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NCERT Notes for Class 12 Chapter 9 Amines: Download PDF

Students can download the NCERT Notes Class 12 Chemistry Chapter 9 Amines pdf from the icon given below to access a clear explanation, important reactions of this chapter. These NCERT notes cover all the key concepts of the Amines.

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NCERT Notes for Class 12 Chapter 9 Amines

Amines Class 12 Chapter 9 notes offer a clear and concise explanation of important concepts, reactions, and properties. These notes help students strengthen their understanding and prepare efficiently for board and competitive exams. Scroll down to access these notes.

Amines

When hydrogens in ammonia are replaced by one or more alkyl or aryl groups, amines are formed. They are derivatives of ammonia.

Structure of amines

• Hybridization: sp³
• Geometry: pyramidal
• No. of lone pairs: 1
• Bond angle: less than 109.5°

Classification of amines

On the basis of the number of hydrogen atoms replaced by alkyl or aryl groups in the ammonia molecule amines can be of 3 types:

• Primary
• Secondary
• Tertiary

Nomenclature

Amines are named alkanamines in the IUPAC system. Di, tri, etc prefixes are used depending on the no. of -NH₂ group attached to the carbon chain.

Examples are given in the tabular form:

Preparation of Amine

Preparation involves various methods to synthesise amines from different starting materials. This topic covers important reactions, techniques, and examples essential for preparation of amines. Students can refer to NCERT Solutions for Class 12 Chemistry Chapter 9 Amines to understand these concepts better through a series of solved question.

Given below the various methods

1. Reduction of nitro compounds

Nitro compounds are reduced to amines in the presence of finely divided Ni, Pd, or Pt & by passing hydrogen gas. Amines can also be prepared by reduction with metals in an acidic medium.

2. Ammonolysis of alkyl halides

Ammonia undergoes a nucleophilic substitution reaction in which a halogen atom is replaced by an amino group. The process by which ammonia molecules cleave the C-X bond is called ammonolysis.

The reactivity order of halides with amines: RI > RBr >RCl

3. Reduction of nitriles

Nitriles on catalytic hydrogenation or reduction with lithium aluminum hydride produce primary amines.

4. Reduction of amides

Amides undergo reduction to produce amines.

5. Gabriel phthalimide synthesis

Gabriel phthalimide synthesis is a reaction that involves the conversion of primary alkyl halides into primary amines using alkyl halides.

6. Hoffmann bromamide degradation reaction

Physical Properties of Amines:

• Primary amines having 3 or more C are liquid & heavier ones are solid.
• Aryl amines are generally colorless but on atmospheric oxidation gain color.
• Amines are polar molecules due to the presence of lone pairs which induce large dipole moments.
• The primary & secondary amines can undergo H bonding. The tertiary amine cannot undergo H bonding but can accept H bonds from N-H or O-H bonds.

• In amines solubility decrease as the molar mass of amines increases. This is because the hydrophobic alkyl part increases in size. Amines with high molecular mass are insoluble in water.
• Boiling point : Ethers (similar molecular weights) < Primary & secondary amine < alcohols ; Primary ≈ Secondary > Tertiary amines

Chemical Reaction of Amines

Amines are reactive due to the electronegativity difference between N and H, as well as the presence of lone pairs. You can download Amines Class 12 Chapter 9 notes for offline access, making it easier to study.

1. Basic character of amines

Amines react with acid to form salts.

Structure-basicity relationship of amines

(a) Alkanamines versus ammonia

The basicity order of amines in the gaseous phase: 3° amine > 2° amine > 1° amine > ammonia

The basicity order of aliphatic amines: 1° > 2° > 3°

The order of basic strength in case of substituted amines:

$\begin{aligned} & \left(\mathrm{C}_2 \mathrm{H}_5\right)_2 \mathrm{NH}>\left(\mathrm{C}_2 \mathrm{H}_5\right)_3 \mathrm{~N}>\mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2>\mathrm{NH}_3 \\ & \left(\mathrm{CH}_3\right)_2 \mathrm{NH}>\mathrm{CH}_3 \mathrm{NH}_2>\left(\mathrm{CH}_3\right)_3 \mathrm{~N}>\mathrm{NH}_3\end{aligned}$

In the aqueous phase, the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows:

(b) Arylamines vs ammonia

In the case of substituted arylamines, electron-releasing groups like –OCH₃, and –CH₃ increase basic strength whereas electron-withdrawing groups like –NO₂, –SO₃H, –COOH, and –X decrease basic strength.

2. Alkylation

3. Acylation

Aliphatic & aromatic primary & secondary amines undergo nucleophilic substitution reactions with acid chlorides, anhydrides & esters.

4. Carbylamine reaction

$\mathrm{R}-\mathrm{NH}_2+\mathrm{CHCl}_3+3 \mathrm{KOH} \xrightarrow{\text { Heat }} \mathrm{R}-\mathrm{NC}+3 \mathrm{KCl}+3 \mathrm{H}_2 \mathrm{O}$

5. Reaction with nitrous acid

(a) Primary aliphatic amines from aliphatic diazonium salts on reaction with nitrous acid.

$\mathrm{R}-\mathrm{NH}_2+\mathrm{HNO}_2 \xrightarrow{\mathrm{NaNO}_2+\mathrm{HCl}}\left[\mathrm{R}-\stackrel{+}{\mathrm{N}_2} \mathrm{Cl}\right] \xrightarrow{\mathrm{H}_2 \mathrm{O}} \mathrm{ROH}+\mathrm{N}_2+\mathrm{HCl}$

(b) Aromatic amines form diazonium salts on reaction with nitrous acid at low temperatures.

$\mathrm{C}_6 \mathrm{H}_5-\mathrm{NH}_2 \xrightarrow[273-278 \mathrm{~K}]{\mathrm{NaNO}_2+2 \mathrm{HCl}} \mathrm{C}_6 \mathrm{H}_5-\stackrel{+}{\mathrm{N}_2} \stackrel{-}{\mathrm{Cl}}+\mathrm{NaCl}+2 \mathrm{H}_2 \mathrm{O}$

Secondary and tertiary amines react with nitrous acid in a different manner.

6. Reaction with aryl sulphonyl chloride

Benzenesulphonyl chloride $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{SO}_2 \mathrm{Cl}\right)$, which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides.

(a) The reaction of benzene sulphonyl chloride yields N-ethylbenzene sulphonyl amide on reaction with a primary amine.

(b) N, N-diethyl benzene sulphonamide is formed in the reaction with a secondary amine with benzene sulphonyl chloride.

(c) Tertiary amines & benzene sulphonyl chloride do not react.

7. Electrophilic substitution

(a) Bromination:

(b) Nitration:

(c) Sulphonation:

Diazonium Salt

The diazonium salts have the general formula – RN₂ + X^- where R stands for an aryl group & X^- ion may be Cl^-, Br^-, HSO₄^-, BF₄^-, etc.

Nomenclature: The name of parent hydrocarbon is added as prefix & name of anion after diazonium. Example: benzenediazonium chloride etc.

1° aliphatic amines form highly unstable alkyl diazonium salts & 1° aromatic amines form arene diazonium salts (stable at low temperature for short time). The stability of arenediazonium ion is explained on the basis of resonance.

Methods of Preparation of Diazonium Salts

The reaction of aniline with nitrous acid produces Benzene diazonium chloride. Diazotization is the term coined for the conversion of primary aromatic amines into diazonium salts.

$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2+\mathrm{NaNO}_2+2 \mathrm{HCl} \xrightarrow{273-278 \mathrm{~K}} \mathrm{C}_6 \mathrm{H}_5 \mathrm{~N}_2^{+} \mathrm{Cl}+\mathrm{NaCl}+2 \mathrm{H}_2 \mathrm{O}$

Physical Properties

• Colorless crystalline solid
• Soluble in water (cold water-stable; warm water - react)
• Decomposes easily in the dry state
• Some diazonium salts like Benzenediazonium fluoroborate are water-insoluble & stable at room temperature.

Chemical Reactions

(A) reactions involving displacement of nitrogen

1. Replacement by halide or cyanide ion:

Also known as the Sandmeyer reaction; involves the nucleophilic substitution of diazonium salt to give aryl halide in presence of Cu(I) ion.

Also known as the Gatterman reaction; involves the nucleophilic substitution of diazonium salt to give aryl halide in the presence of copper powder. Sandmeyer’s reaction is better than Gattermann’s reaction.

2. Replacement by iodide ion:

Iodobenzene is formed when a diazonium salt solution is treated with potassium iodide.

$\mathrm{Ar}_2^{+} \stackrel{-}{\mathrm{N}}_2+\mathrm{KI} \longrightarrow \mathrm{ArI}+\mathrm{KCl}+\mathrm{N}_2$

3. Replacement by fluoride ion:

Arene diazonium fluoroborate is produced on treating arene diazonium chloride with fluoroboric acid which on heating decomposes to yield aryl fluoride.

$\mathrm{ArN}_2^{+} \stackrel{-}{\mathrm{Cl}}+\mathrm{HBF}_4 \longrightarrow \mathrm{Ar}-\stackrel{+}{\mathrm{N}}_2 \mathrm{BF}_4 \xrightarrow{\Delta} \mathrm{Ar}-\mathrm{F}+\mathrm{BF}_3+\mathrm{N}_2$

4. Replacement by H:

Diazonium salts are reduced to arenes when treated with ethanol or hypophosphorous acid.

5. Replacement by hydroxyl group:

6. Replacement by –NO₂ group:

The Diazonium group is replaced by the –NO₂ group when the diazonium fluoroborate is heated with aqueous NaNO₂ in the presence of Cu.

B. Reactions involving retention of diazo group

Coupling reactions:

A reaction between Benzene diazonium chloride & phenol results in coupling between two molecules at the para position of phenol via -N=N- bond. P-hydroxyazobenzene is formed as a product.

Azo coupling of sulfanilamide acid & N-(1-Naphthyl) ethylene di amine

Importance of Diazonium Salts in the Synthesis of Aromatic Compounds

Diazonium salts are a very good intermediate therefore they are helpful in a reaction where direct substitution is not possible. For example, aryl fluorides, aryl iodides, cyanobenzene, & chlorobenzene can not be prepared by direct substitution but are easily prepared when the diazo group is introduced.

Amines Class 12 notes will be helpful to revise the chapter & to get an idea about the main topics covered in the chapter. Also, this NCERT Class 12 Chemistry chapter 9 notes are useful to cover the main topics of the Class 12 CBSE Chemistry Syllabus & also for competitive exams like VITEEE, BITSAT, JEE Core, NEET, etc.

Amines Previous Year Questions and Answers

Selected Previous Year Questions and Answers from Chapter 9 Amines Class 12 NCERT help students practise important questions that often come in exams. They cover basic concepts, properties, reactions, and common uses of amines in a simple way.

Question: The correct order of basic nature on aqueous solution for the bases $\mathrm{NH}_3, \mathrm{H}_2 \mathrm{~N}-\mathrm{NH}_2, \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2$, $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{NH}$ and $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N}$ is :

(1) $\mathrm{NH}_3<\mathrm{H}_2 \mathrm{~N}-\mathrm{NH}_2<\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N}<\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2<$ $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{NH}$

(2) $\mathrm{NH}_3<\mathrm{H}_2 \mathrm{~N}-\mathrm{NH}_2<\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2<\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{NH}<$ $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N}$

(3) $\mathrm{H}_2 \mathrm{~N}-\mathrm{NH}_2<\mathrm{NH}_3<\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N}<\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2<$ $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{NH}$

(4) $\mathrm{NH}_2-\mathrm{NH}_2<\mathrm{NH}_3<\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2<\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N}<$ $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{NH}$

Answer:

Basic strength of amine depends on hydrogen bonding and electronic inductive effect.

$\mathrm{NH}(\mathrm{Et})_2>\mathrm{N}(\mathrm{Et})_3>\mathrm{NH}_2 \mathrm{Et}>\ddot{\mathrm{N}}_3>\mathrm{NH}_2-\mathrm{NH}_2$

Hence, the correct answer is option (4).

Question: When a concentrated solution of sulphanilic acid and 1-naphthylamine is treated with nitrous acid (273 K) and acidified with acetic acid, the mass (g) of 0.1 mole of product formed is : (Given molar mass in g mol–1 H : 1, C : 12, N : 14,O : 16, S : 32)

(1) 343

(2) 330

(3) 33

(4) 66

Answer:

Molar mass of product formed

$\begin{aligned}
& =(16 \times 12)+(14 \times 3)+(16 \times 3)+32+13 \\
& =327 \mathrm{~g}
\end{aligned}$

Mass of 0.1 mol product

$32.7 \mathrm{~g} \approx 33 \mathrm{~g}$
Hence, the correct answer is option (3).

Question: Amines have a lone pair of electrons on nitrogen atom due to which they behave as Lewis base. Greater the value of $K_b$ or smaller the value of $\mathrm{pK}_{\mathrm{b}}$, stronger is the base. Amines are more basic than alcohols, ethers, esters, etc. The basic character of aliphatic amines should increase with the increase of alkyl substitution. But it does not occur in a regular manner as a secondary aliphatic amine is unexpectedly more basic than a tertiary amine in aqueous solutions. Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron releasing groups such as $-\mathrm{CH}_3$, $-\mathrm{OCH}_3,-\mathrm{NH}_2$, etc., increase the basicity while electron-withdrawing substituents such as $-\mathrm{NO}_2,-\mathrm{CN}$, halogens etc., decrease the basicity of amines. The effect of these substitute is more at $\mathrm{p}^{-}$than at $\mathrm{m}^{-}$position.

Why $\mathrm{pK}_{\mathrm{b}}$ of aniline is more than that of methylamine?

Answer: The pKb of aniline is higher than that of methylamine because aniline is a weaker base. The basicity of a compound is related to its ability to accept a proton. Aniline undergoes resonance, and as a result, the lone pair on the N-atom is partially delocalized over the benzene ring. Therefore, the electrons on the N-atom are less available to donate.

On the other hand, in the case of methylamine, due to the +I effect of the methyl group, the electron density on the N-atom is increased, which makes the lone pair more available for protonation.

As a result, aniline is less basic than methylamine. Thus, the pKb of aniline is more than that of methylamine.

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