CBSE Class 12 Chemistry Revision Notes Chapter 9 Amines

Amines are organic compounds formed by replacing hydrogen atoms of ammonia with alkyl or aryl groups.

In CBSE Class 12 Chemistry, this chapter explains amine structure, classification, preparation, basicity, reactions and diazonium salts.

Class 12 Chemistry Chapter 9 Amines is an important organic chemistry chapter. Amines are derivatives of ammonia and contain nitrogen as the main atom of the functional group. They are found in proteins, vitamins, alkaloids, hormones, dyes, medicines and many synthetic materials.

This chapter also covers diazonium salts, which are very useful intermediates in organic synthesis. Students should revise the chapter through structure, classification, preparation methods, basicity, tests and named reactions. Many board questions come from conversions, order of basic strength and differences between primary, secondary and tertiary amines.

Key Takeaways

• Amines: Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups.
• Classification: Amines are classified as primary, secondary and tertiary based on the number of alkyl or aryl groups attached to nitrogen.
• Basicity: Aliphatic amines are generally stronger bases than ammonia, while aromatic amines are weaker due to resonance.
• Diazonium Salts: Diazonium salts help prepare substituted aromatic compounds such as aryl halides, phenols, cyanobenzene and azo dyes.

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Access 30 Minutes Class 12 Chemistry Chapter 9 Amines Notes

Students can use these CBSE Class 12 Chemistry Revision Notes Chapter 9 for quick revision before tests and board exams. Amines is a reaction-heavy chapter, so it is better to revise it in small blocks.

Start with the structure and classification of amines. Then revise preparation methods like reduction of nitro compounds, ammonolysis, Gabriel phthalimide synthesis and Hoffmann bromamide degradation. After that, revise basicity, chemical tests and diazonium salt reactions.

Amines Class 12 Chemistry Chapter 9 Overview

Amines are organic compounds derived from ammonia. When one, two or three hydrogen atoms of ammonia are replaced by alkyl or aryl groups, amines are formed.

Amines are important because they are present in natural and synthetic compounds. Many medicines, dyes, polymers and biologically active substances contain amino groups.

Structure of Amines

In amines, nitrogen is trivalent and has one lone pair of electrons. The nitrogen atom is sp³ hybridised.

Due to this, amines have a pyramidal shape. The bond angle is slightly less than 109.5° because the lone pair repels the bond pairs. For example, trimethylamine has a pyramidal shape.

The lone pair on nitrogen makes amines basic and nucleophilic.

Classification of Amines

Amines are classified as primary, secondary and tertiary depending on how many hydrogen atoms of ammonia are replaced by alkyl or aryl groups.

If all alkyl or aryl groups are the same, the amine is called a simple amine. If the groups are different, it is called a mixed amine.

Aliphatic and Aromatic Amines

Amines may be aliphatic or aromatic.

In aliphatic amines, nitrogen is attached to alkyl groups. Example: CH₃NH₂.

In aromatic amines, nitrogen is directly attached to an aromatic ring. Example: C₆H₅NH₂, known as aniline.

Aniline is one of the most important aromatic amines in this chapter.

Nomenclature of Amines

In common names, aliphatic amines are named by writing the alkyl group before the word amine.

Example:

CH₃NH₂ = Methylamine

C₂H₅NH₂ = Ethylamine

In IUPAC naming, primary amines are named as alkanamines. The final “e” of alkane is replaced by “amine.”

For secondary and tertiary amines, the letter N is used to show groups attached to nitrogen.

Example:

CH₃NHCH₂CH₃ = N-methylethanamine

Preparation of Amines

Amines can be prepared by several methods. These methods are important for conversion-based questions.

Preparation of Amines by Reduction of Nitro Compounds

Nitro compounds are reduced to amines using hydrogen in the presence of nickel, palladium or platinum.

RNO₂ → RNH₂

Nitro compounds can also be reduced using metals in acidic medium.

For aromatic amines, nitrobenzene can be reduced to aniline.

C₆H₅NO₂ → C₆H₅NH₂

This is one of the most common methods for preparing aromatic amines.

Preparation of Amines by Ammonolysis of Alkyl Halides

Alkyl halides react with ethanolic ammonia to form amines. This reaction is called ammonolysis.

RX + NH₃ → RNH₂

The reaction takes place by nucleophilic substitution. The halogen atom is replaced by the amino group.

However, ammonolysis can give a mixture of primary, secondary and tertiary amines along with quaternary ammonium salt. To get primary amine as the major product, excess ammonia is used.

The order of reactivity of alkyl halides is:

RI > RBr > RCl

Preparation of Amines by Reduction of Nitriles

Nitriles are reduced to primary amines using lithium aluminium hydride or catalytic hydrogenation.

RCN → RCH₂NH₂

This method increases the carbon chain by one carbon atom. So it is also useful for ascent of the amine series.

Preparation of Amines by Reduction of Amides

Amides are reduced to amines using lithium aluminium hydride.

RCONH₂ → RCH₂NH₂

This reaction is useful because the carbonyl group of the amide is reduced to a methylene group.

Gabriel Phthalimide Synthesis

Gabriel phthalimide synthesis is used to prepare primary amines.

In this method, phthalimide reacts with ethanolic potassium hydroxide to form potassium phthalimide. This then reacts with an alkyl halide. On alkaline hydrolysis, a primary amine is formed.

This method gives only primary amines.

Important limitation:

Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution easily with the phthalimide anion.

Hoffmann Bromamide Degradation Reaction

Hoffmann bromamide degradation is used to prepare primary amines from amides.

In this reaction, an amide is treated with bromine and sodium hydroxide.

RCONH₂ → RNH₂

The amine formed has one carbon atom less than the starting amide.

Example:

Butanamide gives propanamine.

This reaction is important because it is a step-down reaction.

Physical Properties of Amines

Lower aliphatic amines are gases and have a fishy smell. Primary amines with three or more carbon atoms are liquids, while higher amines are solids.

Aniline and other arylamines are usually colourless, but they may become coloured on storage due to atmospheric oxidation.

Lower amines are soluble in water because they form hydrogen bonds with water. Solubility decreases as molar mass increases because the hydrophobic alkyl part becomes larger.

Boiling Points of Amines

Primary and secondary amines show intermolecular hydrogen bonding because they contain N-H bonds.

Tertiary amines do not show intermolecular hydrogen bonding because they do not have N-H bonds.

For isomeric amines, the order of boiling point is:

Primary amine > Secondary amine > Tertiary amine

Amines have lower boiling points than alcohols of similar molar mass because nitrogen is less electronegative than oxygen and forms weaker hydrogen bonds.

Basic Character of Amines

Amines are basic because nitrogen has a lone pair of electrons. This lone pair can accept a proton.

RNH₂ + H⁺ → RNH₃⁺

Amines react with acids to form ammonium salts.

RNH₂ + HCl → RNH₃⁺Cl⁻

The more easily an amine donates its lone pair, the stronger its basic character.

pKb Values and Basic Strength

Basicity can be compared using Kb and pKb values.

Higher Kb means stronger base.

Lower pKb means stronger base.

So, a compound with smaller pKb value is more basic.

Aliphatic Amines vs Ammonia

Aliphatic amines are stronger bases than ammonia.

This happens because alkyl groups show +I effect. They push electron density towards nitrogen, making the lone pair more available for protonation.

So, alkylamines are generally stronger bases than ammonia.

Basicity of Aliphatic Amines in Aqueous Solution

The basic strength of aliphatic amines in water depends on three factors:

1. +I effect of alkyl groups
2. Solvation of the ammonium ion
3. Steric hindrance

Because of these factors, the order is not always simple.

For ethyl-substituted amines:

(C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃

For methyl-substituted amines:

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

Why Aniline Is Less Basic Than Ammonia

Aniline is less basic than ammonia because the lone pair of electrons on nitrogen is involved in resonance with the benzene ring.

Due to this delocalisation, the lone pair is less available for accepting a proton.

So, aromatic amines like aniline are weaker bases than ammonia.

Electron-releasing groups such as -CH₃ and -OCH₃ increase the basic strength of aniline derivatives. Electron-withdrawing groups such as -NO₂, -COOH and -SO₃H decrease the basic strength.

Chemical Reactions of Amines

Amines react due to the lone pair on nitrogen. They behave as bases and nucleophiles.

Important reactions include:

• Salt formation with acids
• Alkylation
• Acylation
• Carbylamine reaction
• Reaction with nitrous acid
• Hinsberg test
• Electrophilic substitution in aromatic amines

Alkylation of Amines

Amines react with alkyl halides to form higher amines.

Primary amines can give secondary amines, secondary amines can give tertiary amines, and tertiary amines can give quaternary ammonium salts.

This is why alkylation often gives a mixture of products.

Acylation of Amines

Primary and secondary amines react with acid chlorides, anhydrides and esters to form amides.

This reaction is called acylation.

RNH₂ + R’COCl → R’CONHR + HCl

The reaction is carried out in the presence of a base such as pyridine, which removes HCl.

Benzoylation of Amines

When amines react with benzoyl chloride, the reaction is called benzoylation.

For example, methanamine reacts with benzoyl chloride to form N-methylbenzamide.

This is also an acylation reaction.

Carbylamine Reaction

Primary amines react with chloroform and alcoholic potassium hydroxide on heating to form isocyanides or carbylamines.

These products have a very foul smell.

This reaction is called carbylamine reaction or isocyanide test.

Important point:

Only primary amines give this test. Secondary and tertiary amines do not give this test.

Reaction of Amines with Nitrous Acid

Nitrous acid is prepared in situ using sodium nitrite and mineral acid.

Different types of amines react differently with nitrous acid.

Primary Aliphatic Amines

Primary aliphatic amines react with nitrous acid to form unstable aliphatic diazonium salts. These decompose to give alcohols and nitrogen gas.

RNH₂ + HNO₂ → ROH + N₂ + H₂O

Primary Aromatic Amines

Primary aromatic amines react with nitrous acid at 273-278 K to form diazonium salts.

Example:

Aniline gives benzenediazonium chloride.

This reaction is called diazotisation.

Hinsberg Test

Hinsberg test is used to distinguish primary, secondary and tertiary amines.

The reagent used is benzenesulphonyl chloride.

This test is important for identifying different types of amines.

Electrophilic Substitution in Aniline

Aniline is highly reactive towards electrophilic substitution because -NH₂ is an activating group.

The -NH₂ group increases electron density at ortho and para positions. So it is ortho- and para-directing.

Bromination of Aniline

Aniline reacts with bromine water at room temperature to give 2,4,6-tribromoaniline.

This product appears as a white precipitate.

Because aniline is highly activated, substitution occurs at both ortho and para positions.

Nitration of Aniline

Direct nitration of aniline gives a mixture of products and tarry oxidation products.

In strong acidic medium, aniline gets protonated to form anilinium ion. The anilinium ion is meta-directing.

To get mainly para product, the -NH₂ group is first protected by acetylation. Then nitration is carried out, followed by hydrolysis.

Sulphonation of Aniline

Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate.

On heating, it gives p-aminobenzene sulphonic acid, commonly known as sulphanilic acid.

Why Aniline Does Not Undergo Friedel-Crafts Reaction

Aniline does not undergo Friedel-Crafts alkylation or acylation.

This is because aniline forms a salt with aluminium chloride, which is a Lewis acid. Due to this, nitrogen gets a positive charge and the benzene ring becomes strongly deactivated.

Diazonium Salts in Class 12 Chemistry Chapter 9

Diazonium salts have the general formula:

ArN₂⁺X⁻

Here, Ar is an aryl group and X⁻ may be Cl⁻, Br⁻, HSO₄⁻ or BF₄⁻.

Benzenediazonium chloride is an important diazonium salt.

Diazonium salts are useful because the diazonium group can be replaced by many other groups.

Preparation of Diazonium Salts

Benzenediazonium chloride is prepared by treating aniline with nitrous acid at 273-278 K.

Nitrous acid is prepared in the reaction mixture using sodium nitrite and hydrochloric acid.

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

This reaction is called diazotisation.

Diazonium salts are usually used immediately because they are not very stable.

Physical Properties of Diazonium Salts

Benzenediazonium chloride is a colourless crystalline solid. It is soluble in water and stable in cold conditions.

It reacts with water when warmed and decomposes easily in the dry state.

Benzenediazonium fluoroborate is more stable and water-insoluble.

Chemical Reactions of Diazonium Salts

Diazonium salts undergo two main types of reactions:

1. Reactions involving displacement of nitrogen
2. Reactions involving retention of diazo group

Sandmeyer Reaction

In Sandmeyer reaction, the diazonium group is replaced by Cl, Br or CN in the presence of cuprous salts.

Examples:

ArN₂⁺Cl⁻ → ArCl

ArN₂⁺Cl⁻ → ArBr

ArN₂⁺Cl⁻ → ArCN

This reaction is useful for preparing chloro, bromo and cyano aromatic compounds.

Gattermann Reaction

In Gattermann reaction, chlorine or bromine is introduced into the benzene ring by treating diazonium salt with the corresponding halogen acid in the presence of copper powder.

Sandmeyer reaction usually gives better yield than Gattermann reaction.

Replacement by Iodide Ion

When diazonium salt reacts with potassium iodide, iodobenzene is formed.

ArN₂⁺Cl⁻ + KI → ArI + KCl + N₂

This is useful because iodine is not easily introduced directly into the benzene ring.

Replacement by Fluoride Ion

When arenediazonium chloride is treated with fluoroboric acid, diazonium fluoroborate is formed.

On heating, it gives aryl fluoride.

This reaction is useful for preparing fluorobenzene.

Replacement by Hydroxyl Group

When diazonium salt solution is warmed, it gets hydrolysed to phenol.

ArN₂⁺Cl⁻ + H₂O → ArOH + N₂ + HCl

So diazonium salts can be used to prepare phenols.

Replacement by Hydrogen

Diazonium salts can be reduced to arenes using mild reducing agents like hypophosphorous acid or ethanol.

ArN₂⁺Cl⁻ → ArH

This is useful when the amino group is used temporarily and later removed.

Coupling Reactions of Diazonium Salts

Diazonium salts react with phenols or aromatic amines to form azo compounds.

These reactions are called coupling reactions.

For example, benzenediazonium chloride reacts with phenol to form p-hydroxyazobenzene.

It also reacts with aniline to form p-aminoazobenzene.

Azo compounds are often coloured and used as dyes.

Importance of Diazonium Salts in Organic Synthesis

Diazonium salts are very important because they help introduce many groups into the aromatic ring.

The diazonium group can be replaced by:

• -Cl
• -Br
• -I
• -F
• -CN
• -OH
• -NO₂
• -H

Some of these groups cannot be introduced easily by direct substitution. So diazonium salts are very useful intermediates in aromatic chemistry.

Important Named Reactions in Amines

Quick Revision Table for Amines

Common Mistakes in Class 12 Chemistry Chapter 9

Students often think tertiary amines have the highest boiling point, but tertiary amines do not have N-H bonds, so they cannot form intermolecular hydrogen bonds with each other.

Another common mistake is writing the same basicity order for all amines. In aqueous solution, solvation and steric effects also matter.

Students also confuse Gabriel phthalimide synthesis and Hoffmann bromamide degradation. Gabriel synthesis prepares primary aliphatic amines, while Hoffmann bromamide degradation gives amines with one carbon less than the starting amide.

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