Question

isnt hinsberg reagent benzene sulphonyl chloride, how is this P-toluenesulphonyl chloride also showing the same rea

Answer 1

Both benzene sulphonyl chloride and p-toluenesulphonyl chloride can be used as Hinsberg's reagents because they both contain the reactive $-\text{SO}_2\text{Cl}$ group and exhibit the same chemical behavior towards primary, secondary, and tertiary amines, allowing for their distinction based on the solubility of the resulting sulphonamides in alkali.

Answer 2

Yes, you are correct that benzene sulphonyl chloride ($\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$) is traditionally and classically known as Hinsberg's reagent.

However, p-toluenesulphonyl chloride ($\text{CH}_3\text{C}_6\text{H}_4\text{SO}_2\text{Cl}$), often abbreviated as $\text{TsCl}$, is also commonly used and referred to as a Hinsberg-type reagent or even simply "Hinsberg's reagent" in many contexts because it performs the exact same function in the Hinsberg test.

Here's why:

1. Functional Group: Both reagents possess the same reactive functional group, the sulphonyl chloride ($\text{-SO}_2\text{Cl}$) group, which is responsible for reacting with amines via nucleophilic acyl substitution.

2. Mechanism: The mechanism of reaction with primary and secondary amines is identical for both.

   • Primary Amine ($\text{RNH}_2$): Reacts to form an N-substituted sulphonamide ($\text{ArSO}_2\text{NHR}$). This product has an acidic hydrogen attached to the nitrogen, making it soluble in strong alkali (like $\text{KOH}$) by forming a salt.

     $\text{ArSO}_2\text{Cl} + \text{RNH}_2 \rightarrow \text{ArSO}_2\text{NHR} + \text{HCl}$

     $\text{ArSO}_2\text{NHR} + \text{KOH} \rightarrow \text{ArSO}_2\text{N}^-\text{K}^+\text{R} + \text{H}_2\text{O}$ (soluble)

   • Secondary Amine ($\text{R}_2\text{NH}$): Reacts to form an N,N-disubstituted sulphonamide ($\text{ArSO}_2\text{NR}_2$). This product lacks an acidic hydrogen on the nitrogen, so it remains insoluble in alkali.

     $\text{ArSO}_2\text{Cl} + \text{R}_2\text{NH} \rightarrow \text{ArSO}_2\text{NR}_2 + \text{HCl}$

     $\text{ArSO}_2\text{NR}_2 + \text{KOH} \rightarrow \text{No reaction}$ (insoluble)

   • Tertiary Amine ($\text{R}_3\text{N}$): Does not react to form a sulphonamide under these conditions. It might react with the $\text{HCl}$ generated to form a soluble salt, but no sulphonamide is formed.

3. Effect of Methyl Group: The methyl group on the benzene ring of p-toluenesulphonyl chloride is an electron-donating group. While it slightly modifies the electronic properties of the aryl ring, it does not fundamentally alter the reactivity of the sulphonyl chloride group in the context of the Hinsberg test. The acidity of the N-H bond in the primary sulphonamide remains sufficiently acidic to react with strong base.

In practice, p-toluenesulphonyl chloride is often preferred in laboratories due to its ease of handling, crystallinity, and sometimes better yields or more crystalline products. Therefore, for all practical purposes in the context of distinguishing amines, p-toluenesulphonyl chloride functions identically to benzene sulphonyl chloride and is widely accepted as a reagent for the Hinsberg test.

Explanation of the solution:

Hinsberg's reagent traditionally refers to benzene sulphonyl chloride ($\text{C}_6\text{H}_5\text{SO}_2\text{Cl}$). However, p-toluenesulphonyl chloride ($\text{CH}_3\text{C}_6\text{H}_4\text{SO}_2\text{Cl}$) functions identically in the Hinsberg test because both reagents possess the same reactive sulphonyl chloride ($\text{-SO}_2\text{Cl}$) functional group. This group reacts with primary and secondary amines to form sulphonamides. Primary amine sulphonamides have an acidic N-H proton, making them soluble in alkali, while secondary amine sulphonamides lack this acidic proton and are insoluble. The presence of a methyl group in p-toluenesulphonyl chloride does not alter this fundamental reactivity or the acidity of the resulting primary sulphonamide, thus allowing it to perform the same diagnostic function.