A LITTLE LIGHTBULB MOMENT
Substitution and elimination mechanisms
E2: one-step elimination
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Base yanks a proton as the leaving group exits
The idea behind the cue
E2 is a concerted elimination: a base removes a beta hydrogen while the leaving group departs, forming a double bond in one step. It requires the correct geometry, usually anti-periplanar alignment of the C-H and C-leaving group bonds.
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Short & simple
Base yanks a proton as the leaving group exits
Acronym
CBBA — Clumsy Burritos Borrow Accordions.
Story
A referee blows the whistle while a rope is pulled from one side and a bead rolls off the other. In the same instant, the rope straightens into a double line as the crowd leans away.
Connect the pieces
- C (Clumsy) → Concerted: E2 is a one-step elimination: base removal of beta hydrogen and leaving-group departure happen together.
- B (Burritos) → Base: A base pulls off the beta hydrogen to help form the alkene.
- B (Borrow) → Beta: The hydrogen removed is on the beta carbon relative to the leaving group.
- A (Accordions) → Anti-periplanar: E2 usually needs the C-H and C-leaving-group bonds aligned anti-periplanar for proper orbital overlap.
- Clumsy · Concerted: E2 is a one-step elimination: base removal of beta hydrogen and leaving-group departure happen together.
- Burritos · Base: A base pulls off the beta hydrogen to help form the alkene.
- Borrow · Beta: The hydrogen removed is on the beta carbon relative to the leaving group.
- Accordions · Anti-periplanar: E2 usually needs the C-H and C-leaving-group bonds aligned anti-periplanar for proper orbital overlap.
Know the limits
E2 is strongly favored by strong base and suitable geometry; it does not require a carbocation intermediate.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
Which feature is required for a typical E2 reaction?
A. A nucleophile that attacks from the front side
B. Anti-periplanar alignment of the beta hydrogen and leaving group
C. A carbocation intermediate before elimination begins
D. A planar radical intermediate that later loses a proton
Reveal explanation
Anti-periplanar alignment of the beta hydrogen and leaving group
E2 needs the beta hydrogen and leaving group anti-periplanar for optimal orbital overlap. The other choices describe SN1/E1/radical processes or substitution.