What is the Hinsberg reagent?
Hinsberg reagent is called as Benzenesulphonyl chloride. It is used for distinguishing between primary secondary and tertiary amine. The hinsberg reagent formula is C6H5SO2Cl. It is an organosulphur compound. It is colourless and is soluble in organic solvents. Hinsberg reagent reacts with compounds containing –OH and –NH groups. They are useful for the preparation of sulfonamides and sulfonamide esters.
Preparation Of Hinsberg Reagent
Benzenesulphonyl chloride (or Hinsberg reagent) is prepared by the sulfonylation reaction of benzenesulfonic acid with PCl5 or SOCl2.
With Ammonia and Amines
The reaction of sulphonyl chlorides with primary and secondary amines is the basis of the classical Hinsberg test for distinguishing (and separation also) the primary, second and tertiary amines. The treatment of sulfonyl chlorides with ammonia or amines is the general method of preparing sulfonamides.
Nowadays the reagent generally used is p-toluenesulphonyl chloride (tosyl chloride, Hinsberg reagent) in place of benzenesulfonyl chloride, since the substituted sulphonamides thus formed are stable solids which can be easily recrystallized.
The Hinsberg Test can be summarized as follows-
- Primary amines react with benzenesulfonyl chloride to form a precipitate that is soluble in NaOH solution.
- Secondary amines react with benzenesulfonyl chloride to give a precipitate that is insoluble in NaOH solution.
- Tertiary amines do not react with benzenesulfonyl chloride.
Pathway of Hinsberg Reaction
When primary amines are treated with benzenesulphonyl chloride, N-alkyl sulfonamides is obtained having an acidic hydrogen, and are soluble in alkali (aqueous NaOH).
The reaction of benzenesulfonyl chloride with secondary amines gives N, N-dialkyl sulfonamides, having no acidic hydrogen, and is therefore insoluble in alkali (aqueous NaOH).
However, tertiary amines do not react with benzenesulfonyl chloride and are recovered unchanged.
Since tertiary amines do not react with benzenesulfonyl chloride and are usually recovered unchanged, primary, secondary, and tertiary amines can easily be told apart.
However, the hinsberg test is limited for two reasons.
(1) Many N-alkyl sulfonamides in which the alkyl group has six or more carbons are insoluble in alkali, despite their acidic hydrogen, so that a primary amine may appear to be a secondary amine.
(2) If the reaction conditions are not controlled carefully, tertiary amines may not be recovered unchanged.
Separation of primary, secondary and tertiary amine by Hinsberg method is shown below.
The reaction of sulfonyl chlorides with alcohols produce sulfonic esters. When p-toluenesulfonyl chloride is the reagent, the p-toluenesulfonic ester is called a tosylate.