What are elimination reactions?

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What are elimination reactions?

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Answer 1 · 2015-11-17T00:51:51

In general, an elimination reaction (specifically, it's called $β$ $beta$ -elimination) involves the elimination of a proton from the $β$ $beta$ carbon, forming a $π$ $pi$ 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 $E 1$ $"E"1$ and $E 2$ $"E"2$ , 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.

$E 1$ $"E"1$ :

$E 2$ $"E"2$ :

The key features of these two are:

A proton from the $β$ $beta$ carbon leaves, forming a $π$ $pi$ 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:

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

WHEN YOU HAVE A POOR LEAVING GROUP

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

$E1cB$ $"E1cB"$ :

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What are elimination reactions?

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