Bondline / Reactions

Acid-catalysed dehydration (E1)

An alcohol is protonated so water can leave, the carbocation loses a neighbouring proton and an alkene forms. Follows Zaitsev: the more substituted alkene usually wins.

Elimination · 3 steps

Conditions

Concentrated H₂SO₄ or H₃PO₄, heat.

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

OHHOH2+OH2++
Step 1The OH group is protonated, turning it into a good leaving group (water).
OH2+HH2O+
Step 2Water leaves, giving a tertiary carbocation (slow step).
HH2OH3O+H₂O++
Step 3Water removes a proton from a carbon next to C⁺; those electrons form the C=C.

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.

Open in Bondline