Overview of E1 & E2 Elimination Reactions

The most basic elimination reaction:

So what happened?

• Bronsted-Lowry base (OH, in this case) accepts a hydrogen atom.
• Bronsted-Lowry Acid (the CH3 on the left) donates a hydrogen.
• The electrons from the newly-broken bond move over and form a double bond.
• The leaving group (Br) leaves!

There are two different kinds of elimination:

• The C-Br bond is broken first (leaving group leaves first). Then later the hydrogen is accepted and the double bond is formed.
• This is a 2-step reaction, which we call E1.

• The double bond is formed at the same time that the C-Br bond is broken.
• This is a 1-step concerted reaction called E2.

E1 (2-Step Reaction):

A few things to notice:

• The BL base (H2O, in this case) is not involved in the first step at all. The Br falls of on its own!
• A carbocation is formed after the Br falls off (step 2).
• In this example the H2O accepts a hydrogen from the bottom-right CH3, but it can accept a hydrogen from any of the 3 surrounding CH3′s.
• The BL base must have access to a hydrogen it can accept. Steric hindrance can prevent this from happening.

What conditions cause the Br to just fall off?

• A stable carbocation is formed.
• Polar solvent (helps stabilize the carbocation)
• A good leaving group. Br is a good leaving group, as are most halides (I, Cl). These are all stable after they fall off, even with a negative charge.
• Bad leaving groups, unlikely to fall off, include Fluorine (F) and Alcohol (OH).
• Note: these are the exact same conditions as SN1 substitution!

E2 (1-Step Reaction):

What happened:

• The OH grabs the proton, and the extra electrons move over and form a double bond. This “pushes” off the Br, and it all happens in a single step!

What conditions cause the OH to grab the proton, pushing off the Br?

• E2 reactions require a good, strong base, like OH. (Strong base means conjugated acid has a high pKa value)
• Most halides, like Cl, Br, I, are awful bases, and won’t participate in E2.
• Most other anions with a negative charge (OH, F, CN, HS, etc.) can do E2 reactions, but not as fast as OH or OR.
• E2 is not affected much by steric hindrance, carbocation stability, or protic/aprotic solvent.