Question

Give one chemical test to distinguish between the following pairs of compounds. (i) Methylamine and dimethylamine (ii) Secondary and tertiary amines (iii) Ethylamine and aniline (iv) Aniline and benzylamine (v) Aniline and N-methylaniline.

Short answer

Apply Hinsberg test, nitrous acid, bromine water, acetylation, and nitrous acid to distinguish the amine pairs.

Step-by-step solution

Identifying Amines Types

Recognize that methylamine is a primary amine (one hydrogen replaced), while dimethylamine is a secondary amine (two hydrogens replaced).

Assigning a Test for Primary vs Secondary Amines

Use the Hinsberg test, which involves reacting the amine with benzenesulfonyl chloride. Primary amines like methylamine will form a solid sulfonamide soluble in alkali, whereas secondary amines like dimethylamine will form a solid sulfonamide insoluble in alkali.

Recognizing Secondary and Tertiary Amines

Secondary amines have two carbon-containing groups attached to the nitrogen, while tertiary amines have three such groups.

Selecting a Test for Secondary vs Tertiary Amines

React these amines with nitrous acid (NaNO2 and HCl). Secondary amines produce a nitrosamine, which gives a yellow oily layer, whereas tertiary amines do not react.

Differentiating Ethylamine from Aniline

Understand that ethylamine is an aliphatic primary amine and aniline is an aromatic primary amine.

Choosing a Test for Aliphatic vs Aromatic Amines

Use bromine water. Aniline reacts with bromine water to form a white precipitate of 2,4,6-tribromoaniline, while ethylamine does not give any precipitate.

Differentiating Aniline from Benzylamine

Note that aniline is a primary aromatic amine, and benzylamine is a primary aliphatic amine connected via a methylene bridge.

Selecting a Test for Aromatic vs Aliphatic Amines

Use the Acetylation test. Aniline reacts with acetic anhydride to form acetanilide, whereas benzylamine forms N-benzylacetamide. Observing solubility differences can help with identification.

Differentiating Aniline from N-methylaniline

Aniline is a primary aromatic amine, and N-methylaniline is a secondary aromatic amine.

Choose a Distinguishing Test for Aniline vs N-methylaniline

Treat with nitrous acid (NaNO2 and HCl). Aniline forms a diazonium salt in cold conditions, which decomposes rapidly, whereas N-methylaniline forms N-nitroso-N-methylaniline, which is stable.

Key concepts

Hinsberg Test

The Hinsberg test is a popular method used to differentiate between primary, secondary, and tertiary amines based on their ability to form sulfonamide derivatives. The test involves the reaction of an amine with benzenesulfonyl chloride. Primary amines react to form sulfonamides that are soluble in an alkaline solution. This solubility happens because the sulfonamide further reacts with alkali. Secondary amines also form sulfonamides, but these are insoluble in alkali, remaining as a solid. Tertiary amines, however, do not form sulfonamides and remain unreacted.

• The test exploits the different chemical properties of the amine types.
• Methylamine (primary) yields a soluble sulfonamide.
• Dimethylamine (secondary) produces an insoluble one.

Nitrous Acid Test

The nitrous acid test is primarily used for distinguishing secondary from tertiary amines. When treated with nitrous acid, secondary amines form nitrosamines. These nitrosamines appear as a yellow oily layer. Tertiary amines, in contrast, do not form stable diazonium salts with nitrous acid, and hence do not produce this oily layer.

• Secondary amines react to produce a visible oily layer.
• Tertiary amines remain largely unreactive under these conditions.

This makes the nitrous acid test an excellent tool for identifying secondary amines in mixtures of similar compounds.

Bromine Water Test

The bromine water test is a simple and effective way to differentiate between aromatic and aliphatic amines. When bromine water is added to an aromatic amine such as aniline, it reacts to form a white precipitate of 2,4,6-tribromoaniline. This happens because of the bromination of the aromatic ring. Aliphatic amines, on the other hand, do not undergo this transformation and thus show no visible change in bromine water.

• Aniline yields a white precipitate, indicating its aromatic nature.
• Ethylamine shows no visible reaction.

Acetylation Test

The acetylation test is useful in distinguishing between different types of amines by reacting them with acetic anhydride. The test is particularly beneficial for telling apart aromatic and aliphatic amines. When aniline reacts with acetic anhydride, it forms acetanilide, which shows a difference in solubility compared to benzylamine's product, N-benzylacetamide. This solubility variance can be crucial for identifying these compounds.

• Aniline will form acetanilide, an aromatic derivative.
• Benzylamine will form N-benzylacetamide, an aliphatic derivative.

Amines Differentiation

Differentiating between amines often involves using chemical tests to exploit their distinct properties. The nitrous acid test and Hinsberg test are commonly used for this purpose, alongside others like the bromine water and acetylation tests. Understanding the nature of primary, secondary, and tertiary amines—as well as their aromatic and aliphatic classifications—is fundamental to selecting the right test. For instance:

• Primary amines generally react more predictably and provide more distinct results in tests.
• Secondary and aromatic amines can be more challenging but are identifiable with the right conditions.

These tests help chemists and students distinguish between amines in complex mixtures or unknown samples.