What a learner can do afterwards
- Draws the anti-periplanar arrangement the concerted route needs and predicts which alkene it can give
- Explains the Zaitsev preference and states the conditions under which the less substituted alkene wins
- Identifies the substrates and bases that make the carbanion route available
- Uses a ring system to show a case where the geometric requirement blocks the expected product
1 · Read
Substitution swapped one group for another. Elimination removes two groups from neighbouring carbons to build a double bond. Three routes compete for the job. E2 fires in one concerted step, with the base grabbing the proton as the leaving group departs. E1 first loses the leaving group to give a carbocation, then a base plucks a nearby proton. E1cb goes the other way, deprotonating first to a carbanion that then ejects the leaving group.
The concerted route demands geometry. The breaking C-H and C-leaving bonds must sit anti periplanar, 180 degrees apart, so the forming p orbitals align into a pi bond. Only aligned protons can leave. Rings show this. An axial leaving group eliminates cleanly with an axial neighbour, while an equatorial one sits stuck.
With a choice of protons, elimination favours the more substituted alkene. That is the Zaitsev preference, and more alkyl groups around the double bond stabilize it through hyperconjugation. The less substituted Hofmann alkene wins when bulk gets in the way. A giant base cannot reach crowded protons, so it grabs the exposed one.
Read conditions first. Bulky strong bases on hindered halides shout E2. Stable carbocations invite E1. Poor leaving groups beside acidic protons open the E1cb door, and those carbanion paths lean toward the Hofmann product. When a ring holds no proton anti to the leaving group, the expected product cannot form.
Timing splits E2 from E1 from E1cb, geometry gates the concerted route, and bulk negotiates which alkene wins.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.