements happen in E2. Wrong. Rearrangement needs a real carbocation with a lifetime long enough for a hydride shift — only E1 provides one.

Real-World Applications

• Alkene manufacture: industrial dehydration of alcohols to alkenes (E1-like, acid-catalyzed) produces millions of tonnes of ethene and propene feedstocks.
• Pharmaceutical synthesis: E2 eliminations install double bonds with precise stereochemistry in drug molecules, where the anti-periplanar requirement is used deliberately.

E1 goes through a carbocation, is first order, and can rearrange; E2 is concerted, second order, and needs anti-periplanar geometry. The 2× versus 6× rate test settles which mechanism is operating.