The E1 reaction is a type of elimination reaction that follows a two-step, unimolecular mechanism. It is called “E1” because the rate-determining step depends only on one species (the substrate).
Step-by-Step Mechanism
Step 1: Formation of Carbocation (Rate-Determining Step)
- The leaving group departs first, leaving behind a carbocation intermediate.
- This step is slow and controls the overall rate of the reaction.
- Rate law: Rate = k [substrate]
Step 2: Deprotonation
- A base (often the solvent itself) removes a proton from a carbon adjacent to the carbocation (the β-carbon).
- The electrons from this C–H bond form a C=C double bond, giving an alkene.
Key Features
- Carbocation stability matters: Tertiary > secondary >> primary (primary rarely undergoes E1).
- Polar protic solvents (like water, ethanol) stabilize carbocations and favor E1.
- Weak bases are sufficient because the carbocation formation is the slow step.
- Regioselectivity: Often follows Zaitsev’s rule → the more substituted (stable) alkene is the major product.
- Possible rearrangements: Since carbocations are formed, hydride or alkyl shifts may occur, leading to unexpected products.
Comparison with Other Pathways
- Competes directly with SN1 substitution, since both involve carbocation intermediates.
- Differentiation depends on conditions: high temperature favors elimination (E1), while lower temperature favors substitution (SN1).
In summary:
The E1 mechanism involves two steps:
- The leaving group departs → carbocation forms.
- A base removes a proton → double bond (alkene) forms.
It’s favored with tertiary substrates, polar protic solvents, weak bases, and heat.