E1 E2 Comparison

Let's compare E1 and E2 in the following aspects:
  1. Base
  2. Substrate
  3. Solvent
  4. Leaving Group
  5. Kinetics
  6. Stereochemistry
  7. Rearrangements 
Base
E1: As the base abstracts the proton only after the carbocation intermediate is formed, it does not affect the rate of reaction, hence not important.
E2: A strong base is indeed needed to promote the one-step reaction.

Substrate
E1: A more substituted substrate stabilizes the carbocation intermediate. 
E2: A more substituted substrate forms a more substituted alkene. 

 Solvent
E1: A polarizing solvent enhances the rate of ionization as it pulls the cation and anion apart. 
E2: The transition state is less sensitive to the solvent as the transition state has its negative charge shared over the whole molecule. 

Leaving Group
Both reactions need a good leaving group. 

 Kinetics
E1: The rate is only determined by the substrate as it is the only molecule that ionizes.
E2: Both the substrate and the base affects the rate of the concerted reaction.

 Stereochemistry
E1: It actually does not have a specific geometry, but it follows the Zaitsev's rule. 
E2: As it is a concerted reaction, an anti-coplanar orientation between the base and the leaving group is required as it is the lowest energy state.

 Rearrangements
E1: A methyl group or a hydride ion can shift to stabilize the carbocation intermediate before a base attacks.
E2: No rearrangements as there is no intermediate to be stabilized. 

 *Both E1 and E2 follow the Zaitsev's rule: 
 In elimination reactions, the most substituted alkene usually predominates.
E1 E2 Comparison Chart


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