Bondline / Reactions

Dehydration of an alcohol (E1)

Acid turns OH into water, a good leaving group. Water leaves to give a carbocation, which loses a proton from the next carbon to form the alkene.

Elimination · 3 steps

Conditions

Concentrated H₂SO₄ or H₃PO₄ and heat (about 170 °C for ethanol), or the vapour over hot Al₂O₃.

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

OHHOH2+OH2+H2O++
Step 1The OH group is protonated by the acid catalyst.
OH2+H2OHH2O++
Step 2Slow step: water leaves, giving a tertiary carbocation.
HH2OH3O+++
Step 3Water removes a proton from a carbon next to C⁺, and the C–H electrons form the C=C bond.

About elimination reactions

In an elimination, a proton and a leaving group are removed from neighbouring carbons and a C=C forms. Strong bases and heat favour elimination over substitution.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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