E1 and E2 Reactions
First of all, an elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism.
- The one-step mechanism is known as the E2 reaction
- The two-step mechanism is known as the E1 reaction.
Note: The numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively.
E2 is the first step of elimination with a single transition state .
Typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides.
- The reaction rate is second order.
- E2 typically uses a strong base, it needs a chemical strong enough to pull off a weakly acidic hydrogen.
- E2 competes with the SN2 reaction mechanism.
In general, an elimination reaction (specifically, it's called
#beta#-elimination) involves the elimination of a proton from the #beta#carbon, forming a #pi#bond, and ejecting a leaving group.
Note that they don't necessarily all happen in one step.
We have a first-order and a second-order process associated with elimination. These are called
#"E"1#and #"E"2#, respectively. First-order simply means the rate-limiting step involves one molecule only. Second-order would mean that the rate-limiting step involves two molecules.
E1 REACTIONS vs. E2 REACTIONS
Some examples can be seen below.
The key features of these two are:
- A proton from the
#beta#carbon leaves, forming a #pi#bond across a carbon-carbon bond such that the most substituted product is made (Zaitsev's Rule).
- Favorable at higher temperatures.
- Favorable for higher steric hindrance on the electrophile and/or nucleophile.
The key differences are:
#"E"1#has no need for an antiperiplanar orientation, but #"E"2#does. #"E"1#has a carbocation intermediate, which allows for 1,2-hydride shifts or 1,2-alkyl shifts, but #"E"2#does not have or allow either.
WHEN YOU HAVE A POOR LEAVING GROUP
#"E"2#, we have one special reaction called #"E1cB"#, meaning "first-order elimination, forming a conjugate base intermediate". This only happens if the leaving group is especially poor. #"E1cB"#:
- A proton from the