E1 and E2 Reactions

Key Questions

  • First of all, an elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism.

    • The one-step mechanism is known as the E2 reaction
    • The two-step mechanism is known as the E1 reaction.

    Note: The numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively.

    What is E2 mechanism?

    • E2 is the first step of elimination with a single transition state .

    • Typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides.

    • The reaction rate is second order.
    • E2 typically uses a strong base, it needs a chemical strong enough to pull off a weakly acidic hydrogen.
    • E2 competes with the SN2 reaction mechanism.
  • In general, an elimination reaction (specifically, it's called #beta#-elimination) involves the elimination of a proton from the #beta# carbon, forming a #pi# bond, and ejecting a leaving group.

    Note that they don't necessarily all happen in one step.


    We have a first-order and a second-order process associated with elimination. These are called #"E"1# and #"E"2#, respectively. First-order simply means the rate-limiting step involves one molecule only. Second-order would mean that the rate-limiting step involves two molecules.


    Some examples can be seen below.



    The key features of these two are:

    • A proton from the #beta# carbon leaves, forming a #pi# bond across a carbon-carbon bond such that the most substituted product is made (Zaitsev's Rule).
    • Favorable at higher temperatures.
    • Favorable for higher steric hindrance on the electrophile and/or nucleophile.

    The key differences are:

    • #"E"1# has no need for an antiperiplanar orientation, but #"E"2# does.
    • #"E"1# has a carbocation intermediate, which allows for 1,2-hydride shifts or 1,2-alkyl shifts, but #"E"2# does not have or allow either.


    Lastly, under #"E"2#, we have one special reaction called #"E1cB"#, meaning "first-order elimination, forming a conjugate base intermediate". This only happens if the leaving group is especially poor.