Basics of E1 elimination reaction

The E1 elimination is a type of elimination reaction in organic chemistry. It takes place in two separate steps, but the rate of the reaction is only limited by one of the reactants. Figure 1 here provides an example of an E1 elimination.

1)

The leaving group - bromide - takes the electrons from the detaches from the main molecule, which becomes a carbocation. Carbocations are not stable though, so some further reaction must take place!

2)

A base abstracts a proton from the main molecule, which stabilizes the main molecule via the formation of a double bond.

Figure 1. Example of the first and second step in an E1 elimination reaction, using 2-bromo-2-methylbutane as the main reactant.

Important factors that influence E1 reactivity

These factors generally increase reactivity towards an E1 elimination:

  • A good leaving group in the main molecule

  • Degree of substitution: The more substituted the (potential) carbocation is, the more stable and therefore likely to form it is. E1 actually almost only occurs at tertiary carbon atoms!

  • Base: A strong base that can pull off that hydrogen. But E1 can actually take place even with a weak base.

  • Temperature: The rate of elimination reactions generally increase with temparature. When in competition with substitution reactions, adding heat to the reaction will therefore favor the elimination path.

  • The solvent in which the reaction takes place can have a huge effect. How this affect elimination reactions are beyond this simulation, but you can check out this page for an overview of how solvents are normally categorized.

Stereochemistry of the reaction product

Because the reaction goes through a planar carbocation intermediate, there is no specific selectivity towards cis or trans for the product, and usually the result will be a mix of the two.