Hinsberg Reagent and Test

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What is Hinsberg Reagent?

Hinsberg reagent can be described as an alternate name for benzene sulfonyl chloride. This name is given for the usage of its Hinsberg test to detect and distinguish primary, secondary, tertiary amines of a given sample. This reagent is an organosulfur compound, and the Hinsberg reagent formula (chemical formula) C6H5SO2Cl. The Hinsberg reagent's is a colourless oil, which is viscous in nature, and it is soluble in organic solvents.

This reagent undergoes reaction with compounds containing N-H, O-H bonds, which are reactive in nature. We can also use it in the preparation of sulfonamide esters (via reaction with alcohol) and sulfonamides (via reaction with amines).

Preparation of Hinsberg Reagent

The benzene sulfonic acid chlorination or the benzene sulfonic acid salts with phosphorus oxychloride (POCl3) produces the required reagent.

Another way to prepare the necessary Hinsberg Reagent can be done by reacting benzene with the chloro sulfuric acid (which has a chemical formula HSO3Cl). Let us look at the preparation methods of the required reagent including the structure of Hinsberg reagent below.

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Hinsberg Test

Hinsberg test can be described as a chemical reaction that can distinguish between primary, secondary, and tertiary amine types. This reaction was first described in 1890 by the German chemist named "Oscar Heinrich Daniel Hinsberg".

In this type of test, the amines are allowed to act as nucleophiles and attack electrophile (also called sulfonyl chloride). This leads to chloride displacement and the generation of sulfonamides. When both primary amines and secondary amines produce sulfonamides, this sulfonamide product cannot be soluble and precipitates from the solid solution.

Hinsberg Reaction Pathways

The benzene sulfonyl chloride reaction with primary amines results in a sulfonamide product, which is soluble in alkali. This reaction is illustrated below.

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The benzene sulfonyl chloride reaction with the secondary amines results in a sulfonamide product, which can NOT be soluble in alkali. One of the examples for this reaction type can be given below.

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No such reaction takes place between the benzene sulfonyl chloride reagent and the tertiary amine. Tertiary amines help in sulfonyl chloride hydrolysis. This reaction produces salts, which can be soluble in water.

Therefore, the Hinsberg reagent is used differently to react with primary, secondary, tertiary amines. These differences can be observed in the sulfonamide product solubility in alkali.

Determination of Amines Using Hinsbergs Test

The reagent of Hinsberg is the benzene sulphonyl chloride, and it can be used for distinguishing between 1°, 2°, 3° amines.

Whereas, the primary amines contain 2 active Hydrogen ions, so easy they react with benzene sulphonyl chloride and can produce salt, which can be dissolved in NaOH because of remaining another active Hydrogen. The secondary amines contain only 1 hydrogen ion, so it reacts with Benzene sulphonyl chloride but does not dissolve in NaOH. Tertiary amines do not hold active Hydrogen, and thus they don't react with Benzene sulphonyl chloride.

Order of Basctity of Amines

Let us assume that the terms a, b, and c indicate primary, secondary, and tertiary amines, respectively.

The Groups attached to these amines can be denoted by 'R.'

In the aqueous phase, if R is taken as a methyl group (the total Rs, except hydrogen), then the order of basicity can be given as b > a > c.

Whereas, if R is taken as any group other than a methyl group, then the order of basicity can be given as b > c > a

At the same time, coming to the gas phase, the order of basicity can be given as c > b > a.

Note: While, if it is not mentioned in the questions, we can assume it to be in the aqueous phase as a default case.

Differentiate Methylamine and Trimethylamine

(i) The terms methylamine and dimethylamine can be differentiated using the carbylamine test.

Carbylamine Test: The primary amines of aliphatic and aromatic, on heating with ethanolic potassium hydroxide and chloroform, forms either foul-smelling isocyanides or carbylamines. Being a primary amine of aliphatic, Methylamine produces a positive carbylamine test, whereas dimethylamine does not.

(ii) While the secondary and tertiary amines can be differentiated by allowing them to react with the reagent of Hinsberg (benzene sulphonyl chloride).

The secondary amines react with the reagent of Hinsberg to form a product, which is insoluble in an alkali. For suppose, N, N - diethylamine reacts with Hinsberg's reagent to produce N and N – diethyl benzene sulphonamide, which can be insoluble in an alkali. But, the tertiary amines will not react with the Hinsberg's reagent.

Preparation of Para-Nitroaniline from Benzene

To achieve this, we can start with benzene, where the first step is to nitrate it with a mixture of sulfuric acid and nitric acid to produce nitrobenzene. If we could nitrate this again, the nitro group would undergo meta-position because the nitro group is a meta director. Hence, we have to reduce it.

There are many ways to perform this, but the tin or HCl method used is fine and will produce aniline. Where aniline is nitrated directly, but it is much reactive, and by so, we will lose selectivity and probably get some over nitration. So, it is suggested to first convert it to acetanilide and then nitrate. This will mainly give us the p-nitro acetanilide product (which is a dash of the ortho product, and it can be removed by recrystallization). Now, finally, hydrolyze off the acetyl group, and we will get p-nitroaniline.

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Did You Know?

  • Ethers are considered to be less reactive because of the absence of charge separation or polarity. One of the reasons for the common use of ether as a solvent is the lack of reactivity of the ether functional group.

  • Ethers are less reactive to that of alcohol but are substantially reactive to that of alkanes.

  • They are inert to a few elements such as bases, active methylene group, alkali metals.

FAQ (Frequently Asked Questions)

1. Explain the Solubility of Benzoic Acid in NaOH.

Answer: It is Zero. We cannot easily dissolve a solid in a solid.

The solubility of benzoic acid in an aqueous solution (NaOH solution) is just represented as the solubility of sodium benzoate in water, which is a bit over 60 g of sodium benzoate per 100 ml.

To translate the same to benzoic acid, which contains a less molar mass, it would be nearly 51 g per 100 ml of the NaOH solution, assuming we had enough NaOH in the aqueous solution to react with all the benzoic acid.

2. What does the "Carbylamine Test" for Amines mean?

Answer: It is often called "Hofmann's Carbylamine Test," where the test can be useful to detect and identify between the amines of primary or secondary tertiary.

Primary amines form the isocyanates, which are foul-smelling, and can be detected easily. At the same time, the secondary and tertiary amines do not give the test at all.

3. Explain the Hinsberg Test Mechanism.

Answer: First, the amine reacts with benzene sulfonyl chloride in an additional elimination reaction on the highly electrophilic sulfonyl chloride derivative. The resulting product can be said as a sulfonamide salt of sodium after the stepwise chlorine loss and a proton from the amine in the sodium hydroxide presence.