Question

MCAT Tutorial: E1/E2 Reactions

Answer 1

\({\bf{E1~Reaction:}}\) First Order Elimination
> only depends on the concentration of substrate, not the solvent/base
> unimolecular reaction
> requires the formation of a cation intermediate
> reaction favors tertiary > secondary > primary intermediates in that order
> susceptible to re-arrangements (see hydride shifts)
> favors polar protic solvents

> slow step: the formation of the cation, leaving group leaves
> fast step: loss of proton on the atom adjacent to the cation

Answer 2

note: this reaction will compete with SN1 but can be pushed towards E1 using heat or a poorly nucleophilic base

Answer 3

\({\bf{E2~Reaction:}}\) Second Order Elimination
> depends on both the concentration of the substrate and the concentration of the solvent
> bimolecular reaction
> does not require cation intermediate (exit of leaving group and attack of new group happen in same step)
> leaving group and the exiting hydrogen must be anti (see conformational analysis)
> transition state must be antiperiplanar
> favors strong base
> leaving group does not have to be as strong as E1 reactions (no intermediate necessary)



special note: t-butyl bromide tends to favor E2

Answer 4

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT