Elimination — bimolecular
E2: A strong base eliminates HBr from bromoethane
How the E2 works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: NaOEt / EtOH (strong, unhindered base). Conditions: Heat, ethanol. Bonds formed: C=C π bond; O–H; bonds broken: C–H (β), C–Br. Stereochemistry: Anti-periplanar H and Br required. Rate and selectivity: Rate = k[RBr][base]; Zaitsev product usually major.
At a glance
Step 1: Base approaches the β-hydrogen
Ethoxide (a strong, small/unhindered base) reaches for a hydrogen on the carbon next to the one bearing bromine — the β-hydrogen.
Step 2: One concerted step, three arrows
Three things move at once: the base grabs the β-H, that C–H pair becomes the new π bond, and the C–Br pair leaves with bromide. One concerted step, three curved arrows.
Step 3: Alkene, ethanol and bromide
The result is ethylene plus ethanol and bromide — a double bond forms exactly where the hydrogen was removed.
Abbreviations and terms
- EtOH
- Ethanol. Polar protic solvent.
- NaOEt
- Sodium ethoxide. Alkoxide base matched to ethyl esters (avoids transesterification).
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
- E2
- Bimolecular elimination. Concerted base removal of β-H and loss of the leaving group (anti-periplanar).
- Zaitsev
- Zaitsev’s rule. Elimination usually gives the more substituted (more stable) alkene.
How it compares with related reactions
| E2 | Hofmann elimination of a quaternary ammonium salt | Aldol condensation | |
|---|---|---|---|
| Reagents | NaOEt / EtOH (strong, unhindered base) | (CH₃)₃N⁺–R I⁻ → Ag₂O/H₂O (gives HO⁻) | NaOH or KOH (cat.) |
| Conditions | Heat, ethanol | Heat 100–200 °C | H₂O/EtOH, heat 50–100 °C |
| Stereochemistry | Anti-periplanar H and Br required | Anti-periplanar H and N⁺; Hofmann (less substituted) alkene | Mostly E-enal (thermodynamic) |
| Rate / selectivity | Rate = k[RBr][base]; Zaitsev product usually major | E2: rate = k[R–NMe₃⁺][HO⁻] | E1cB; loss of HO⁻ from the enolate is rate-determining |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the E2?
NaOEt / EtOH (strong, unhindered base). Conditions: Heat, ethanol.
Which bonds are formed and which are broken?
Formed: C=C π bond; O–H. Broken: C–H (β), C–Br.
What is the stereochemical outcome?
Anti-periplanar H and Br required
What controls the rate or the selectivity?
Rate = k[RBr][base]; Zaitsev product usually major
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go