Recrystallize a second time from a smaller amount of hexane. A hot filtration is not necessary this time. The insoluble urea side product has the following structure. The Hinsberg test is used to distinguish primary, secondary, and tertiary amines. The same procedure can be used to make the benzenesulfonamide derivative except that the derivative is made in larger amounts and purified.
Primary amines form a derivative that is soluble in base, insoluble in acid.
Secondary amines form a derivative that is insoluble in base and acid. Tertiary amines do not react to form the derivative. They themselves are insoluble in base and soluble in acid. The classes of amines can also be identified by IR analysis. SFC, pp 335-336 has a general procedure.