E2 reaction

An E2 reaction is a concerted elimination in which a base removes a β-hydrogen as a leaving group departs, forming a carbon–carbon double bond in one step. The reacting bonds usually align anti-periplanar.

Anti-periplanar conformation: A molecular arrangement in which two bonds lie in the same plane and point in opposite directions. This alignment lets the C–H bond electrons form a π bond as the leaving group departs.
β-Elimination: A reaction that removes a group from one atom and a hydrogen from an adjacent atom, forming a multiple bond. E2 is a concerted member of this broader reaction family.
Zaitsev's rule: A guideline predicting that elimination often favors the more substituted alkene product. It helps predict the major alkene when multiple β-hydrogens are available.
Cyclohexane: A six-membered saturated carbon ring that commonly adopts a chair conformation. In cyclohexanes, E2 typically requires trans-diaxial β-hydrogen and leaving-group bonds.
E2 rate law: The rate equation for an E2 reaction, proportional to both substrate and base concentrations. Both reactants participate in the single rate-determining event.
β-Hydrogen: A hydrogen attached to a carbon adjacent to the carbon bearing a leaving group. Its removal creates the second bond of the alkene.
Hofmann elimination: An elimination that commonly yields the less substituted alkene, often from a quaternary ammonium substrate. Its substrate and leaving-group features can reverse the usual alkene regioselectivity.
Trans-diaxial elimination: An elimination in a cyclohexane chair where the leaving group and β-hydrogen are axial and opposite in direction. It is the ring-specific geometry that satisfies the E2 anti-periplanar requirement.
Transition state: A high-energy molecular configuration at the peak of a reaction's activation barrier. E2 bond breaking and bond formation occur together in its transition state.
Alkyl halide: An organic compound in which a halogen is bonded to an sp³-hybridized carbon. Many E2 reactions use an alkyl halide as the substrate and halide as leaving group.

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