Identification

Benzenesulfonyl chloride (C₆H₅SO₂Cl) is added to the unknown amine in aqueous NaOH. A 1° amine reacts to form a sulfonamide (R–NH–SO₂C₆H₅) that is soluble in excess NaOH because the remaining N–H is acidic (pKa ≈ 10) and can be deprotonated to give a soluble sodium salt. On acidification, the salt precipitates as an insoluble sulfonamide. A 2° amine forms an N,N-disubstituted sulfonamide (R₂N–SO₂C₆H₅) with no acidic N–H, so it is insoluble in NaOH from the outset — a precipitate forms immediately and does not dissolve in base. A 3° amine has no N–H bond and cannot form a stable sulfonamide; it does not react with benzenesulfonyl chloride, so the amine layer simply separates or no reaction is observed. Decision rule: dissolves in NaOH → 1°; precipitate insoluble in NaOH → 2°; no reaction → 3°.

The unknown amine is heated with chloroform (CHCl₃) and ethanolic KOH. If a primary amine (R–NH₂) is present, a foul-smelling isocyanide (carbylamine, R–N≡C:) is produced. The mechanism begins with KOH dehydrohalogenating CHCl₃ to form the highly reactive dichlorocarbene (:CCl₂). The carbene inserts into the N–H bond of the 1° amine to give an intermediate that loses two HCl molecules (assisted by KOH) to yield the isocyanide. The characteristic horrific odour of the isocyanide is the positive result. Crucially, 2° and 3° amines do NOT give this test because 2° amines lack a second N–H needed for double dehydrohalogenation, and 3° amines have no N–H at all. The carbylamine test is therefore SPECIFIC for primary amines — both aliphatic and aromatic.

Nitrous acid (HNO₂) is generated in situ by mixing NaNO₂ with cold dilute HCl. The response depends on amine class. (A) Primary aliphatic amines (R–NH₂): react vigorously to form highly unstable aliphatic diazonium salts (R–N₂⁺) that decompose immediately with effervescence of N₂ gas and produce a mixture of alcohols, alkenes, and halides — a brisk evolution of nitrogen gas at room temperature is the positive result. (B) Primary aromatic amines (Ar–NH₂): react at 0–5 °C to form stable arenediazonium salts (Ar–N₂⁺ X⁻) because the positive charge is stabilised by resonance with the ring; the solution stays clear and on warming liberates N₂. (C) Secondary amines (R₂NH): react with HNO₂ to form N-nitrosamines (R₂N–N=O), which are yellow oily liquids or solids insoluble in water. (D) Tertiary amines: do not react with HNO₂ in a useful way; they merely form soluble ammonium salts that dissolve in the acidic medium and no distinguishing product forms. The complete decision tree is: N₂ gas at RT → 1° aliphatic; stable diazonium at 0°C → 1° aromatic; yellow oil/nitrosamine → 2°; no reaction → 3°.

• \ce{C6H5-NH2} — Aniline (phenylamine) (mol): A primary aromatic amine; the lone pair on nitrogen attacks the nitrosonium ion (NO⁺) generated from NaNO₂ + HCl.
• \ce{NaNO2} — Sodium nitrite (mol): Source of nitrous acid (HNO₂) when mixed with HCl; HNO₂ generates the electrophilic nitrosonium ion NO⁺ in situ.
• \ce{C6H5-N2^{+}Cl^{-}} — Benzenediazonium chloride (mol): Stable diazonium salt formed at 0–5 °C; resonance with the aromatic ring delocalises the positive charge, preventing immediate decomposition.

Problem. An unknown liquid amine is subjected to three tests: (1) With benzenesulfonyl chloride in NaOH, a precipitate forms that does NOT dissolve in excess NaOH. (2) The carbylamine test gives NO foul smell. (3) With NaNO₂/HCl, a yellow oily layer forms. Identify the class of the amine and explain each result.

Answer. The unknown amine is a secondary (2°) amine.

• Amines: Structure and Nomenclature — Understanding the structural difference between 1°, 2°, and 3° amines is a prerequisite for interpreting all three identification tests mechanistically.
• Amines: Basicity and Nucleophilicity — The lone pair on nitrogen that makes amines basic also drives their nucleophilic reactions with benzenesulfonyl chloride and nitrosonium ion in the identification tests.
• Diazonium Salts and Coupling Reactions — The nitrous acid test produces diazonium salts from primary aromatic amines; understanding their stability and subsequent reactions deepens interpretation of the test.
• Nucleophilic Substitution (SN1 and SN2) — The Hinsberg test involves nucleophilic substitution of Cl by the amine nitrogen on the sulfonyl chloride; this mechanism is directly explained by SN2 theory.
• Synthesis of Amines — Knowing how amines are synthesised — from nitriles, nitro reduction, or alkylation of ammonia — helps predict what class of amine a given synthetic route will yield, which must then be confirmed by identification tests.