The Amine Distinguishing-Tests Toolbox

This section gathers the chapter's tests so you can tell amine classes apart.

Is it a primary amine? → Carbylamine test: heat with CHCl3_3 + alcoholic KOH. A foul, offensive smell (isocyanide) appears only with 1° amines (aliphatic or aromatic).

1° vs 2° vs 3°? → Hinsberg test with benzenesulphonyl chloride, then KOH:

  • 1° → sulphonamide soluble in KOH (acidic N-H).
  • 2° → sulphonamide insoluble (precipitate, no N-H).
  • 3° → no reaction.

Reaction with nitrous acid (NaNO2_2/HCl, cold):

  • 1° aliphatic → alcohol + N2_2 gas (effervescence).
  • 1° aromatic → cold-stable diazonium salt (→ can be coupled to give an orange/red dye).
  • 2° → yellow oily N-nitrosamine.
  • 3° → no characteristic reaction.

Key Point: carbylamine → 1° only; Hinsberg → 1° soluble / 2° precipitate / 3° no reaction; HNO2_2 → 1° aliphatic gives N2_2, 1° aromatic gives a diazonium salt (dye test), 2° gives a nitrosamine.

Worked Distinctions (the exam classics)

Methylamine vs dimethylamine vs trimethylamine (1° vs 2° vs 3°): use the Hinsberg test — methylamine (1°) gives a KOH-soluble product, dimethylamine (2°) gives an insoluble precipitate, trimethylamine (3°) does not react. (The carbylamine test would only flag the 1° amine.)

Aniline (1° aromatic) vs N-methylaniline (2°): carbylamine test — aniline gives the foul isocyanide smell; N-methylaniline does not. Also, aniline gives a diazonium salt with cold HNO2_2 that couples with 2-naphthol to give a red dye, while the 2° amine gives a nitrosamine.

Ethylamine (1° aliphatic) vs aniline (1° aromatic): both are primary (both give carbylamine), but with cold HNO2_2 ethylamine releases N2_2 (gives ethanol) while aniline forms a stable diazonium salt (which then couples to a dye) — distinguishing aliphatic from aromatic primary amines.

Key Point: combine tests — carbylamine spots any 1° amine; Hinsberg separates 1°/2°/3°; cold HNO2_2 + a coupling reagent distinguishes a 1° aromatic amine (dye) from a 1° aliphatic amine (N2_2 gas).

Uses & Importance of Amines

Amines and their derivatives are everywhere in medicine, industry and biology:

  • Drugs: many medicines are amines or contain amino groups — sulpha drugs (from sulphanilic acid), adrenaline, antihistamines, and countless others; amines' basicity lets them form soluble salts used in formulations.
  • Dyes: aromatic amines are the building blocks of azo dyes (via diazonium coupling) — a huge industry of colours for textiles, inks and food.
  • Polymers: diamines such as hexamethylenediamine are used to make nylon (polyamides).
  • Agrochemicals & others: amines are used in making herbicides, surfactants and rubber chemicals.
  • Biology: amino acids (the building blocks of proteins), many vitamins, hormones (e.g. adrenaline) and alkaloids (quinine, morphine, nicotine) all contain amine groups.

Key Point: amines matter in drugs (sulpha drugs, adrenaline), dyes (azo dyes), polymers (nylon), and biology (amino acids, vitamins, hormones, alkaloids).

Solved Examples

Example 1: Confirm a primary amine

Which test confirms an unknown is a primary amine, and what is observed?

Solution: The carbylamine test (CHCl3_3 + alcoholic KOH) — a foul, offensive smell of the isocyanide confirms a primary amine.

Example 2: Tell apart 1°, 2°, 3°

Which single test distinguishes all three classes of amine?

Solution: The Hinsberg test: 1° → KOH-soluble product; 2° → insoluble precipitate; 3° → no reaction.

Example 3: Aliphatic vs aromatic primary amine

How would you distinguish ethylamine from aniline?

Solution: Treat each with cold HNO2_2 (NaNO2_2/HCl): ethylamine gives N2_2 gas (effervescence) and ethanol; aniline forms a stable diazonium salt, which on coupling (e.g. with 2-naphthol) gives a red/orange dye.

Example 4: Dye test

What positive test specifically indicates a primary aromatic amine?

Solution: Diazotisation (cold HNO2_2) followed by coupling with an alkaline 2-naphthol/phenol to give a brightly coloured (orange/red) azo dye — characteristic of a primary aromatic amine.

Example 5: 2° amine identification

What does a 2° amine give with nitrous acid, and how does it look?

Solution: A yellow oily N-nitrosamine (R2_2N-N=O) — distinct from the gas (1° aliphatic) or the diazonium dye (1° aromatic).

Example 6: Distinguish aniline from N-methylaniline

Give one test to tell aniline (1°) from N-methylaniline (2°).

Solution: The carbylamine test — aniline gives the foul isocyanide smell (it is 1°); N-methylaniline (2°) does not respond.

Example 7: A use of amines in medicine

Give one important medicinal class derived from an aromatic amine.

Solution: The sulpha drugs (sulphonamides), derived from sulphanilic acid/sulphanilamide, are antibacterial agents.

Example 8: A polymer from a diamine

Name a common polymer made using a diamine.

Solution: Nylon (a polyamide) — for example nylon-6,6 is made from hexamethylenediamine and adipic acid.

Example 9: Biological amines

Give two examples of biologically important nitrogen compounds containing amine groups.

Solution: Amino acids (the building blocks of proteins) and alkaloids such as quinine, morphine or nicotine (also hormones like adrenaline).

Example 10: Choosing a test scheme

You have three bottles: a 1° aliphatic amine, a 1° aromatic amine and a 3° amine. Outline a test scheme.

Solution: (1) Carbylamine test flags the two primary amines (foul smell); the 3° amine is the one that does not respond. (2) Treat the two primaries with cold HNO2_2: the one giving N2_2 gas is the 1° aliphatic amine; the one forming a diazonium salt → dye on coupling is the 1° aromatic amine.