What are elimination reactions?

1 Answer
Truong-Son N.
Nov 17, 2015

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.

REACTION ORDER

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.

E1 REACTIONS vs. E2 REACTIONS

Some examples can be seen below.

#"E"1#:

#"E"2#:

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.

WHEN YOU HAVE A POOR LEAVING GROUP

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.

#"E1cB"#:

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