Bondline / Reactions

Alcohol to chloroalkane (SN1)

A tertiary alcohol is protonated, loses water to form a carbocation, and chloride attacks. This is why tertiary alcohols turn Lucas reagent cloudy at once.

Substitution · 3 steps

Conditions

Concentrated HCl at room temperature for tertiary alcohols; secondary and primary alcohols need ZnCl₂ (Lucas reagent) or PCl₅/SOCl₂.

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

OHHClOH2+Cl−++
Step 1A lone pair on the OH oxygen takes a proton from HCl, turning OH into a good leaving group (water).
OH2+Cl−Cl−++
Step 2Slow step: water leaves, giving a planar tertiary carbocation.
Cl−Cl+
Step 3Chloride attacks the carbocation.

About substitution reactions

In a substitution, a nucleophile replaces a leaving group on a saturated carbon. Whether it happens in one step (SN2) or through a carbocation (SN1) depends on the substrate, the nucleophile and the solvent.

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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