Todd:Chem3x11 ToddL11: Difference between revisions
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[[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. | 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. You've probably seen a very common example of E1 elimination where tertiary alcohols give alkenes in the presence of acid. | ||
[[Image:Good E1 Substrates.png|thumb|center|500px| '''Scheme 3:''' Groups that are Good at Eliminating with an E1 Mechanism]] | [[Image:Good E1 Substrates.png|thumb|center|500px| '''Scheme 3:''' Groups that are Good at Eliminating with an E1 Mechanism]] | ||
These substrates could also eliminate via an E2 mechanism (which we'll do in a minute) - it's just a question of selectivity. There is also usually a possibility of substitution reactions in many reactions. We just have to keep this in mind. As a rule of thumb we often see more elimination rather than substitution at higher temperatures. | |||
Other groups are either less likely to react E1 or are unable to do so: | Other groups are either less likely to react E1 or are unable to do so: | ||
[[Image:Less Good E1.png|thumb|center|500px| '''Scheme 3:''' Groups that are Less Good at Eliminating with an E1 Mechanism]] | |||
Revision as of 05:32, 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. You've probably seen a very common example of E1 elimination where tertiary alcohols give alkenes in the presence of acid.
These substrates could also eliminate via an E2 mechanism (which we'll do in a minute) - it's just a question of selectivity. There is also usually a possibility of substitution reactions in many reactions. We just have to keep this in mind. As a rule of thumb we often see more elimination rather than substitution at higher temperatures.
Other groups are either less likely to react E1 or are unable to do so:
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