E1 Reaction Mechanism: Step-by-Step

1The key idea

E1 has two main stages: leaving-group departure generates a carbocation, then a weak base removes a beta hydrogen and the electrons form a C=C bond. The ionization step is typically rate determining.

2Conditions and intermediates

Tertiary carbocations are often accessible in polar protic media with weak bases. Acid-catalyzed dehydration of tertiary alcohols can also proceed by E1 after protonation converts OH into water, a better leaving group.

3Draw the arrows in the right order

First draw a C–leaving group bond arrow to the leaving group. In a separate step, draw a base lone-pair arrow to the beta hydrogen and a C–H bond arrow toward the carbocation center to form the alkene.

4Products and competition

Often, the more substituted alkene is favored, but rearrangements, beta-hydrogen availability, and competing nucleophilic substitution can alter outcomes. Higher temperatures may increase elimination relative to substitution without guaranteeing a single exclusive product.

5Worked example

Heating 2-methyl-2-butanol with an acid catalyst gives mainly 2-methyl-2-butene within the alkene mixture. Protonate OH, lose water to form a tertiary carbocation, then remove a beta hydrogen to create C=C.

Common mistakes

• Drawing direct OH departure from an alcohol without prior protonation under acidic dehydration conditions.
• Showing all E1 electron-flow arrows in one concerted elementary step.
• Forgetting competing SN1 and possible carbocation rearrangements.
• Claiming temperature alone identifies a unique reaction mechanism.

Quick concept check

What mechanistic feature do E1 and SN1 share?