Details of Elimination Mechanisms
1. The similarity in rate determining steps between E1 and SN1 helps to explain why the same factors govern whether a mechanism is E1 or E2 (E1CB disregarded for the time being) – those being; possibility of a stabilised cation and a polar solvent which can solvate said cation.
Therefore, the E1 order of reactivity for alkyl halides runs tertiary > secondary > primary.
2. Where there is a choice of protons to lose in the second step of the mechanism, the major product will be that with the most substituted alkene (thermodynamically more stable). For example;
This example illustrates a couple of points – firstly note that the carbocation intermediate has a choice of three protons to eliminate, secondly note that elimination from the methyl group directly adjacent will give the least substituted of the possible products – this is very unlikely to happen. Elimination occurs as shown for two reasons – it gives an alkene substituted at all positions (elimination from the iPr would also achieve this), but crucially it gives an alkene that is conjugated with the phenyl ring – the most favourable of possible outcomes.
An added complication is that hydride/alkyl shifts may occur in the carbocation – although this will only happen if a more highly substituted alkene is made possible by doing so. An example of this is given below;
The two possible routes are drawn out – route a is the standard elimination route without any migration, and results in a di-substituted alkene; route b involves a methyl migration (this doesn’t only work for methyl groups – it could be any alkyl/aryl group, or hydride), then the elimination to give a much better alkene (more substituted and conjugated).
1. For this mechanism to occur rather than E2, several conditions must be in place; the proton to be removed must be suitably acidic, the carbanion created must be stabilised and the leaving group must be a poor one.
2. As previously mentioned, actual examples of this reaction type are very rare (the example given in the previous page is hypothetical!). Two known examples are; elimination of HCN from cyanohydrins (shown below) and HF from Cl2CHCF3.
1. As was mentioned in the nucleophilic substitution section, species which are basic are often also nucleophilic. With this in mind, a second look at the E2 example on the previous page will reveal that a nucleophilic substitution reaction could easily happen instead of an elimination – in fact this applies to all eliminations. This will be dealt with in more detail on a subsequent page (Mechanistic Comparison: SN vs. E).
2. In E2, the RDS involves removal of the proton at the same time as the leaving group departs – therefore both base strength and leaving group ability are important for the rate of E2. For the base it is fairly simple – the stronger it is, the faster the reaction (for a given LG). Solvent plays a part again – like SN2, an aprotic solvent gives best results – there will be a very strongly hydrogen bound cage of solvent around the base if it is protic. The role of the leaving group is slightly more complex than in SN2 however – because of its involvement in the all-encompassing TS, it can have an effect on the reaction by altering the strength of the C-H bond – so not just the C-LG bond is important. This is, however, very difficult to quantify, so mainly similar qualities to an nucleophilic substitution leaving group will be observed.