Organic Reactions & Mechanisms (CHEM 220) · Module 5 of 9
E2 Elimination in Depth
The concerted elimination pathway: geometry requirements, product selectivity, and stereospecificity.
The Concerted E2 Mechanism
- E2 (elimination bimolecular) is a single concerted step: a strong base removes a proton (a beta-hydrogen) while the leaving group departs simultaneously, forming a new pi bond, with rate = k[substrate][base].
What is the rate law for E2, and what does it imply about the mechanism?
Solution. rate = k[substrate][base], a second-order rate law implying both the substrate and base are involved in a single concerted rate-determining step.
Anti-Periplanar Geometry
- E2 requires the leaving group and the beta-hydrogen being removed to be anti-periplanar (a 180° dihedral angle) so their orbitals can align to form the new pi bond as the mechanism proceeds.
Why must the hydrogen and leaving group be anti-periplanar for E2 to occur efficiently?
Solution. This geometry allows the C–H sigma bond's electrons and the departing C–leaving-group's antibonding orbital to align properly, letting the new pi bond form smoothly as both groups leave simultaneously.
Zaitsev vs. Hofmann Products
- E2 typically favors the Zaitsev product — the more substituted, more stable alkene — when using a small, unhindered strong base.
- A bulky base, like potassium tert-butoxide, favors the Hofmann product (the less substituted alkene) because it preferentially removes a more sterically accessible hydrogen rather than the one leading to the most stable alkene.
Why does using a bulky base like potassium tert-butoxide favor the Hofmann (less substituted) product instead of the Zaitsev product?
Solution. The bulky base has difficulty reaching a more hindered, more substituted beta-hydrogen, so it preferentially removes a more accessible, less hindered hydrogen, leading to the less substituted alkene.
Stereospecificity of E2
- E2 is stereospecific: a specific stereoisomer of the starting material, due to the anti-periplanar requirement, produces a specific alkene geometry (E or Z), rather than a mixture.
What does it mean that E2 is "stereospecific"?
Solution. It means a particular stereoisomer of starting material yields a particular, predictable alkene stereochemistry (E or Z), because of the strict anti-periplanar geometric requirement of the mechanism — different starting stereoisomers give different alkene products.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the rate law for E2?
2. What geometric relationship must the hydrogen and leaving group have for E2?