Lucas test template

ZnCl₂ binds the OH oxygen, the C–O bond breaks to a tertiary carbocation and chloride captures it; the insoluble tert-butyl chloride makes the solution turbid immediately.

Mechanism step by step

ZnCl₂ in conc. HCl converts a tertiary alcohol to its chloride at once (SN1)

1. Lewis acid binds O – The OH oxygen gives a lone pair to ZnCl₂
2. Ionisation (slow) – C–O breaks: a tertiary carbocation
3. Chloride adds (fast) – Cl⁻ captures the carbocation

Key points

• Rate follows carbocation stability: 3° turbid at once, 2° in about 5 minutes, 1° no turbidity at room temperature.
• The alcohol dissolves in the reagent; the alkyl chloride does not, hence the cloudiness.
• ZnCl₂ is a Lewis acid: it turns OH into a better leaving group.
• Works for alcohols soluble in the reagent (up to about six carbons).

Drawn with 54 atoms, 7 arrows, 7 shapes – every one editable in MolDraw.

Sources

• Furniss et al., Vogel's Textbook of Practical Organic Chemistry, 5th ed., Longman 1989 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook