8.3 E1/E2 Summary
The comparison between E1 and E2, in terms of the rate law, mechanism, reaction condition, etc., can be summarized in the following table.
|
E1 |
E2 |
|
| Rate law |
Rate = k×[substrate] |
Rate = k×[substrate]×[base] |
| Mechanism |
multiple steps with carbocation intermediate |
one step, concerted |
| Product |
More substituted, more stable alkenes
|
small base: more substituted alkenes (Zaitsev’s rule)
bulky base: less substituted alkenes (Hoffmann rule) |
| Substrate | tertiary 3° > secondary 2° > primary 1° (no E1) |
tertiary 3° > secondary 2° > primary 1° |
| Base | weak base, (H2O, ROH) | strong base (OH–, RO–, etc.) |
The competition between E1 and E2, or whether a substrate goes through E1 or E2, mainly depends on the nature of the substrate, that is:
- Primary 1º substrates go with E2 only, because primary carbocations are too unstable to be formed.
- Secondary 2º and tertiary 3º substrates can go with either an E1 or E2 reaction, and appropriate reaction conditions are necessary to facilitate a specific mechanism. The E2 reaction is favored by a high concentration of a strong base (OH–, RO–, or NH2–) and a polar aprotic solvent. The E1 reaction is favored by a weak base, and a polar protic compound, H2O, ROH, can be both a base and a solvent (solvolysis).
For study purposes, a comparison between the E1 and E2 mechanisms helps us understand the two processes in depth. In practice, however, the competition between E1 and E2 will not be an issue because they require rather different reaction conditions. More important actually, it is a competition between elimination and substitution. Next, we will have detailed discussions on the comparison and competition between all four types of reactions: SN1, SN2, E1 and E2.
