Acid-catalysed dehydration (E1): practice and mechanism
Concentrated acid and heat remove water (E1 via the carbocation); following Zaitsev, the more substituted alkene forms.
Example
The acid protonates the OH group, water leaves and a carbocation forms. Loss of a proton gives the alkene; following Zaitsev, the more substituted one is favoured.
Mechanism step by step
- 1
The acid protonates the OH group to –OH₂⁺, a good leaving group.
- 2
Water leaves and a carbocation forms (for primary alcohols more E2-like).
- 3
A base (e.g. HSO₄⁻ or H₂O) removes a proton from the neighbouring carbon: the alkene forms, preferably the more substituted one.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- This alkene is less substituted. Acid-catalysed elimination favours the more stable, more substituted alkene (Zaitsev).
Often confused with: Alcohol to haloalkane (HBr)
Frequently asked questions
Why does no ether form?
At lower temperature an ether can indeed form (e.g. diethyl ether from ethanol at ~140 °C). Higher temperature favours elimination (entropy).
Can carbocations rearrange?
Yes. If a hydride or methyl shift gives a more stable carbocation, it happens, and the product skeleton changes.