Question

What are elimination reactions?

Answer 1

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"`: