An amine reacts with benzenesulfonyl chloride in aqueous $\mathrm{KOH}$ to give a solid that does not dissolve in excess alkali. The amine is:

A secondary amine gives an N,N-disubstituted sulfonamide with no hydrogen on nitrogen, which is therefore not acidic and is insoluble in alkali. Primary amines ($\mathrm{CH}{}_3\mathrm{NH}{}_2$, $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{NH}{}_2$) give sulfonamides with an acidic N-H, which dissolve in alkali. Tertiary amines do not react.