Todd:Chem3x11 ToddL11

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Chem3x11 Lecture 11

Being constructed Mon May 28 - will be ready shortly

This lecture is about elimination reactions.

(Back to the main teaching page)

Key concepts

• The two most common elimination mechanisms, E1 and E2, can often both occur for a given substrate
• The orbital requirements for the reactions tend to give selective, or in some cases specific, stereochemical outcomes

Eliminations

An elimination reaction is where a small molecule leaves a carbon-containing molecule via the generation of a double bond.

Scheme 1: General Definition of an Elimination

We will consider three mechanisms. They differ in the sequence of loss of the two components.

The E1 Reaction Mechanism

The X- leaves first, giving a carbocation, which then loses a proton to give the double bond. Rate limiting step is initial loss of anion. Hence rate only depends on concentration of starting material.

Scheme 2: The E1 Reaction

This mechanism is promoted when the intermediate carbocation is stable, e.g. when the carbocation is highly substituted, or in an allylic/benzylic position, or stabilised by an adjacent heteroatom.

Scheme 3: Groups that are Good at Eliminating with an E1 Mechanism

(...though these substrates could also eliminate via an E2 mechanism (which we'll do in a minute) - it's just a question of selectivity)

Other groups are either less likely to react E1 or are unable to do so:

The Licence for This Page

Is CC-BY-3.0 meaning you can use whatever you want, provided you cite me.