Nitrogen-bearing organic compounds — chiefly amines, nitro compounds, nitriles and aryl diazonium salts — are a favourite hunting ground for NEET question-setters because they connect basicity (physical chemistry), distinguishing tests (qualitative reasoning) and conversion chains (synthesis). Mastering this small family lets you solve almost every "identify X, Y, Z" and "which is the strongest base" question.
Classification, Structure and Nomenclature
Nitrogen has five valence electrons, so in neutral organic molecules it usually forms three bonds and retains one lone pair. That lone pair is the single most important feature of this chapter: it makes amines basic and nucleophilic, and its involvement in resonance or in a positive centre destroys that reactivity.
The main classes you must recognise:
- Amines – ammonia derivatives, R–NH₂ (primary, 1°), R₂NH (secondary, 2°), R₃N (tertiary, 3°). Classification depends on how many carbons are attached to nitrogen, not on the nature of the carbon skeleton. Thus tert-butylamine, (CH₃)₃C–NH₂, is a primary amine even though the carbon is tertiary.
- Nitro compounds – R–NO₂, where nitrogen carries a formal positive charge and the group is a powerful electron-withdrawer.
- Nitriles (cyanides) – R–C≡N, joined through carbon; isocyanides (carbylamines) R–N⁺≡C⁻, joined through nitrogen. Same molecular formula, different attachment: a classic functional isomerism pair.
- Diazonium salts – Ar–N₂⁺X⁻, containing two nitrogens; aryl diazonium salts are moderately stable at low temperature, alkyl ones decompose instantly.
- Amides (R–CONH₂) also contain nitrogen but behave as carboxylic acid derivatives, and are essentially non-basic.
Nomenclature: in the common system the alkyl groups are named and the word "amine" is suffixed, as in ethylmethylamine. In IUPAC nomenclature amines are alkanamines — CH₃NH₂ is methanamine, CH₃CH₂CH(NH₃⁺… ) skeletons are numbered so nitrogen gets the lowest locant, and substituents on nitrogen carry the prefix N- (for example CH₃–NH–C₂H₅ is N-methylethanamine). When NH₂ is not the senior group it appears as the prefix amino-, e.g. 3-aminopropanoic acid. In 1° amines the nitrogen is roughly sp³ hybridised, and the molecule is pyramidal with the C–N–H angle a little compressed (about 108° in methylamine) because the lone pair repels the bond pairs more strongly.
Preparation of Amines
Six routes cover almost everything asked:
- Ammonolysis of alkyl halides: R–X + NH₃ → RNH₂, an SN2 attack by nitrogen. Because the product amine is itself nucleophilic, the reaction continues to give 2°, 3° amines and finally the quaternary ammonium salt. A large excess of ammonia favours the primary amine. Reactivity of halides follows R–I > R–Br > R–Cl. Aryl halides do not respond under ordinary conditions.
- Reduction of nitro compounds: Ar–NO₂ → Ar–NH₂ using H₂/Ni (or Pd, Pt), or Sn/HCl or Fe + HCl. The Fe/HCl variant is industrially preferred because only a small amount of HCl is needed (FeCl₂ formed is hydrolysed to regenerate the acid).
- Reduction of nitriles: R–CN + 4[H] → R–CH₂–NH₂ using LiAlH₄ or H₂/Ni (sodium in alcohol = Mendius reaction). Note the chain grows by one carbon.
- Reduction of amides: R–CONH₂ + LiAlH₄ → R–CH₂–NH₂, with no change in carbon number.
- Hofmann bromamide degradation: R–CONH₂ + Br₂ + 4NaOH → R–NH₂ + Na₂CO₃ + 2NaBr + 2H₂O. Only primary amines are formed, and the product has one carbon less than the amide.
- Gabriel phthalimide synthesis: potassium phthalimide + R–X, followed by alkaline hydrolysis, gives an exclusively primary amine free from 2°/3° contamination. It fails for aromatic amines because aryl halides will not undergo the nucleophilic substitution step.
A quick mental test: if the question demands a pure 1° amine, think Gabriel or Hofmann; if the carbon count must increase by one, think nitrile reduction; if it must decrease by one, think Hofmann degradation.
Physical Properties and Basic Strength
Lower amines are gases or volatile liquids with a fishy odour. Primary and secondary amines form intermolecular hydrogen bonds (N–H···N), so their boiling points exceed those of comparable tertiary amines, which have no N–H bond. For a fixed molar mass the order is alcohols > amines > alkanes, because N is less electronegative than O and its hydrogen bonds are weaker. All classes of lower amines — including tertiary — dissolve appreciably in water since they can accept hydrogen bonds from water; solubility falls as the hydrocarbon part lengthens.
Amines act as Lewis/Brønsted bases: R₃N + H₂O ⇌ R₃NH⁺ + OH⁻. A larger K_b (smaller pK_b) means a stronger base. Three factors compete in aqueous solution:
- +I effect of alkyl groups increases electron density on N (favours 2°, 3°).
- Solvation/H-bonding stabilisation of the ammonium ion: more N–H bonds means better hydration (favours 1°).
- Steric crowding around nitrogen hinders protonation (opposes 3°).
The resultant experimental orders in water are:
- Methyl series: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃
- Ethyl series: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃
In the gas phase, where solvation is absent, only inductive and steric effects operate and the order becomes cleanly 3° > 2° > 1° > NH₃.
Aromatic amines are much weaker bases than ammonia or aliphatic amines (aniline pK_b ≈ 9.4). The nitrogen lone pair is delocalised into the benzene ring, so it is less available; moreover the anilinium ion loses this stabilisation, making protonation unfavourable. Substituents adjust this: electron-donating groups (–CH₃, –OCH₃, –NH₂) strengthen the base, while electron-withdrawing groups (–NO₂, –CN, –X) weaken it, the effect being strongest from the ortho and para positions. Every ortho-substituted aniline is anomalously weak (the ortho effect, a steric/inhibition-of-resonance phenomenon). Overall: alkylamines > NH₃ > arylamines > diarylamines > triarylamines; amides and pyrrole-type nitrogens are essentially non-basic.
Chemical Reactions of Amines
(a) Alkylation. Successive attack on alkyl halides converts 1° → 2° → 3° amine → quaternary ammonium salt.
(b) Acylation. 1° and 2° amines react with acid chlorides, anhydrides or esters (in presence of a base like pyridine to remove HCl) giving substituted amides: C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ (acetanilide). Tertiary amines have no replaceable hydrogen on N and do not acylate. Because the amide nitrogen is deactivated, acylation is used as a protecting strategy (see below).
(c) Reaction with benzenesulphonyl chloride – Hinsberg's test.
- 1° amine → N-alkylbenzenesulphonamide, which still has an acidic N–H (activated by two electron-withdrawing groups) and hence dissolves in alkali.
- 2° amine → N,N-dialkyl sulphonamide with no N–H; insoluble in alkali.
- 3° amine → no reaction.
(d) Carbylamine (Hofmann isocyanide) test. Only aliphatic primary amines heated with chloroform and alcoholic KOH give foul-smelling isocyanides: R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O. Secondary and tertiary amines give no such product.
(e) Reaction with nitrous acid (NaNO₂ + HCl, 273–278 K).
- Aliphatic 1° amine → unstable diazonium salt → N₂ gas + alcohol (brisk effervescence).
- Aromatic 1° amine → benzenediazonium chloride, which is stable enough to be kept (with cooling) at 273–278 K, because the positive charge on the terminal nitrogen is delocalised into the ring. This diazotisation reaction is the single most important step in aromatic nitrogen chemistry, since it opens the door to replacing –NH₂ with almost any other group.
- Secondary amines (1° or 2°, aliphatic or aromatic) give N-nitrosoamines, yellow oily compounds, R₂N–N=O.
- Tertiary aliphatic amines simply form a water-soluble salt; tertiary aromatic amines undergo ring nitrosation to give a para-nitroso product (or ortho if para is blocked).
Diazonium Salts: Preparation and the Two Great Families of Reactions
Benzenediazonium chloride, C₆H₅N₂⁺Cl⁻, is made by diazotisation of aniline: NaNO₂ + HCl generate nitrous acid in situ, which reacts with the amine at 273–278 K (temperatures above this decompose the salt to phenol and N₂). Its reactions split cleanly into two groups.
Reactions with loss of nitrogen (replacement reactions) — the diazonium group is exchanged for another substituent, letting a chemist place a group on the ring exactly where an –NH₂ used to be, which is often impossible by direct electrophilic substitution:
- Sandmeyer reaction: with CuCl or CuBr (plus the corresponding HX) → chlorobenzene or bromobenzene.
- Gattermann reaction: the same transformation using Cu powder with HCl/HBr directly, avoiding the separate preparation of the cuprous halide.
- With KI (no catalyst needed): → iodobenzene.
- Balz–Schiemann reaction: the diazonium salt is first converted to the fluoroborate, ArN₂⁺BF₄⁻, which on gentle heating decomposes to the aryl fluoride — the standard route to C–F bonds on a ring, since direct fluorination is too violent to control.
- With water (warming in dilute acid): → phenol + N₂.
- With KCN/CuCN: → aryl cyanide (nitrile), extending the ring by one carbon.
- With H₃PO₂ (hypophosphorous acid) and water: → benzene, with H replacing N₂⁺. This deamination sequence (amine → diazonium → benzene) is a clever way to remove an –NH₂ group entirely after it has served its purpose as an activator/director during synthesis.
Reactions with retention of nitrogen (coupling reactions) — the diazonium ion behaves as a weak electrophile and attacks a strongly activated ring (phenols or aromatic amines) at the position para to the activating group, forming a brightly coloured azo compound, Ar–N=N–Ar′. Benzenediazonium chloride with phenol in weakly alkaline solution gives p-hydroxyazobenzene; with N,N-dimethylaniline it gives p-(dimethylamino)azobenzene (butter yellow). This azo-coupling chemistry is the industrial basis of azo dyes and of acid–base indicators such as methyl orange.
Nitro Compounds and Nitriles: Two Smaller but Testable Families
Nitroalkanes (R–NO₂) are prepared by treating an alkyl halide with AgNO₂ (silver, being a soft, ambidentate-selective cation, favours N-attack, giving the nitro compound), whereas KNO₂ favours O-attack and gives the alkyl nitrite instead — a subtle reagent-choice question NEET likes to ask alongside the analogous KCN/AgCN pair. The nitro group is a strong resonance hybrid with two equivalent N–O bonds and a full positive formal charge on nitrogen; this powerful electron withdrawal makes any α-hydrogen unusually acidic (nitroalkanes with an α-H show tautomeric aci-nitro forms and dissolve in NaOH), and makes aromatic nitro compounds important intermediates because the group is reduced cleanly to –NH₂ under a variety of conditions.
Nitriles (R–C≡N) are made from alkyl halides with KCN (chain length unchanged, carbon attached through C), by dehydration of an amide with P₂O₅ or POCl₃, or from the reaction of an aldehyde/ketone-derived cyanohydrin. Hydrolysis under acid or base gives a carboxylic acid via the amide; reduction (LiAlH₄ or H₂/Ni) gives a primary amine with the chain extended by one carbon; a Grignard reagent adds across the C≡N to give, after hydrolysis, a ketone. Isocyanides (R–N≡C), made only via the carbylamine reaction or Ag salt of KCN with alkyl halide, are extremely foul-smelling and toxic, and hydrolyse under acid to a primary amine plus methanoic (formic) acid — the opposite connectivity from nitrile hydrolysis, which is exactly why the carbylamine test is diagnostic only for primary amines and not for anything downstream of it.
Common Mistakes and Exam Traps
- Classifying amines by the carbon skeleton instead of by nitrogen's substituents. tert-Butylamine is a primary amine because only one carbon is bonded to N, even though that carbon itself is tertiary — a classic trap distinct from alkyl-halide classification.
- Applying the aqueous basicity order (2° > 1° > 3° > NH₃ for methylamines) in the gas phase. In the gas phase, with no solvation effects, the order cleanly follows inductive/steric reasoning alone: 3° > 2° > 1° > NH₃. Mixing up which order applies in which phase is one of the most common NEET errors in this chapter.
- Forgetting the ortho effect on aniline derivatives. Any ortho-substituted aniline is anomalously less basic than the para isomer regardless of whether the substituent is electron-donating or withdrawing, because steric inhibition of resonance and hydrogen bonding effects dominate at that position.
- Confusing which nucleophile attack gives nitrile vs isocyanide, and nitro vs nitrite. KCN (through carbon) gives the nitrile, AgCN (through nitrogen) gives the isocyanide; similarly AgNO₂ (through nitrogen) gives the nitroalkane, KNO₂ (through oxygen) gives the alkyl nitrite — remembering which metal favours which end of the ambident ion is essential for synthesis questions.