E1

1. The similarity in rate determining steps between E1 and S_N1 helps to explain why the same factors govern whether a mechanism is E1 or E2 (E1_CB 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).

E1_CB

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.

E2

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: S_N 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 S_N2, 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 S_N2 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.