Amines

An amine is what you get when one or more hydrogen atoms of ammonia are replaced by alkyl or aryl groups. Because it is made from ammonia, its shape is that of ammonia. The nitrogen is sp³ hybridised: three of the four hybrid orbitals form σ bonds (to H or to carbon) and the fourth holds the lone pair. The molecule is therefore pyramidal, not planar.

The C–N–E angle (E is C or H) is less than the tetrahedral 109.5° because the lone pair repels the bonding pairs more strongly than they repel each other. NCERT quotes trimethylamine: the C–N–C angle is 108°.

Everything in the chapter comes back to this lone pair: it makes amines bases, it attacks acyl chlorides, alkyl halides and chloroform in the name reactions, and in aniline it is delocalised into the ring, which makes aniline both a weaker base and an ortho, para director. In the exam draw the pyramid and show the lone pair; a flat drawing is wrong.

Amines are classified by how many hydrogens of ammonia have been replaced. A primary (1°) amine has one alkyl or aryl group on nitrogen (R–NH₂), a secondary (2°) amine has two (R₂NH), a tertiary (3°) amine has three (R₃N). Note that this is different from the classification of alcohols and halides, where 1°, 2°, 3° refer to the carbon carrying the group. In amines the count is on the nitrogen. So (CH₃)₃C–NH₂ (tert-butylamine) is a primary amine even though the carbon attached to N is tertiary, and (CH₃)₂CH–NH₂ is also primary.

Secondary and tertiary amines are further called simple when all the groups on nitrogen are identical, for example (CH₃)₂NH, and mixed when they differ, for example CH₃NHC₂H₅. If nitrogen carries four groups it is a quaternary ammonium salt, R₄N⁺X⁻, which is an ionic compound, not an amine.

Aliphatic amines have nitrogen on an sp³ carbon (ethanamine); aromatic amines have nitrogen directly on the ring (aniline). Benzylamine, C₆H₅CH₂NH₂, has a ring in it but the nitrogen sits on a CH₂, so it behaves as an aliphatic amine: it is a much stronger base than aniline and cannot be diazotised to a stable salt. This distinction is tested every year, usually in an order-of-basicity question.

In the common system an aliphatic amine is named by adding 'amine' to the alkyl group: methylamine, diethylamine. In the IUPAC system amines are named as alkanamines: drop the final 'e' of the alkane and add 'amine', with a locant for the carbon carrying nitrogen. CH₃CH₂NH₂ is ethanamine; CH₃CH(NH₂)CH₃ is propan-2-amine; H₂N–CH₂CH₂CH=CH₂ is but-3-en-1-amine (the double bond takes the lower number only after the amine suffix has been placed as low as possible).

For secondary and tertiary amines the longest chain attached to nitrogen is the parent, and the other groups are prefixed with the locant N. CH₃NHCH₂CH₃ is N-methylethanamine; (CH₃CH₂)₂NCH₃ is N-ethyl-N-methylethanamine; (CH₃)₂N–CH₂CH₃ is N,N-dimethylethanamine; CH₃NHCH(CH₃)₂ is N-methylpropan-2-amine. Two amino groups on a chain give 'diamine' and the 'e' of the alkane is kept: H₂N(CH₂)₆NH₂ is hexane-1,6-diamine. When –NH₂ is not the principal group it is the prefix 'amino'.

Aromatic amines: C₆H₅NH₂ is aniline (an accepted IUPAC name; benzenamine is the systematic one). CH₃C₆H₄NH₂ with the methyl ortho to NH₂ is 2-methylaniline (o-toluidine); m-BrC₆H₄NH₂ is 3-bromoaniline; C₆H₅NHCH₃ is N-methylaniline. The board also expects the four kinds of isomers of C₄H₁₁N: four primary (butan-1-amine, butan-2-amine, 2-methylpropan-1-amine, 2-methylpropan-2-amine), three secondary (N-methylpropan-1-amine, N-methylpropan-2-amine, N-ethylethanamine) and one tertiary (N,N-dimethylethanamine). Primary–secondary–tertiary pairs are functional isomers; butan-1-amine and 2-methylpropan-1-amine are chain isomers; butan-1-amine and butan-2-amine are position isomers; N-methylpropan-1-amine and N-ethylethanamine are metamers.

NCERT gives six routes. Learn each with its reagent and the class of amine it delivers.

1. Reduction of nitro compounds. H₂/Pd in ethanol, or a metal and acid: Sn/HCl or Fe/HCl. C₆H₅NO₂ + 3H₂ → C₆H₅NH₂ + 2H₂O. Fe/HCl is preferred because the FeCl₂ formed is hydrolysed to release HCl, so only a small amount of acid is needed. This gives primary amines.

2. Ammonolysis of alkyl halides. Ammonia attacks R–X by nucleophilic substitution: RX + NH₃ → RNH₃⁺X⁻, which a base converts to RNH₂. The primary amine is itself a nucleophile and reacts further, so the product is a mixture of 1°, 2°, 3° amines and finally the quaternary salt R₄N⁺X⁻. Excess ammonia favours the primary amine. Reactivity of halides: RI > RBr > RCl.

3. Reduction of nitriles. R–C≡N + 4[H] → RCH₂NH₂ using H₂/Ni, LiAlH₄ or Na(Hg)/C₂H₅OH. The amine has one carbon more than the halide you started from.

4. Reduction of amides. RCONH₂ with LiAlH₄ then H₂O gives RCH₂NH₂, same number of carbons.

5. Gabriel phthalimide synthesis. Phthalimide + KOH → potassium phthalimide; + R–X → N-alkylphthalimide; alkaline hydrolysis (NaOH, aq) → primary amine + sodium phthalate. It gives a pure primary amine with no 2° or 3° contamination. It cannot make aromatic amines because aryl halides do not undergo nucleophilic substitution with the phthalimide anion.

6. Hofmann bromamide degradation. R–CONH₂ + Br₂ + 4NaOH → R–NH₂ + Na₂CO₃ + 2NaBr + 2H₂O. The alkyl or aryl group migrates from carbonyl carbon to nitrogen, so the amine has one carbon less than the amide. This is the descent-of-series route and works for aromatic amides too: benzamide gives aniline.

The lower aliphatic amines are gases with a fishy smell; higher ones are liquids, then solids. Aniline is colourless when pure but turns brown in air because of oxidation.

Solubility. Lower aliphatic amines dissolve in water because they hydrogen-bond with it. Solubility falls as the hydrophobic alkyl part grows, and aniline is practically insoluble because the large phenyl group outweighs the –NH₂. The NCERT order of increasing solubility for the exam question is C₆H₅NH₂ < (C₂H₅)₂NH < C₂H₅NH₂: the primary amine has the smaller hydrocarbon part and forms more hydrogen bonds. Amines are soluble in organic solvents like alcohol, ether and benzene.

Boiling points. Primary and secondary amines have N–H bonds and associate through intermolecular hydrogen bonding; tertiary amines have no N–H and cannot. So for comparable molecular mass the order is 1° > 2° > 3°: butan-1-amine 350.8 K, N-ethylethanamine 329.3 K, N,N-dimethylethanamine 310.5 K. Nitrogen is less electronegative than oxygen, so N–H···N hydrogen bonds are weaker than O–H···O bonds and amines boil below alcohols of similar mass (butan-1-ol 390.3 K) but above alkanes (pentane 309.1 K). This is why CH₃NH₂ boils higher than (CH₃)₃N even though it is lighter, and why the board's favourite increasing order is (CH₃)₂NH < C₂H₅NH₂ < C₂H₅OH.

Amines are basic because the lone pair on nitrogen can accept a proton: R–NH₂ + H₂O ⇌ R–NH₃⁺ + OH⁻. With mineral acids they form water-soluble ammonium salts, CH₃CH₂CH₂NH₂ + HCl → CH₃CH₂CH₂NH₃⁺Cl⁻ (propan-1-aminium chloride), and NaOH regenerates the amine. Basic strength is measured by K_b or pK_b = −log K_b; a smaller pK_b means a stronger base. The pK_b values to remember: NH₃ 4.75, CH₃NH₂ 3.38, (CH₃)₂NH 3.27, (CH₃)₃N 4.22, C₂H₅NH₂ 3.29, (C₂H₅)₂NH 3.00, (C₂H₅)₃N 3.25, C₆H₅CH₂NH₂ 4.70, C₆H₅NH₂ 9.38, C₆H₅NHCH₃ 9.30, C₆H₅N(CH₃)₂ 8.92.

Alkyl amines vs ammonia. The +I effect of alkyl groups raises electron density on nitrogen, so alkylamines are stronger bases than ammonia. In the gas phase, where only this electronic effect operates, the order is 3° > 2° > 1° > NH₃.

In aqueous solution three factors compete: the +I effect, the solvation (hydration) of the substituted ammonium ion by hydrogen bonding, and steric hindrance around nitrogen. More N–H bonds on the cation mean better solvation, which stabilises the conjugate acid and favours basicity; a tertiary ammonium ion has only one N–H and is poorly solvated, and its three alkyl groups hinder the approach of a proton. The net result is that the tertiary amine drops down the order. For ethyl groups the aqueous order is (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃; for the smaller methyl groups it is (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. Secondary is always the strongest in water.

Aromatic amines. In aniline the lone pair is delocalised into the ring (five resonance structures for aniline; in the anilinium ion only two Kekulé structures, with no lone pair to delocalise). The amine is more stabilised than its conjugate acid, so it holds its lone pair tightly and is a far weaker base than ammonia or alkylamines. Electron-releasing groups on the ring (–CH₃, –OCH₃) raise basicity, electron-withdrawing groups (–NO₂, –Cl) lower it: p-toluidine > aniline > p-nitroaniline. Benzylamine has no such delocalisation (N is on the CH₂) and is nearly as basic as alkylamines. A standard order: (C₂H₅)₂NH > C₂H₅NH₂ > C₆H₅CH₂NH₂ > NH₃ > C₆H₅NH₂.

Alkylation. Amines react with alkyl halides in the same way ammonia does, giving 2° and 3° amines and finally the quaternary salt. Aniline with excess CH₃I goes to N,N-dimethylaniline and then N,N,N-trimethylanilinium iodide, C₆H₅N⁺(CH₃)₃I⁻; with Na₂CO₃ the salt is converted to the hydroxide.

Acylation. Primary and secondary amines react with acid chlorides, anhydrides and esters: the lone pair attacks the carbonyl carbon and a hydrogen on N is replaced by an acyl group. CH₃NH₂ + CH₃COCl → CH₃NHCOCH₃ (N-methylethanamide) + HCl, carried out in pyridine, a stronger base that removes the HCl and shifts the equilibrium forward. With acetic anhydride aniline gives acetanilide (N-phenylethanamide) and acetic acid. Reaction with benzoyl chloride is benzoylation: C₆H₅NH₂ + C₆H₅COCl → C₆H₅NHCOC₆H₅ (N-phenylbenzamide, benzanilide) + HCl. The amide product is much less basic and its –NHCOCH₃ is less activating than –NH₂, which is exactly why acetylation is used to control substitution in aniline (see 9.6.7). Tertiary amines have no N–H and are not acylated.

Carbylamine reaction. Aliphatic and aromatic primary amines heated with chloroform and alcoholic KOH give isocyanides (carbylamines), which are foul-smelling: R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O. Secondary and tertiary amines do not react, so this is the standard test for a primary amine and the standard way to distinguish, for example, aniline from N-methylaniline or methylamine from dimethylamine.

Nitrous acid is generated in situ from NaNO₂ and HCl. Aliphatic primary amines form unstable alkyldiazonium salts, which lose N₂ at once to give alcohols: R–NH₂ + HNO₂ → [R–N₂⁺Cl⁻] → R–OH + N₂ + HCl. The nitrogen is evolved quantitatively, so the reaction is used to estimate amino groups. Aromatic primary amines behave differently: at 273–278 K they give arenediazonium salts that are stable enough to be used, C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O. This is diazotisation, the door to the whole of 9.7–9.8.

Hinsberg test uses benzenesulphonyl chloride, C₆H₅SO₂Cl (Hinsberg's reagent), followed by aqueous KOH, and separates the three classes of amine. A primary amine gives an N-alkylbenzenesulphonamide, C₆H₅SO₂NHR. The hydrogen on nitrogen is acidic because it sits next to the strongly electron-withdrawing sulphonyl group, so the product dissolves in alkali as its potassium salt. A secondary amine gives C₆H₅SO₂NR₂, which has no N–H and is therefore insoluble in alkali; it separates as a solid or oil. A tertiary amine has no N–H at all and does not react with the reagent. NCERT notes that p-toluenesulphonyl chloride is now used in place of benzenesulphonyl chloride, with the same logic.

The –NH₂ group is strongly activating and ortho, para directing because its lone pair raises electron density at those positions. This makes aniline so reactive that controlling substitution is the main theme.

Bromination. Aniline with bromine water at room temperature gives a white precipitate of 2,4,6-tribromoaniline at once; you cannot stop at one bromine. To make p-bromoaniline you first protect the group by acetylation with acetic anhydride to give acetanilide. In –NHCOCH₃ the lone pair is shared with the carbonyl oxygen by resonance, so the group is less activating and the bulky acetamido group sends bromine to the para position. Bromination then gives p-bromoacetanilide, and hydrolysis (H⁺ or OH⁻, water) removes the acetyl group to give p-bromoaniline. This three-step control is a classic 3-mark question.

Nitration. Direct nitration with conc. HNO₃/H₂SO₄ at 288 K gives 51% p-nitroaniline, 47% m-nitroaniline and only 2% ortho, plus oxidation products and tar. The reason for the large meta product: in strongly acidic medium aniline is protonated to the anilinium ion, C₆H₅NH₃⁺, whose –NH₃⁺ group is deactivating and meta directing. Protection by acetylation, nitration, and hydrolysis gives p-nitroaniline as the main product.

Sulphonation. Aniline with conc. H₂SO₄ forms anilinium hydrogensulphate, which on heating at 453–473 K rearranges to p-aminobenzenesulphonic acid (sulphanilic acid), which exists as a zwitterion, ⁺H₃N–C₆H₄–SO₃⁻.

No Friedel–Crafts reaction. Aniline does not undergo Friedel–Crafts alkylation or acylation because the Lewis acid catalyst AlCl₃ coordinates with the nitrogen lone pair (aniline is a base). The resulting positively charged nitrogen deactivates the ring towards further electrophilic attack.

Diazonium salts have the general formula R–N₂⁺X⁻, where R is an aryl group and X⁻ is Cl⁻, Br⁻, HSO₄⁻ or BF₄⁻. The –N₂⁺ group is the diazonium group. Benzenediazonium chloride is C₆H₅N₂⁺Cl⁻; the hydrogensulphate is C₆H₅N₂⁺HSO₄⁻.

Preparation: diazotisation. Aniline is treated with nitrous acid (NaNO₂ + HCl) at 273–278 K: C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O. The salt is never isolated; the cold solution is used directly for the next step because it decomposes on standing, evolving nitrogen.

Stability. Arenediazonium salts are far more stable than alkyldiazonium salts because the positive charge is dispersed over the benzene ring by resonance: the –N₂⁺ group conjugates with the π system. An alkyldiazonium ion has no such stabilisation and loses N₂ immediately to give a carbocation, which is why aliphatic amines give alcohols with HNO₂.

Physical properties. Benzenediazonium chloride is a colourless crystalline solid, readily soluble in water, stable in cold but reacting with water when warmed. In the dry state it is unstable and explosive. Benzenediazonium fluoroborate, C₆H₅N₂⁺BF₄⁻, is water-insoluble and stable at room temperature, which is why it is the salt used for the fluoro and nitro replacements.

There are two families of reaction: those in which N₂ is lost and replaced by another group, and coupling, in which the diazonium group is kept and becomes an azo bridge.

A. Replacement of the diazo group (N₂ evolved).

• By Cl or Br, Sandmeyer reaction: ArN₂⁺Cl⁻ + Cu₂Cl₂/HCl → ArCl + N₂; with Cu₂Br₂/HBr → ArBr. Cuprous salts are the catalyst.
• By CN, also Sandmeyer: CuCN/KCN → ArCN + N₂. The nitrile is the route to benzoic acid (hydrolysis) and benzylamine (reduction).
• Gattermann reaction: copper powder with the halogen acid, Cu/HCl → ArCl, Cu/HBr → ArBr. Yields are lower than Sandmeyer.
• By I: no catalyst needed, KI → ArI + KCl + N₂. This is the only good way to put iodine on a benzene ring, since direct iodination is reversible.
• By F: HBF₄ gives the fluoroborate, which on heating gives ArF + BF₃ + N₂.
• By H: hypophosphorous acid or ethanol reduces the salt: ArN₂⁺Cl⁻ + H₃PO₂ + H₂O → ArH + N₂ + H₃PO₃ + HCl; with C₂H₅OH the products are ArH + N₂ + CH₃CHO + HCl. Used to remove an amino group after it has directed substitution (deamination).
• By OH: warming the aqueous solution to 283 K gives phenol: ArN₂⁺Cl⁻ + H₂O → ArOH + N₂ + HCl.
• By NO₂: the fluoroborate heated with aqueous NaNO₂ and copper gives nitrobenzene: ArN₂⁺BF₄⁻ + NaNO₂/Cu, Δ → ArNO₂ + N₂ + NaBF₄.

B. Coupling reactions (N₂ retained). The diazonium ion is a weak electrophile and attacks only strongly activated rings, at the para position. With phenol in alkaline medium it gives p-hydroxyazobenzene, an orange dye; with aniline in mildly acidic medium it gives p-aminoazobenzene, a yellow dye. The product contains the –N=N– azo linkage, an extended conjugated system, which is why azo compounds are coloured. C₆H₅N₂⁺Cl⁻ + C₆H₅OH → (OH⁻) C₆H₅–N=N–C₆H₄–OH (p) + HCl.

Diazonium salts matter because they let you put groups on a benzene ring that cannot be introduced by direct electrophilic substitution. Fluorine and iodine cannot be attached directly (direct iodination is reversible), a cyano group cannot be put in by nucleophilic substitution on an aryl halide, and a hydroxyl group needs harsh conditions by any other route. Through the diazonium salt, all of these come from the amino group, and the amino group itself comes from a nitro group, which is easy to introduce by nitration. So the standard synthetic logic is: benzene → nitrobenzene (conc. HNO₃/H₂SO₄) → aniline (Sn/HCl or Fe/HCl) → benzenediazonium chloride (NaNO₂/HCl, 273–278 K) → the target group.

A second use is that –NH₂ is a strong ortho, para director that can be removed afterwards by H₃PO₂. That gives access to substitution patterns that are otherwise impossible: brominate aniline to 2,4,6-tribromoaniline, diazotise, then remove the amino group with H₃PO₂ to get 1,3,5-tribromobenzene, a meta pattern from a bromine that is itself ortho, para directing. Similarly, aniline to 2,4,6-tribromofluorobenzene uses bromination, diazotisation and HBF₄/heat.

The third use is dyes: coupling gives azo dyes. In a conversion question the diazonium salt is almost always the middle step; write the reagent and condition above each arrow.