The E2 reaction is a highly predictable and useful method for creating carbon-carbon double bonds in organic synthesis. Unlike the two-step E1 mechanism, the E2 pathway is a concerted process, meaning that all bond-breaking and bond-forming events happen at the exact same time. A strong base attacks a proton on a carbon atom adjacent to the one holding the leaving group.
As the base pulls the proton away, the carbon-hydrogen bond electrons fold inward to form a new pi bond, which simultaneously ejects the leaving group from the molecule. This synchronized molecular dance requires both the substrate and the base to be present in the transition state.
Because the transition state involves both reactants, the kinetics of the E2 reaction are second-order. This means that if you double the concentration of either the strong base or the substrate, the overall rate of the reaction will double. For this mechanism to occur efficiently, the departing proton and the leaving group must be arranged in a specific geometric orientation known as anti-periplanar.
This means they must point in opposite directions on the same plane, which allows the electron orbitals to overlap properly and form the new double bond as the old bonds break apart.
The choice of base and the structure of the substrate both heavily influence the outcome of an E2 reaction. Bulky, sterically hindered bases tend to remove the most accessible protons on the outside of the molecule, often leading to the less substituted double bond as the major product.
Conversely, smaller bases usually yield the more stable, highly substituted alkene, in accordance with Zaitsev’s rule. While tertiary substrates undergo E2 reactions rapidly because they form highly stable alkene products, secondary and primary substrates can also undergo E2 eliminations if a sufficiently strong base is used.