# What are elimination reactions?

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 $\text{E} 1$ and $\text{E} 2$, 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.

$\text{E} 1$:

$\text{E} 2$:

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:

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

WHEN YOU HAVE A POOR LEAVING GROUP

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

$\text{E1cB}$: