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==The E1 Reaction Mechanism== | ==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 | 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. | ||
[[Image:General E1 Mechanism.png|thumb|center|500px| '''Scheme 2:''' The E1 Reaction]] | [[Image:General E1 Mechanism.png|thumb|center|500px| '''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. | |||
[[Image:Good E1 Substrates.png|thumb|center|500px| '''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: | |||
Revision as of 05:18, 28 May 2012
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.
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.
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.
(...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.
