Distinguishing Primary, Secondary and Tertiary Amines: the Hinsberg Test

The Hinsberg test distinguishes primary, secondary and tertiary amines by

exploiting a difference in the ACIDITY of the sulphonamide products formed when each is

treated with benzenesulphonyl chloride ($\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$) in

the presence of excess aqueous $\text{KOH}$.

Primary amine. $\text{R}-\text{NH}_2$ reacts to give $N$-alkylbenzenesulphonamide,

$\text{C}_6\text{H}_5\text{SO}_2-\text{NHR}$, which still has ONE hydrogen on the

sulphonamide nitrogen. That hydrogen is unusually acidic -- much more acidic than an

ordinary amine $\text{N}-\text{H}$ -- because the resulting anion,

$\text{C}_6\text{H}_5\text{SO}_2-\text{NR}^-$, is strongly stabilised by the powerful

electron-withdrawing sulphonyl group right next to it. This acidity is exactly what

alcohols/amides are NOT acidic enough to show, and it is enough for the sulphonamide to

DISSOLVE in the excess aqueous $\text{KOH}$ present, forming a water-soluble potassium

salt.

Secondary amine. $\text{R}_2\text{NH}$ reacts to give $N,N$-dialkylbenzenesulphonamide,

$\text{C}_6\text{H}_5\text{SO}_2-\text{NR}_2$ -- but here BOTH of nitrogen's original

hydrogens have already been replaced (one by each alkyl group of the starting secondary

amine), so there is NO acidic $\text{N}-\text{H}$ left at all. This product is therefore

INSOLUBLE in excess $\text{KOH}$ and separates as a solid or oily precipitate.

Tertiary amine. $\text{R}_3\text{N}$ has no hydrogen on nitrogen to begin with, so it

does not react with benzenesulphonyl chloride at all under these conditions. The

unreacted tertiary amine is initially insoluble in the neutral/basic reaction mixture

(it forms a separate liquid layer or is only sparingly soluble), but -- unlike the

secondary amine's sulphonamide -- it dissolves readily when the mixture is subsequently

ACIDIFIED, because protonation of its still-basic nitrogen converts it to a …