Unit 16

Organic Chemistry

Class 12 Chemistry

Amines

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NEB/CDC syllabus scope: Unit 16 is a 7-teaching-hour chapter covering aliphatic amines and the aromatic amine aniline. Required topics include introduction, nomenclature, classification and isomerism; separation of primary, secondary and tertiary amines by Hoffmann’s method; preparation of primary amines from haloalkanes, nitriles, nitroalkanes and amides; physical properties; basicity and comparison of 1°, 2° and 3° amines; reactions of primary amines with chloroform, concentrated HCl, R–X, RCOX and nitrous acid; nitrous-acid test of 1°, 2° and 3° amines; preparation of aniline from nitrobenzene and phenol; basicity of aniline; alkylation, acylation, diazotization, carbylamine and coupling reactions; electrophilic substitution—nitration, sulphonation and bromination; and uses of aniline.

1. Introduction to Amines

Definition Amines are organic derivatives of ammonia, NH₃, in which one or more hydrogen atoms are replaced by alkyl and/or aryl groups.

Primary amine

R–NH₂

Secondary amine

R₂NH

Tertiary amine

R₃N

Aromatic amine

Ar–NH₂

The nitrogen atom of an amine possesses a lone pair of electrons. This lone pair is responsible for the basic and nucleophilic character of amines.

Diagram 1: Primary, secondary and tertiary amines

2. Classification of Amines

2.1 Based on Number of Organic Groups on Nitrogen

Type	General formula	Example
Primary (1°)	RNH₂	C₂H₅NH₂
Secondary (2°)	R₂NH	(C₂H₅)₂NH
Tertiary (3°)	R₃N	(C₂H₅)₃N

2.2 Aliphatic and Aromatic Amines

Aliphatic amine: nitrogen bonded to an alkyl group, e.g. CH₃NH₂.
Aromatic amine: nitrogen directly bonded to an aromatic ring, e.g. C₆H₅NH₂ (aniline).

Aniline vs Benzylamine C₆H₅NH₂ is an aromatic amine because N is directly attached to the ring. C₆H₅CH₂NH₂ is benzylamine, an aliphatic-type primary amine because –NH₂ is on a side-chain sp³ carbon.

3. Nomenclature and Isomerism

3.1 Common Names

Name the alkyl group(s) attached to nitrogen followed by “amine”.

3.2 IUPAC Names

Use the suffix –amine with the parent hydrocarbon. Substituents directly attached to nitrogen are shown using the prefix N-.

Formula	IUPAC name	Common name
CH₃NH₂	Methanamine	Methylamine
CH₃CH₂NH₂	Ethanamine	Ethylamine
(CH₃)₂NH	N-Methylmethanamine	Dimethylamine
CH₃NHCH₂CH₃	N-Methylethanamine	Ethylmethylamine
C₆H₅NH₂	Benzenamine	Aniline

3.3 Isomerism

Amines may show chain, position, metamerism and functional/class isomerism among 1°, 2° and 3° amines where molecular formula permits.

Example: C₃H₉N CH₃CH₂CH₂NH₂ = propan-1-amine CH₃CH(NH₂)CH₃ = propan-2-amine CH₃NHCH₂CH₃ = N-methylethanamine (CH₃)₃N = trimethylamine

4. Separation of 1°, 2° and 3° Amines by Hoffmann’s Method

A mixture of primary, secondary and tertiary amines is treated with diethyl oxalate. The three classes behave differently because primary and secondary amines contain replaceable N–H hydrogen, while tertiary amines do not.

Amine	Reaction with diethyl oxalate	Nature of product
1° amine, RNH₂	Forms N,N′-dialkyloxamide	Solid
2° amine, R₂NH	Forms dialkyl oxamic ester	Liquid
3° amine, R₃N	No reaction	Remains unchanged

2RNH₂ + C₂H₅OCO–COOC₂H₅ → RNHCO–CONHR + 2C₂H₅OH R₂NH + C₂H₅OCO–COOC₂H₅ → R₂NCO–COOC₂H₅ + C₂H₅OH

The products are separated using differences in physical state/solubility, and the derivatives can be hydrolysed to regenerate the corresponding amines.

Diagram 2: Hoffmann method of amine separation

5. Preparation of Primary Amines

5.1 From Haloalkanes

Haloalkanes react with excess alcoholic ammonia to form primary amines. Excess NH₃ helps reduce further alkylation.

R–X + 2NH₃ → RNH₂ + NH₄X

Example

C₂H₅Br + 2NH₃ → C₂H₅NH₂ + NH₄Br

5.2 From Nitriles

R–C≡N + 4[H] → R–CH₂NH₂

Reduction may be achieved by catalytic hydrogenation or suitable hydride/reducing systems.

5.3 From Nitroalkanes

RNO₂ + 6[H] → RNH₂ + 2H₂O

5.4 From Amides — Hofmann Bromamide Reaction

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

Carbon-count rule Hofmann bromamide degradation gives an amine containing one carbon atom fewer than the starting amide.

Diagram 3: Preparation routes of primary amines

6. Physical Properties of Aliphatic Amines

Property	General trend / reason
State	Lower amines are gases or volatile liquids; higher members become liquids/solids.
Odour	Lower amines often have strong ammonia-like or fishy odours.
Polarity	Amines are polar because C–N and N–H bonds are polar and nitrogen has a lone pair.
Hydrogen bonding	1° and 2° amines can form intermolecular N–H hydrogen bonds; 3° amines cannot donate N–H hydrogen bonds.
Boiling point	Generally higher than comparable hydrocarbons but lower than corresponding alcohols.
Water solubility	Lower amines are soluble because they can hydrogen-bond with water; solubility decreases with larger alkyl groups.

Diagram 4: Intermolecular hydrogen bonding in amines

7. Basic Nature of Amines

Why amines are basic The nitrogen lone pair can accept a proton. Thus amines behave as Brønsted–Lowry bases and Lewis bases.

RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻ RNH₂ + HCl → RNH₃⁺Cl⁻

7.1 Comparative Basicity of 1°, 2° and 3° Aliphatic Amines

Alkyl groups donate electron density toward nitrogen (+I effect), tending to increase basicity. In aqueous solution, however, solvation and steric effects also matter.

School-level aqueous trend for lower alkyl amines 2° amine > 1° amine > 3° amine > NH₃

This order is a useful Grade 12 approximation for common lower alkyl amines in water. Exact basicity can vary with structure, solvent and substituents.

7.2 Gas-Phase / Pure Electron-Donation Idea

If only electron donation by alkyl groups is considered, more alkyl groups tend to increase electron density at nitrogen. In solution, hydration/steric effects modify this simple trend.

Diagram 5: Main factors controlling amine basicity

8. Chemical Reactions of Primary Aliphatic Amines

8.1 With Chloroform — Carbylamine Reaction

Primary amines react with chloroform and alcoholic KOH on heating to form isocyanides (carbylamines).

RNH₂ + CHCl₃ + 3KOH → RNC + 3KCl + 3H₂O

Safety Isocyanides have extremely unpleasant odours and chloroform is hazardous. This is an exam reaction, not a home experiment.

8.2 With Concentrated HCl

RNH₂ + HCl → RNH₃Cl

An ammonium salt is formed.

8.3 Alkylation with R–X

RNH₂ + R′X → RR′NH + HX

Further alkylation can produce tertiary amines and finally quaternary ammonium salts.

8.4 Acylation with RCOX

RNH₂ + R′COCl → R′CONHR + HCl

A substituted amide is formed.

8.5 With Nitrous Acid

Primary aliphatic amines form unstable aliphatic diazonium intermediates that decompose to alcohols with evolution of nitrogen gas.

RNH₂ + HNO₂ → ROH + N₂↑ + H₂O

Diagram 6: Major syllabus reactions of primary amines

9. Nitrous Acid Test for 1°, 2° and 3° Amines

Nitrous acid is prepared in situ using sodium nitrite and hydrochloric acid:

NaNO₂ + HCl → HNO₂ + NaCl

Amine class	Reaction with HNO₂	Observation
1° aliphatic, RNH₂	Forms unstable diazonium intermediate → alcohol	Brisk evolution of N₂ gas
2° aliphatic, R₂NH	Forms N-nitrosamine	Yellow/oily nitrosamine, no N₂ evolution
3° aliphatic, R₃N	Mainly forms soluble ammonium nitrite-type salt under acidic conditions	No N₂; no characteristic nitrosamine oil of 2° amine

RNH₂ + HNO₂ → ROH + N₂↑ + H₂O R₂NH + HNO₂ → R₂N–N=O + H₂O

Diagram 7: Distinguishing amines with nitrous acid

10. Aromatic Amine: Aniline

Aniline Aniline (C₆H₅NH₂) is the simplest important aromatic primary amine. The –NH₂ group is bonded directly to a benzene ring.

10.1 Preparation from Nitrobenzene

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Typical reducing systems include Sn/HCl or Fe/HCl; the initially formed anilinium salt is treated with base to liberate aniline.

10.2 Preparation from Phenol

C₆H₅OH + NH₃ → C₆H₅NH₂ + H₂O

This conversion requires suitable catalytic/high-temperature industrial conditions.

Diagram 8: Two syllabus routes to aniline

10.3 Physical Properties of Aniline

Pure aniline is a colourless to pale oily liquid but darkens on exposure to air/light because of oxidation impurities.
It has a characteristic odour and is toxic.
It is only slightly soluble in water but dissolves in many organic solvents.
It has a relatively high boiling point because of polarity and N–H hydrogen bonding.

Safety Aniline is toxic and can be absorbed through skin. Odour should never be used as a handling or identification method.

11. Basicity of Aniline

Aniline is a weaker base than common aliphatic amines and weaker than ammonia in water.

Typical basicity: aliphatic amine > NH₃ > aniline

Why Is Aniline Less Basic?

The nitrogen lone pair is conjugated with the benzene ring and becomes delocalized by resonance. It is therefore less available to accept H⁺.

Diagram 9: Resonance lowers aniline basicity

12. Chemical Properties of Aniline

12.1 Salt Formation

C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻

12.2 Alkylation

C₆H₅NH₂ + CH₃I → C₆H₅NHCH₃ + HI

Further alkylation can occur.

12.3 Acylation

C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ + HCl

The product is acetanilide.

12.4 Carbylamine Reaction

C₆H₅NH₂ + CHCl₃ + 3KOH → C₆H₅NC + 3KCl + 3H₂O

Diagram 10: Salt formation, alkylation, acylation and carbylamine reaction

13. Diazotization of Aniline

Diazotization Conversion of a primary aromatic amine into a diazonium salt using nitrous acid at low temperature is called diazotization.

Nitrous acid is produced in situ from NaNO₂ and HCl. The temperature is maintained at about 0–5 °C because benzenediazonium salts decompose more readily when warmed.

NaNO₂ + HCl → HNO₂ + NaCl C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

Diagram 11: Formation of benzenediazonium chloride

14. Azo Coupling Reaction

Benzenediazonium chloride acts as a weak electrophile and couples with strongly activated aromatic rings such as phenol or aniline, producing coloured azo compounds containing –N=N–.

14.1 Coupling with Phenol

C₆H₅N₂⁺Cl⁻ + C₆H₅OH → p-HOC₆H₄–N=N–C₆H₅ + HCl

Coupling with phenol is performed in alkaline medium; para coupling is usually favoured when the para position is free.

14.2 Coupling with Aniline

C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → p-H₂NC₆H₄–N=N–C₆H₅ + HCl

Diagram 12: Coupling reactions of benzenediazonium salt

15. Electrophilic Substitution Reactions of Aniline

The –NH₂ group strongly donates electron density into the aromatic ring by resonance. It is therefore a strongly activating, ortho/para-directing group.

15.1 Bromination

Aniline reacts rapidly with bromine water to give a white precipitate of 2,4,6-tribromoaniline.

C₆H₅NH₂ + 3Br₂ → 2,4,6-C₆H₂Br₃NH₂↓ + 3HBr

15.2 Sulphonation

With concentrated H₂SO₄, aniline initially forms anilinium hydrogen sulphate. On strong heating, rearrangement/substitution gives mainly sulphanilic acid (p-aminobenzenesulphonic acid).

C₆H₅NH₂ → p-H₂NC₆H₄SO₃H (conc. H₂SO₄, heat)

15.3 Nitration

Direct nitration of aniline in strongly acidic nitrating mixture is complicated because –NH₂ is protonated to –NH₃⁺, which is deactivating and meta directing. In preparative chemistry, the amino group is often protected by acylation before nitration.

Exam Important Free –NH₂ is strongly activating and ortho/para directing, but in strongly acidic nitrating medium aniline becomes protonated. This is why controlled nitration is commonly discussed through protection as acetanilide.

Diagram 13: Required electrophilic substitutions of aniline

16. Uses of Aniline

Important intermediate in the manufacture of dyes and pigments, especially azo-dye chemistry.
Used in the synthesis of pharmaceuticals and fine chemicals.
Used in manufacture of rubber-processing chemicals.
Important feedstock for polyurethane-related industrial chemicals.
Used as an intermediate for numerous aromatic nitrogen compounds.

Safety Note Aniline is toxic and can affect blood oxygen transport after significant exposure. Industrial usefulness does not imply that it is safe for direct handling without appropriate controls.

17. High-Yield Reaction Summary

Topic	Reaction / result	Key point
Haloalkane → amine	RX + 2NH₃ → RNH₂ + NH₄X	Use excess NH₃
Nitrile reduction	RCN → RCH₂NH₂	Adds/retains nitrile carbon
Nitroalkane reduction	RNO₂ → RNH₂	Primary amine
Hofmann bromamide	RCONH₂ → RNH₂	One fewer carbon
Basicity	RNH₂ + H⁺ → RNH₃⁺	Lone pair accepts proton
Carbylamine	1° amine + CHCl₃/KOH → RNC	Test for 1° amines
Acylation	RNH₂ + R′COCl → R′CONHR	Amide formation
1° amine + HNO₂	→ ROH + N₂	Gas evolution
2° amine + HNO₂	→ N-nitrosamine	Yellow oily product
Nitrobenzene → aniline	C₆H₅NO₂ + 6[H] → C₆H₅NH₂	Reduction
Diazotization	Aniline + NaNO₂/HCl → ArN₂⁺Cl⁻	0–5 °C
Azo coupling	ArN₂⁺ + activated ring → Ar–N=N–Ar′	Coloured products
Bromination of aniline	+ 3Br₂ → 2,4,6-tribromoaniline	White ppt.
Sulphonation	Aniline → sulphanilic acid	Conc. H₂SO₄ / heat

18. Common Exam Mistakes

Classifying an amine by the carbon bearing nitrogen rather than by the number of organic groups attached directly to N.
Confusing aniline, C₆H₅NH₂, with benzylamine, C₆H₅CH₂NH₂.
Forgetting that tertiary amines contain no N–H bond.
Writing that all 1°, 2° and 3° amines react identically with diethyl oxalate in Hoffmann separation.
Forgetting the one-carbon loss in Hofmann bromamide degradation.
Assuming a single basicity order is universal in all solvents. Solvation and steric effects matter.
Writing aniline as more basic than methylamine. Resonance makes aniline much less basic.
Applying the carbylamine test to secondary or tertiary amines. It is characteristic of primary amines.
Forgetting nitrogen evolution when a primary aliphatic amine reacts with nitrous acid.
Confusing diazotization of aniline with the reaction of a primary aliphatic amine; aromatic diazonium salts are sufficiently stable at 0–5 °C for further reactions.
Forgetting low temperature in diazotization.
Writing azo coupling without the –N=N– linkage.
Writing –NH₂ as meta directing in ordinary neutral aniline. Free –NH₂ is strongly ortho/para directing.
Ignoring protonation during direct nitration of aniline in strongly acidic medium.
Writing bromination of aniline as simple monobromination in bromine water; the normal syllabus product is 2,4,6-tribromoaniline.

19. Worked Examples

Worked Example 1: Classify (CH₃)₂NH

Nitrogen is attached to two methyl groups and one H.

Answer: secondary (2°) amine.

Worked Example 2: Prepare ethylamine from bromoethane C₂H₅Br + 2NH₃ → C₂H₅NH₂ + NH₄Br

Use excess ammonia to favour the primary amine.

Worked Example 3: Predict Hofmann bromamide product

Starting amide: CH₃CH₂CONH₂ (propanamide).

CH₃CH₂CONH₂ → CH₃CH₂NH₂

Product: ethylamine; the product has one fewer carbon atom.

Worked Example 4: Identify amine by HNO₂ test

An unknown aliphatic amine gives brisk N₂ gas with NaNO₂/HCl.

Inference: it is a primary aliphatic amine.

Worked Example 5: Why is aniline weakly basic?

The nitrogen lone pair is delocalized into the aromatic π system. Protonation would remove this lone pair from conjugation, so aniline has less tendency to accept H⁺ than an aliphatic amine.

Worked Example 6: Aniline → diazonium salt C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

Maintain 0–5 °C.

20. Important Exam Questions

Short-Answer Questions

Define amine and classify 1°, 2° and 3° amines.
Differentiate aliphatic and aromatic amines.
Give IUPAC names of CH₃NH₂, C₂H₅NH₂ and CH₃NHCH₂CH₃.
State the principle of Hoffmann’s method of separation.
What does a primary amine form with diethyl oxalate?
Why does a tertiary amine not react in Hoffmann’s separation?
How is ethylamine prepared from bromoethane?
How is a primary amine prepared from a nitrile?
How is a primary amine prepared from a nitroalkane?
State Hofmann bromamide reaction.
Why are amines basic?
Compare basicity of lower 1°, 2° and 3° alkyl amines in water.
What is the carbylamine reaction?
Write the reaction of a primary amine with HCl.
What is acylation of an amine?
What happens when a primary aliphatic amine reacts with nitrous acid?
How does a secondary amine react with nitrous acid?
How is aniline prepared from nitrobenzene?
How is aniline prepared from phenol?
Why is aniline less basic than ammonia?
What is diazotization?
Why is diazotization carried out at 0–5 °C?
What is azo coupling?
What is formed when aniline reacts with bromine water?
What is sulphanilic acid?
State important uses of aniline.

Long-Answer Questions

Explain nomenclature, classification and isomerism of aliphatic amines.
Describe separation of 1°, 2° and 3° amines by Hoffmann’s method.
Describe four methods of preparing primary amines.
Explain physical properties and basic nature of aliphatic amines.
Discuss factors affecting the comparative basicity of 1°, 2° and 3° amines.
Explain reactions of primary amines with CHCl₃, HCl, R–X, RCOX and HNO₂.
Describe nitrous-acid test for 1°, 2° and 3° amines.
Describe preparation and physical properties of aniline.
Explain why aniline is less basic than aliphatic amines and ammonia.
Explain alkylation, acylation, diazotization and carbylamine reactions of aniline.
Explain coupling reaction of benzenediazonium chloride with phenol or aniline.
Describe nitration, sulphonation and bromination of aniline.

Conversion Questions

Bromoethane → ethylamine.
Ethanenitrile → ethylamine.
Nitroethane → ethylamine.
Propanamide → ethylamine.
Nitrobenzene → aniline.
Phenol → aniline.
Aniline → acetanilide.
Aniline → benzenediazonium chloride.
Benzenediazonium chloride + phenol → azo dye.
Aniline → 2,4,6-tribromoaniline.
Aniline → sulphanilic acid.

Diagram Questions

Draw classification of 1°, 2° and 3° amines.
Draw Hoffmann’s separation flowchart.
Draw four preparation routes of primary amines.
Draw hydrogen bonding in primary amines.
Draw factors controlling amine basicity.
Draw reaction map of primary amines.
Draw nitrous-acid test flowchart.
Draw preparation routes to aniline.
Draw resonance explanation of aniline basicity.
Draw aniline reaction map.
Draw diazotization reaction.
Draw azo coupling reaction.
Draw electrophilic substitution map of aniline.

Exam Strategy Split the chapter into aliphatic amines and aniline. For aliphatic amines, master Hoffmann separation, four preparations, basicity and nitrous-acid test. For aniline, master resonance/basicity, diazotization, coupling and electrophilic substitution.

21. One-Minute Revision

Amines are derivatives of NH₃ with alkyl/aryl groups replacing H.
1° = RNH₂; 2° = R₂NH; 3° = R₃N.
Aniline is C₆H₅NH₂; benzylamine is C₆H₅CH₂NH₂.
Hoffmann separation uses diethyl oxalate.
1° amine gives a solid oxamide derivative; 2° gives a liquid oxamic ester; 3° does not react.
Haloalkane + excess NH₃ gives primary amine.
RCN reduction gives RCH₂NH₂.
RNO₂ reduction gives RNH₂.
Hofmann bromamide converts RCONH₂ to RNH₂ with one fewer carbon.
Amines are basic because nitrogen has a lone pair.
Typical lower aqueous alkyl-amine order: 2° > 1° > 3° > NH₃.
Primary amines give carbylamines with CHCl₃/KOH.
Primary amine + HNO₂ gives alcohol + N₂ gas.
Secondary amine + HNO₂ gives N-nitrosamine.
Aniline is prepared by reduction of nitrobenzene or from phenol + NH₃ under suitable conditions.
Aniline is less basic because its lone pair is delocalized into the benzene ring.
Aniline + CH₃COCl gives acetanilide.
Diazotization uses NaNO₂/HCl at 0–5 °C.
Benzenediazonium chloride couples with phenol/aniline to form azo compounds.
Free –NH₂ strongly activates benzene and is ortho/para directing.
Aniline + bromine water gives white 2,4,6-tribromoaniline.
Aniline + conc. H₂SO₄/heat gives sulphanilic acid.

22. Diagram Practice

Students should practice these labelled diagrams for the NEB examination:

Primary, secondary and tertiary amines.
Hoffmann separation using diethyl oxalate.
Four routes for preparation of primary amines.
Hydrogen bonding in amines.
Factors affecting amine basicity.
Reaction map of primary amines.
Nitrous-acid test of 1°, 2° and 3° amines.
Preparation of aniline from nitrobenzene and phenol.
Resonance explanation of aniline basicity.
Reaction map of aniline.
Diazotization of aniline.
Azo coupling with phenol/aniline.
Electrophilic substitution reactions of aniline.

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