E1 and E2 Reactions
Key Questions
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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.
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E2 is the first step of elimination with a single transition state .
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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.
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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.
REACTION ORDER
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.
#"E"1# :
#"E"2# :
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
Lastly, under
#"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