NEET ChemistryNCERT Class 12Chapter 9

Amines: NEET notes

Amines are ammonia with its hydrogens swapped for alkyl or aryl groups, keeping nitrogen's lone pair. The chapter names and classifies them, shows six ways to make them, explains their hydrogen bonding and why their base strength depends on inductive, solvation and resonance effects, and covers the reactions that tell 1°, 2° and 3° amines apart. It ends with diazonium salts: how they are made cold, how the –N₂⁺ group is swapped for halogens, –CN, –OH, –H or –NO₂, and how coupling makes azo dyes.

What NEET asks

NEET asks for basicity orders (gas phase versus water, alkyl versus aryl, substituted anilines) and pKb reading, carbon counting in Hoffmann and nitrile routes, why Gabriel fails for aniline, the carbylamine and Hinsberg tests, aniline's bromination, nitration (51/47/2) and missing Friedel-Crafts reaction, and diazonium conversions (Sandmeyer, Gatterman, KI, HBF₄, H₃PO₂, coupling). Marks are lost on the aqueous basicity order, on the Hinsberg solubilities and on the temperature window 273–278 K.

1. Structure, classes and names

NCERT §9.1–9.3

Amines are ammonia with one, two or all three hydrogens swapped for alkyl or aryl groups. They turn up in proteins, vitamins, alkaloids and hormones, and in synthetic polymers, dyes and drugs.
Examples from medicine: adrenaline and ephedrine, both secondary amines, raise blood pressure; novocain, a synthetic amino compound, is a dental anaesthetic; the antihistamine benadryl has a tertiary amino group. Quaternary ammonium salts serve as surfactants.
As in ammonia, the nitrogen is trivalent and keeps one unshared electron pair. Its orbitals are sp³ hybridised: three form bonds to H or C and the fourth holds the lone pair, so the molecule is pyramidal.
The lone pair squeezes the C–N–E angle (E = C or H) below 109.5°; in trimethylamine it is 108°.
Classes: replacing one H of NH₃ gives a primary (1°) amine, RNH₂ or ArNH₂; two gives a secondary (2°) amine, R–NHR′; three gives a tertiary (3°) amine. An amine is 'simple' when all its groups are the same and 'mixed' when they differ.
Common names put the alkyl group in front of 'amine' as one word (methylamine), with di- or tri- for repeated groups. IUPAC names primary amines as alkanamines by replacing the '-e' of the alkane with 'amine': CH₃NH₂ is methanamine.
With two or more –NH₂ groups, locants and di-, tri- are used and the 'e' of the hydrocarbon stays: H₂N–CH₂–CH₂–NH₂ is ethane-1,2-diamine. Groups on nitrogen get the locant N: CH₃NHCH₂CH₃ is N-methylethanamine and (CH₃CH₂)₃N is N,N-diethylethanamine.
In arylamines –NH₂ sits directly on the ring. C₆H₅NH₂ is aniline, also an accepted IUPAC name, or benzenamine.
IUPAC names for common ones: propan-1-amine (n-propylamine), propan-2-amine (isopropylamine), N-methylethanamine (ethylmethylamine), N,N-dimethylmethanamine (trimethylamine), prop-2-en-1-amine (allylamine), hexane-1,6-diamine (hexamethylenediamine), 2-methylaniline (o-toluidine), 4-bromobenzenamine (p-bromoaniline).

2. Preparing amines by reduction and ammonolysis

NCERT §9.4

Reduction of nitro compounds: hydrogen over finely divided Ni, Pd or Pt, or a metal in acid, turns nitrobenzene into aniline; nitroalkanes give alkanamines the same way.
Iron scrap with hydrochloric acid is the preferred metal–acid pair. The FeCl₂ formed hydrolyses and gives back HCl, so only a little acid is needed to start the reaction.
Ammonolysis: an alkyl or benzyl halide heated with ethanolic ammonia in a sealed tube at 373 K undergoes nucleophilic substitution, –NH₂ replacing the halogen.
The primary amine formed is itself a nucleophile, so it reacts with more halide to give secondary and tertiary amines and finally a quaternary ammonium salt. A strong base frees the amine from its ammonium salt.
Ammonolysis therefore gives a mixture; using a large excess of ammonia makes the primary amine the main product. Halides react in the order RI > RBr > RCl.
Reduction of nitriles with LiAlH₄ or by catalytic hydrogenation gives primary amines. Since a nitrile comes from an alkyl halide and cyanide, this route adds one carbon (ascent of the amine series).
Reduction of amides with LiAlH₄ also gives amines.
Worked: CH₃CH₂Cl with ethanolic NaCN gives propanenitrile, which is reduced to propan-1-amine. Benzyl chloride gives phenylethanenitrile, and H₂/Ni turns it into 2-phenylethanamine.

3. Gabriel and Hoffmann methods

NCERT §9.4

Gabriel phthalimide synthesis gives primary amines only. Phthalimide with ethanolic KOH forms potassium phthalimide; heating this with an alkyl halide gives an N-alkylphthalimide, and alkaline hydrolysis releases the primary amine.
Gabriel synthesis cannot give aniline or any aromatic primary amine: an aryl halide will not be attacked by the phthalimide anion in a substitution.
Hoffmann bromamide degradation (NCERT's spelling): an amide is treated with bromine in aqueous or ethanolic NaOH. An alkyl or aryl group migrates from the carbonyl carbon to nitrogen, and the primary amine formed has one carbon fewer than the amide.
Equation: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O.
Worked: propanamine (three carbons) comes from butanamide (four); benzamide (seven carbons) gives aniline (six).
Carbon count at a glance: nitrile reduction adds one carbon to the alkyl halide, Hoffmann degradation removes one from the amide, and Gabriel synthesis and ammonolysis keep the halide's carbon count.

4. Physical properties of amines

NCERT §9.5

The lower aliphatic amines are gases with a fishy smell. Primary amines with three or more carbons are liquids, and higher ones are solids.
Aniline and other arylamines are usually colourless when pure but darken on storage as air oxidises them.
Lower aliphatic amines dissolve in water by hydrogen bonding with it. Solubility falls as the water-repelling alkyl part grows, and higher amines are essentially insoluble. Amines dissolve in alcohol, ether and benzene.
Nitrogen (electronegativity 3.0) is less electronegative than oxygen (3.5), so alcohols are more polar than amines and form stronger hydrogen bonds. Butan-1-ol is therefore more soluble in water than butan-1-amine.
Primary and secondary amines associate through N–H···N hydrogen bonds. Primary amines have two N–H hydrogens and associate more; tertiary amines have none and do not associate. Boiling points of isomers: primary > secondary > tertiary.
Boiling points (molar mass, K): n-C₄H₉NH₂ (73) 350.8; (C₂H₅)₂NH (73) 329.3; C₂H₅N(CH₃)₂ (73) 310.5; C₂H₅CH(CH₃)₂ (72) 300.8; n-C₄H₉OH (74) 390.3.

5. Basic character of amines

NCERT §9.6

The lone pair on nitrogen makes amines Lewis bases. They form salts with acids, and NaOH gives the amine back. Amine salts dissolve in water but not in ether, which lets amines be separated from non-basic organic compounds.
Base strength is measured by Kb, or pKb = −log Kb; the larger Kb (smaller pKb), the stronger the base. Ammonia has pKb 4.75.
pKb in water. Methyl series: 1° methanamine 3.38, 2° N-methylmethanamine 3.27, 3° N,N-dimethylmethanamine 4.22. Ethyl series: 1° ethanamine 3.29, 2° N-ethylethanamine 3.00, 3° N,N-diethylethanamine 3.25. Aromatic: benzenamine (aniline) 9.38, N-methylaniline 9.30, N,N-dimethylaniline 8.92; phenylmethanamine (benzylamine) 4.70.
Alkyl groups push electrons (+I) onto nitrogen, making the lone pair more available, and spread the positive charge of the ammonium ion. So aliphatic amines are stronger bases than ammonia; their pKb values run from 3 to 4.22.
In the gas phase basicity follows the +I effect: tertiary > secondary > primary > NH₃.
In water, the ammonium ions are also stabilised by hydrogen bonding to water (solvation). Smaller ions with more N–H hydrogens are solvated best, favouring primary > secondary > tertiary, the reverse of the inductive order. Groups bigger than –CH₃ also hinder that hydrogen bonding sterically.
The balance of inductive effect, solvation and steric hindrance gives the aqueous orders (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ and (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.
Aniline is a much weaker base than ammonia: its lone pair is conjugated with the ring. Aniline has five resonance structures, the anilinium ion only two (Kekulé), so protonation costs aniline stability.
On substituted anilines, electron-releasing groups (–OCH₃, –CH₃) raise basic strength; electron-withdrawing groups (–NO₂, –SO₃H, –COOH, –X) lower it.
Worked order, strongest first: (C₂H₅)₂NH > C₂H₅NH₂ > NH₃ > C₆H₅NH₂.

6. Acylation, alkylation and the carbylamine test

NCERT §9.6

Alkylation: amines react with alkyl halides, as in ammonolysis, moving up to the quaternary salt.
Acylation: any primary or secondary amine, aliphatic or aromatic, swaps an N–H hydrogen for an acyl group when treated with an acid chloride, an anhydride or an ester. The reaction is a nucleophilic substitution and the product is an amide.
Acylation is run with a base stronger than the amine, such as pyridine, which removes the HCl formed and pushes the equilibrium forward.
Examples: ethanamine with ethanoyl chloride gives N-ethylethanamide; aniline with ethanoic anhydride gives N-phenylethanamide (acetanilide).
Benzoylation is acylation with benzoyl chloride, C₆H₅COCl: methanamine gives N-methylbenzamide.
With carboxylic acids at room temperature, amines simply form salts.
Carbylamine reaction (isocyanide test): a primary amine, aliphatic or aromatic, heated with chloroform and ethanolic KOH gives a foul-smelling isocyanide. Secondary and tertiary amines do not react, so this is a test for primary amines.
Equation: R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O (on heating).

7. Nitrous acid and Hinsberg's reagent

NCERT §9.6

Nitrous acid is made in the reaction mixture from sodium nitrite and a mineral acid. The three classes of amine react with it differently.
Primary aliphatic amines give unstable aliphatic diazonium salts, which break down to an alcohol with quantitative release of nitrogen gas. Measuring that nitrogen is used to estimate amino acids and proteins.
Aromatic primary amines react with nitrous acid at 273–278 K to form diazonium salts, which are the starting point for many aromatic compounds.
Secondary and tertiary amines react with nitrous acid in other ways (not detailed in this unit).
Hinsberg's reagent is benzenesulphonyl chloride, C₆H₅SO₂Cl. A primary amine gives an N-alkylbenzenesulphonamide whose N–H is strongly acidic, because the sulphonyl group withdraws electrons, so the product dissolves in alkali.
A secondary amine gives an N,N-dialkylbenzenesulphonamide with no H on nitrogen, which is not acidic and does not dissolve in alkali. Tertiary amines do not react.
These differences are used to tell 1°, 2° and 3° amines apart and to separate a mixture of them. Nowadays p-toluenesulphonyl chloride is used in place of benzenesulphonyl chloride.

8. Electrophilic substitution in aniline

NCERT §9.6

The resonance structures of aniline pile electron density onto the ortho and para positions, so –NH₂ is a powerful activating, ortho- and para-directing group.
Bromination: aniline and bromine water at room temperature give a white precipitate of 2,4,6-tribromoaniline at once.
To stop at one substituent, –NH₂ is protected by acetylation with acetic anhydride (pyridine), the substitution is done on acetanilide, and the amide is then hydrolysed back to the amine. Acetanilide with Br₂ in CH₃COOH gives mainly the para product, and hydrolysis gives 4-bromoaniline.
In acetanilide the nitrogen lone pair is also shared with the carbonyl oxygen by resonance, so it is less available to the ring and –NHCOCH₃ activates less than –NH₂.
Nitration: direct nitration (HNO₃, H₂SO₄, 288 K) gives tarry oxidation products as well as nitroanilines. In strong acid much of the aniline becomes the anilinium ion, which is meta-directing, so the product mix is 51% para, 47% meta and 2% ortho.
Nitrating acetanilide instead, then hydrolysing, gives p-nitroaniline as the main product.
Sulphonation: aniline with concentrated H₂SO₄ forms anilinium hydrogensulphate, which on heating at 453–473 K gives p-aminobenzenesulphonic acid (sulphanilic acid) as the main product; it exists as a zwitter ion.
Aniline gives no Friedel-Crafts reaction: it forms a salt with the AlCl₃ catalyst, and the positively charged nitrogen then strongly deactivates the ring.

9. Diazonium salts: making and handling

NCERT §9.7–9.8

Diazonium salts have the formula R–N₂⁺X⁻, where R is an aryl group and X⁻ may be Cl⁻, Br⁻, HSO₄⁻, BF₄⁻ and so on. The –N₂⁺ group is the diazonium group.
Names add 'diazonium' to the parent hydrocarbon and then name the anion: C₆H₅N₂⁺Cl⁻ is benzenediazonium chloride and C₆H₅N₂⁺HSO₄⁻ is benzenediazonium hydrogensulphate.
Primary aliphatic amines give very unstable alkyldiazonium salts. Arenediazonium salts last for a short time in cold solution (273–278 K); resonance with the ring explains their extra stability.
Diazotisation is the conversion of a primary aromatic amine into its diazonium salt. Benzenediazonium chloride is made from aniline and nitrous acid (NaNO₂ + HCl) at 273–278 K: C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O.
Being unstable, the salt is usually used as soon as it is made rather than stored.
Benzenediazonium chloride is a colourless crystalline solid, readily soluble in water. It is stable in the cold, reacts with water on warming and decomposes easily when dry. Benzenediazonium fluoroborate is insoluble in water and stable at room temperature.

10. Replacing the diazonium group

NCERT §9.9

Diazonium reactions fall into two groups: those that displace nitrogen and those that keep the diazo group. The diazonium group is an excellent leaving group; the N₂ it forms escapes as a gas.
Sandmeyer reaction: with Cu(I) salts, Cl⁻, Br⁻ and CN⁻ replace the diazonium group: Cu₂Cl₂/HCl gives ArCl, Cu₂Br₂/HBr gives ArBr, CuCN/KCN gives ArCN.
Gatterman reaction (NCERT's spelling): the diazonium salt with the halogen acid and copper powder gives ArCl or ArBr. The Sandmeyer reaction gives the better yield.
Iodide: warming the diazonium salt solution with potassium iodide gives iodobenzene; iodine is hard to put on the ring directly.
Fluoride: fluoroboric acid precipitates the arenediazonium fluoroborate, which on heating gives the aryl fluoride, BF₃ and N₂.
Replacement by H: mild reducing agents, hypophosphorous acid (phosphinic acid) or ethanol, reduce the salt to the arene and are themselves oxidised to phosphorous acid and ethanal.
Replacement by –OH: if the diazonium solution warms up to 283 K, it hydrolyses to phenol, N₂ and HCl.
Replacement by –NO₂: the diazonium fluoroborate heated with aqueous NaNO₂ and copper gives the nitroarene.

11. Coupling and uses in synthesis

NCERT §9.9–9.10

Coupling reactions keep both nitrogens. The azo products join two aromatic rings through –N=N–, forming an extended conjugated system; they are often coloured and are used as dyes.
Benzenediazonium chloride couples with phenol at phenol's para position, in alkaline (OH⁻) solution, to give p-hydroxyazobenzene, an orange dye.
With aniline, in acid (H⁺), the product is p-aminoazobenzene, a yellow dye. Coupling is an electrophilic substitution on the phenol or amine ring.
Diazonium salts can introduce –F, –Cl, –Br, –I, –CN, –OH and –NO₂ onto a ring. Aryl fluorides and iodides cannot be made by direct halogenation, and –CN cannot replace Cl in chlorobenzene by substitution, but all come easily from the diazonium salt.
So diazonium chemistry reaches substituted aromatics that direct substitution on benzene or its derivatives cannot give, such as 1,3,5-tribromobenzene from aniline (brominate, diazotise, then replace by H).
Tertiary amines such as trimethylamine are used as insect attractants.

Must-know facts

Amine N: sp³, pyramidal, one lone pair; C–N–C in trimethylamine 108°.
1° RNH₂, 2° R₂NH, 3° R₃N. IUPAC: alkanamine; N-locants for groups on nitrogen; aniline = benzenamine.
Nitro → amine: H₂ with Ni/Pd/Pt or metal + acid; Fe scrap + HCl preferred (FeCl₂ regenerates HCl).
Ammonolysis: ethanolic NH₃, sealed tube, 373 K; gives a mixture up to R₄N⁺X⁻; large excess NH₃ favours 1°; RI > RBr > RCl.
Nitrile + LiAlH₄ or H₂/Ni → 1° amine, one carbon more than the halide.
Gabriel: 1° aliphatic amines only; aryl amines cannot be made (aryl halides resist substitution).
Hoffmann bromamide: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O; one carbon fewer.
b.p. (K), mass 72–74: n-butylamine 350.8 > diethylamine 329.3 > ethyldimethylamine 310.5 > 2-methylbutane 300.8; butan-1-ol 390.3.
Isomeric amine b.p.: 1° > 2° > 3° (N–H hydrogen bonding).
pKb NH₃ 4.75; aliphatic amines 3–4.22 (stronger bases); aniline 9.38 (weaker).
Gas phase: 3° > 2° > 1° > NH₃. Water: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ and (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.
Aniline: 5 resonance structures vs anilinium's 2 → weaker base than NH₃. EDG (–OCH₃, –CH₃) raise, EWG (–NO₂, –COOH, –X) lower basicity.
Acylation needs pyridine; aniline + (CH₃CO)₂O → acetanilide; benzoylation uses C₆H₅COCl.
Carbylamine (CHCl₃ + ethanolic KOH, heat): foul isocyanide, 1° amines only.
HNO₂: 1° aliphatic → alcohol + N₂ (quantitative); 1° aromatic at 273–278 K → diazonium salt.
Hinsberg (C₆H₅SO₂Cl): 1° product soluble in alkali; 2° insoluble; 3° no reaction. p-Toluenesulphonyl chloride now used.
Aniline + Br₂ water → 2,4,6-tribromoaniline (white ppt). Protect as acetanilide for monosubstitution.
Aniline nitration (288 K): 51% p, 47% m, 2% o, because anilinium ion is meta-directing.
Sulphonation at 453–473 K → sulphanilic acid; aniline gives no Friedel-Crafts reaction.
Diazotisation: ArNH₂ + NaNO₂ + 2HCl at 273–278 K; use at once; fluoroborate stable at room temperature.
Sandmeyer: Cu₂Cl₂/HCl, Cu₂Br₂/HBr, CuCN/KCN. Gatterman: Cu powder + HX. Sandmeyer yield is better.
KI → ArI; HBF₄ then heat → ArF; H₃PO₂ or ethanol → ArH; warm to 283 K → phenol; NaNO₂/Cu on fluoroborate → ArNO₂.
Coupling: with phenol (OH⁻) → p-hydroxyazobenzene (orange); with aniline (H⁺) → p-aminoazobenzene (yellow).

Common traps

Classifying an amine as primary, secondary or tertiary by the carbon that holds nitrogen, as for alcohols.

Amines are classified by how many groups are on nitrogen: (CH₃)₃CNH₂ is a primary amine even though its carbon is tertiary.

Making aniline by the Gabriel phthalimide synthesis.

Aryl halides do not undergo nucleophilic substitution with the phthalimide anion, so Gabriel gives only aliphatic primary amines.

Keeping the carbon count in the Hoffmann bromamide reaction.

The amine has one carbon fewer than the amide: butanamide gives propanamine, benzamide gives aniline.

Ranking aqueous basicity purely by the +I effect (3° > 2° > 1°).

That order holds only in the gas phase. In water solvation and steric effects give (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ and (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃.

Reading a larger pKb as a stronger base.

Smaller pKb means larger Kb and a stronger base: diethylamine (3.00) is stronger than ammonia (4.75), aniline (9.38) far weaker.

Expecting benzylamine to be as weak a base as aniline because both have a ring.

In phenylmethanamine the –NH₂ is on a CH₂, not the ring, so its lone pair is not delocalised: pKb 4.70 against aniline's 9.38.

Using the carbylamine test to pick out a secondary amine.

Only primary amines, aliphatic or aromatic, give the foul isocyanide. Use Hinsberg's reagent to separate 2° from 3°.

Saying the Hinsberg product of a secondary amine dissolves in alkali.

It has no H on nitrogen, so it is not acidic and stays insoluble. The primary amine's sulphonamide, with an acidic N–H, dissolves.

Expecting direct nitration of aniline to give only ortho and para products.

In strong acid aniline is protonated to the meta-directing anilinium ion: 51% para, 47% meta, 2% ortho, plus tarry oxidation products.

Running Friedel-Crafts alkylation or acylation on aniline.

Aniline forms a salt with AlCl₃; the positive nitrogen deactivates the ring and no reaction occurs.

Storing a diazonium salt or letting it warm up.

Arenediazonium chlorides last only a short time at 273–278 K; above about 283 K the solution hydrolyses to phenol. They are used at once.

Formulas

Base strength

pKb = −log Kb; Kb ratio = 10^(ΔpKb)

Ammonia (4.75) vs aniline (9.38): 10^4.63 ≈ 4.3 × 10⁴.

Hoffmann bromamide degradation

RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O

The amine has one carbon fewer than the amide.

Carbylamine reaction

R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O

Heat; primary amines only; foul-smelling isocyanide.

Diazotisation

C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

At 273–278 K; use the salt at once.

Nitrous acid on a 1° aliphatic amine

R–NH₂ + HNO₂ → [R–N₂⁺Cl⁻] → ROH + N₂ + HCl

NaNO₂ + HCl; N₂ released quantitatively.

Key terms

Amine: A derivative of ammonia with one or more H replaced by alkyl or aryl groups.
Ammonolysis: Cleavage of a C–X bond by ammonia, giving amines and finally a quaternary salt.
Quaternary ammonium salt: R₄N⁺X⁻, nitrogen carrying four carbon groups.
Gabriel phthalimide synthesis: Primary aliphatic amines from potassium phthalimide, an alkyl halide and alkaline hydrolysis.
Hoffmann bromamide degradation: Amide + Br₂ + NaOH → primary amine with one carbon fewer.
pKb: −log Kb; the smaller it is, the stronger the base.
Acylation: Replacing an H on nitrogen by an acyl group to give an amide.
Isocyanide (carbylamine): R–NC, the foul-smelling product of the carbylamine test.
Hinsberg's reagent: Benzenesulphonyl chloride, used to tell 1°, 2° and 3° amines apart.
Acetanilide: N-Phenylethanamide, aniline protected by acetylation.
Diazotisation: Converting a primary aromatic amine to its diazonium salt with HNO₂ at 273–278 K.
Sandmeyer reaction: Replacing –N₂⁺ by Cl, Br or CN using Cu(I) salts.
Coupling reaction: A diazonium salt joining a phenol or arylamine through –N=N– to form an azo dye.

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