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.

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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. You've probably seen a very common example of E1 elimination where tertiary alcohols give alkenes in the presence of acid.

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:

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

There is a regiochemical issue we ought to address. If there is a choice of double bonds being formed in the final step, we tend to see the more substituted double bond being formed. This arises from the more substituted transition state, containing the more substituted slightly charged carbon atoms of the double bond being formed. Here we see that the observed product is favoured kinetically and thermodynamically.

Scheme 4: E1 Gives More Substituted Double Bonds Where there is a Choice

There is also a stereochemical issue we ought to address. E1 will tend to give E rather than Z products. Here we appeal to the difference in energies of the intermediate carbocations that must form (a steric argument) and extrapolate to the stabilities of the transition states going to the relevant products.

Scheme 5: E1 Gives E-isomers of Double Bonds Where there is a Choice

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