The E1 Reaction Mechanism

The E1 reaction mechanism is a 2-step reaction that also forms an alkene. The “1” in E1 refers to the rate law, which only depends on the concentration of the substrate.

In the first step of an E1 reaction mechanism, the leaving group leaves, forming a carbocation. In the second step, the base deprotonates a neighboring hydrogen to form a double bond between the carbocation and carbon that was just deprotonated.

A figure showing the 2-step E1 reaction mechanism and the E1 rate law

Notice that the base here is water, which has no negative charge and is a very weak base. This is consistent with most E1 reactions as E1 reaction mechanisms generally use a weak base. E1 reactions also tend to occur when the leaving group is on a tertiary carbon. An E1 reaction will never occur if the leaving group is on a primary carbon and will only rarely occur if the leaving group is on a secondary carbon.

 

Keep it Simple

In the E1 reaction mechanism, a carbocation intermediate is formed just like in an SN1 reaction. But in the SN1 mechanism section, we really focused on the carbocation rearrangement by hydride and methyl shifts. Why don’t we make such a big deal about that in the E1 section?

It’s because E1 reactions almost exclusively occur on tertiary carbons, so the molecule can’t ever rearrange to a more stable carbocation. SN1 reactions also prefer to occur on tertiary carbons, but they can also take place on secondary carbons, which opens up the door for rearrangements.

We also do not focus on the leaving group being anti-periplanar to a hydrogen. Because of the carbocation intermediate formed, this is not something to worry about; however, the hydrogen still must neighbor the leaving group.

 

Summary Points for E1 Reaction Mechanisms

  • 2-step mechanism that goes through a carbocation intermediate at the end of step 1.
  • Occurs almost exclusively when the leaving group is on a tertiary carbon and when the base involved is very weak.