Amines & Biomolecules
Syllabus Unit: Organic Chemistry
Why it matters
Chapter overview
Amines are basic, nitrogen-centred compounds whose diazonium salts unlock an entire family of aromatic transformations otherwise impossible by direct substitution, while biomolecules — carbohydrates, proteins, nucleic acids and vitamins — show how the same organic reactions (hemiacetal formation, condensation, hydrogen bonding) build the machinery of life. NEET tests classification and basicity of amines, the specific conditions each diazonium reaction needs, the structural proof behind glucose's cyclic form, and the base-pairing and vitamin-classification facts that recur across biology and chemistry alike.
1Amines: Classification & Reactions
Amines are classified by how many carbon groups sit on nitrogen — not, confusingly, the same rule used for alcohols — and that classification is exactly what Hinsberg's test exploits to identify them.
Learning Objectives
- Classify amines as 1°, 2° or 3° based on the number of carbon groups attached to nitrogen, and contrast this with the alcohol classification rule.
- Describe Gabriel phthalimide synthesis as a method to prepare a pure primary amine, avoiding the over-alkylation problem of direct alkylation.
- Explain Hinsberg's test (benzenesulfonyl chloride, then KOH) and the distinct result it gives for 1°, 2° and 3° amines.
- Describe the carbylamine (isocyanide) test as a way to specifically detect primary amines.
Key Terms
Amines (R-NH2, R2NH, R3N) are classified by how many alkyl/aryl groups are attached to nitrogen — 1°, 2° or 3° — which is the opposite convention to alcohols, where classification depends on the carbon bearing -OH, not the oxygen itself.
Preparing a pure primary amine
Directly alkylating ammonia with a haloalkane tends to over-alkylate, giving a mixture of 1°, 2°, 3° amines and a quaternary ammonium salt. Gabriel phthalimide synthesis avoids this: potassium phthalimide is alkylated with a primary haloalkane (SN2), then hydrolysed (or treated with hydrazine), releasing a pure 1° amine with no possibility of further alkylation.
Identifying amines by class
- Hinsberg's test: 1° amine + benzenesulfonyl chloride → sulfonamide with one N-H remaining, soluble in KOH (forms an anion). 2° amine → sulfonamide with no N-H remaining, insoluble in KOH. 3° amine → no reaction (no N-H to react with the sulfonyl chloride).
- Carbylamine test: only 1° amines react with CHCl3/alcoholic KOH to form an isocyanide (R-NC), identifiable by its distinctive unpleasant smell; 2° and 3° amines give no reaction.