Hinsberg test template

Ethylamine attacks sulfur and displaces chloride; the N-ethylsulfonamide still has an N–H made acidic by the sulfonyl group, so it dissolves in aqueous KOH as its salt.

Mechanism step by step

Benzenesulfonyl chloride tells primary, secondary and tertiary amines apart

1. Attack at sulfur – The amine N attacks S; chloride leaves
2. Deprotonation – Hydroxide removes a proton: the sulfonamide
3. Dissolves in alkali – The remaining N–H is acidic (pKa ≈ 10): a soluble salt forms

Key points

• The strongly electron-withdrawing SO₂ group stabilises the negative charge on N, so the N–H is acidic.
• 1° amine: sulfonamide dissolves in KOH; adding acid precipitates it again.
• 2° amine: R₂N–SO₂Ph has no N–H, so it stays insoluble in KOH.
• 3° amine: no N–H, no stable sulfonamide; the amine is unchanged (dissolves in acid).
• p-Toluenesulfonyl chloride is often used in place of benzenesulfonyl chloride.

Drawn with 106 atoms, 10 arrows, 9 shapes – every one editable in MolDraw.

Sources

• Furniss et al., Vogel's Textbook of Practical Organic Chemistry, 5th ed., Longman 1989 Textbook