E2: Bimolecular Elimination

Here is what is going on in an elimination reaction:
  1. base abstracts proton
  2. pi bond forms
  3. leaving group leaves
 In E2, everything happens in one step (concerted reaction again just as in SN2). Just like SN2, there is no intermediate formed and there is only one transition state (2 transition state for E1 and SN1).
E2 Mechanism

Base & Substrate
Here, a strong base is required instead of a strong nucleophile because a proton is abstracted (not an electrophilic carbon atom being attacked). Note that a strong nucleophile is usually a strong base. If the substrate is a primary, SN2 may take place instead. Therefore, we want the substrate to be tertiary/ more substituted. This also has to do with the transition state and the alkene formed. In the transition state, a more substituted substrate is more stable, hence in a lower energy state. For the alkene formed, a more substituted alkene is the major product (Zaitsev's rule); when there is a choice of proton, the one that will form a more substitued alkene is favorably abstracted.

Stereochemistry
 As the product is an alkene, the reaction must involve two carbons and the bond formation between them. There is a carbon that has the leaving group attached and the other has the proton being abstracted. Like SN2 (always back-side attack), E2 is a stereo-specific reaction too. The proton being abstracted has to be anti-coplanar (staggered conformation) to the leaving group.

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