12.4. Determining if a Reaction Follows an E2 or E1 Mechanism

Because of the similarities between them, usually factors that favour S_N2 also favour E2 and factors that favour S_N1 also favour E1 (see Section 11.4). However, there are far more exceptions and deviations from these trends in elimination reactions than in nucleophilic substitution reactions.

The two mechanisms, E1 and E2, are distinct. However, for many compounds the two reaction pathways have similar activation energies. In these cases some of the molecules follow the E1 mechanism and some follow the E2 mechanism. It is usually not possible to accurately predict which pathway will be preferred, or by how much, for a given compound. It is important to recognize that in most cases the major product will be the same regardless of which elimination mechanism occurs. If the mechanism could affect the structure of the major product this text will clearly indicate which mechanism is followed to allow the student to accurately predict the product.

There are some compounds for which the elimination mechanism can be accurately predicted. If following an E1 mechanism would generate a primary carbocation without resonance stabilization, then the mechanism will instead always be E2 (Figure 12.5).

Figure 12.5 – Using the Structure of the Potential Carbocation Intermediate to Predict the Mechanism of Elimination.