What is the Difference Between E1 and E2 Reactions?

The main difference between E1 and E2 reactions lies in their mechanisms, rate laws, and reaction conditions. Here are six key differences between the two types of elimination reactions:

1. Number of Steps: E1 reactions take place in two steps and have a carbocation intermediate, while E2 reactions occur in a single step with no intermediate.
2. Rate Law: E1 reactions follow a first-order rate law (rate = k[substrate]), where the rate depends only on the concentration of the substrate. In contrast, E2 reactions follow a second-order rate law (rate = k[substrate][base]), where the rate depends on both the substrate and the deprotonating base.
3. Stereochemistry: E1 reactions have a carbocation intermediate, which can lead to a mixture of configurations in the products. On the other hand, E2 reactions have a single step with no intermediate, resulting in retention of the starting configuration.
4. Base: E1 mechanisms require a weak base, such as H2O or ROH, while E2 mechanisms require a strong base, such as OH-, RO-, or NH2.
5. Solvent: Polar protic solvents, like H2O or ROH, favor E1 mechanisms by stabilizing the carbocation. E2 reactions are favored by polar aprotic solvents.
6. Substrate Structure: Primary alkyl halides can only participate in E2 reactions, as primary carbocations are too unstable to form. Secondary and tertiary alkyl halides can undergo either E1 or E2 reactions.

In summary, E1 and E2 reactions differ in their mechanisms, rate laws, stereochemistry, base requirements, solvent preferences, and substrate structures.

Comparative Table: E1 vs E2 Reactions

The E1 and E2 reactions are both elimination reactions in organic chemistry, but they have distinct differences. Here is a table comparing the key aspects of each reaction:

Please note that the E2 reaction is an anti-coplanar reaction, meaning that the base attacks at an anti-periplanar orientation compared to the leaving group. Additionally, the transition state would be advanced in the E2 reaction if the Lewis base attacks from the retro-syn low energy configuration at the required conformation.