Elimination Reactions
Elimination is where stereochemistry stops being decoration and starts determining the answer. An E2 reaction will not happen at all unless the hydrogen and the leaving group can reach the right geometry, which is why two diastereomers of the same compound can give different alkenes — or one of them nothing at all. This part covers the stereoelectronic requirement in detail, then the full mechanistic spectrum from E1 through E2 to E1cb, and ends with the question the exam actually asks: substitution or elimination? Two layers on every section: a slow, hand-held beginner path and a research-grade advanced/reference path.
The 10 sections in Part 5
- 1The E2 mechanism and its rate law Free below
- 2The anti-periplanar requirement and its orbital basis
- 3Zaitsev versus Hofmann — which alkene?
- 4The variable transition state, the E1–E2–E1cb spectrum, and kinetic isotope effects
- 5The E1 mechanism and its relationship to SN1
- 6The E1cb mechanism
- 7Syn eliminations and their cyclic transition states
- 8Bredt’s rule — alkenes that cannot be isolated
- 9Substitution or elimination? The complete decision procedure
- 10Where Part 5 leaves you, and what Part 6 does with it
The E2 mechanism and its rate law
Free extractSection E.1 of Part 5, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
One step, three arrows, second-order kinetics — and the four experimental facts that force that picture.
Take bromoethane and sodium ethoxide in ethanol. The ethoxide has two things it can do: attack the carbon bearing bromine (that is SN2, Part 4) or remove a hydrogen from the neighbouring carbon. If it does the second, ethene is formed:
The word concerted does the real work in the definition. Everything happens in a single step, through a single transition state: the base starts to take the β-hydrogen, the C–H electrons start to become the π bond, and the leaving group starts to depart, all at once. There is no intermediate. Nothing is ever formed that could be trapped or observed.
Mechanism — E2 — concerted bimolecular elimination
- Single step. The base removes the β-hydrogen while the Cβ–H electrons move into the Cα–Cβ bond and the Cα–X electrons leave with X. Three curly arrows, one transition state.
- In the transition state the C–H and C–X bonds are both partly broken, the π bond is partly formed, and both carbons are rehybridising from sp³ towards sp². Negative charge is spread over the base, the two carbons and the leaving group.
- Products. Alkene + conjugate acid of the base + leaving group anion. The base is consumed stoichiometrically: one equivalent per equivalent of alkene.
The rate law is the first and best evidence. It is first order in substrate and first order in base, so both appear in the transition state of the rate-determining step. Since E2 has only one step, its transition state is the rate-determining transition state. Double the ethoxide concentration and the rate doubles — which immediately rules out any mechanism in which the substrate ionises first, because that step would not care how much base was present.
The four pieces of evidence for a concerted mechanism
| Observation | What it rules out | What it supports |
|---|---|---|
| Second-order kinetics, first order in base | any mechanism with a slow ionisation first (E1) | base present in the rate-determining transition state |
| No rearranged products, even with skeletons that rearrange instantly as cations (neopentyl-type, 3,3-dimethyl-2-butyl) | a free carbocation intermediate | the leaving group never fully departs before the alkene forms |
| Large primary kinetic isotope effect, kH/kD typically 3–8 for the β-hydrogen | any mechanism in which C–H breaking happens after the rate-determining step (E1) | the C–H bond is being broken in the rate-determining step |
| Stereospecificity — two diastereomers give two different alkenes | any freely rotating intermediate (cation or carbanion) | a single transition state with a defined geometry — the subject of E.2 |
The third and fourth entries carry the most weight and each gets a section of its own: isotope effects in E.4, stereospecificity in E.2. The second is worth dwelling on for a moment because it is easy to under-rate. If E2 went through a carbocation, then every substrate whose cation rearranges rapidly would give rearranged alkenes. They do not. Under E2 conditions the alkene skeleton is the substrate skeleton, every time. Under E1 conditions on the same substrate, rearranged alkenes appear. That contrast, on one compound, is a complete mechanistic argument.
Sodium ethoxide (0.10 M) and 2-bromobutane (0.10 M) in ethanol react at a certain rate. What happens to the rate of elimination if (a) [EtO⁻] is doubled, (b) both concentrations are doubled, (c) the ethoxide is replaced by the same concentration of the much weaker base ethanol? Easy
⚠ Common mistakes & exam traps
- The base in an E2 reaction is a reagent, not a catalyst. One mole of base is consumed per mole of alkene. Writing a catalytic amount of KOH over the arrow for an E2 dehydrohalogenation is a chemistry error, not a bookkeeping one.
- “Bimolecular” means two species in the rate-determining transition state, not two steps. E2 has exactly one step. The 2 in E2 is a molecularity, matching the 2 in SN2.
- Check for a β-hydrogen before writing any elimination. Neopentyl halides, benzyl halides, aryl and vinyl halides, and (CH₃)₃CCH₂X have none on an sp³ carbon adjacent to the leaving group, so E2 is impossible however strong the base.
- Do not draw the alkene π bond forming from the leaving group’s electrons. The π bond comes from the C–H bonding pair. The C–X pair leaves with X. Reversing those two arrows is the commonest arrow error in the whole topic.
Charge distribution in the transition state, the Hammett evidence, and why the mechanism is concerted rather than merely fast.
Calling the E2 transition state “concerted” conceals a question worth asking: is the concert enforced or merely preferred? A concerted mechanism is enforced when the stepwise alternative would need an intermediate with no significant lifetime — a species that would fall apart faster than a bond vibration. For a simple secondary halide with a good leaving group, both the unstabilised carbanion and the secondary carbocation are exactly that, so concert is not a choice but the only option left. Stabilise one end — a carbonyl or nitro group on Cβ, or a tertiary Cα in an ionising medium — and a real intermediate becomes possible, and the mechanism slides towards E1cb or E1. E.4 makes this quantitative.
Where is the charge in the E2 transition state?
The transition state carries partial negative charge on the base, on Cβ (which is losing a proton faster than it is gaining π bonding) and on the departing leaving group. The distribution is not fixed. The standard probe is a Hammett study on 2-arylethyl substrates ArCH₂CH₂X, where a substituent on the ring senses charge developing on Cβ.
| System (2-arylethyl-X, EtO⁻/EtOH) | Hammett ρ | Interpretation |
|---|---|---|
| X = Br | about +2 | moderate negative charge on Cβ — a fairly ‘central’ E2 transition state |
| X = OTs | somewhat larger than for Br | poorer leaving group, so C–X breaking lags — more carbanion character |
| X = N(CH₃)₃⁺ | about +3 to +4 | a very poor (and positively charged) leaving group: the transition state is strongly E1cb-like, with substantial negative charge on Cβ |
A second probe is the leaving-group element effect: the ratio kBr/kCl for otherwise identical substrates. If the C–X bond is breaking in the rate-determining step, the ratio is large (tens); if it is not breaking at all in that step, the ratio is near 1. This becomes the decisive test for E1cb in E.6, and it is worth carrying the idea forward now: an isotope effect tells you about the bond to the isotope; an element effect tells you about the bond to the leaving group. You need both to place a mechanism.
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Read the rest of Part 5
The remaining 9 sections of this part — The anti-periplanar requirement and its orbital basis, Zaitsev versus Hofmann — which alkene?, The variable transition state, the E1–E2–E1cb spectrum, and kinetic isotope effects, The E1… — and all nine parts of Reaction Mechanisms are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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