NCERT Solution / Notes Class 12 Chemistry Chapter 13 Amines

NCERT Solutions, Question Answer and Mind Map for Class 12 Chemistry Chapter 13, “Amines,” is a study material package designed to help students understand the properties, classification, and reactions of amines.

NCERT Solutions provide detailed explanations and answers to the questions presented in the chapter. The solutions cover all the topics in the chapter, including the structure, nomenclature, physical properties, and methods of preparation of amines. They also provide tips on how to answer different types of questions, including short answer, long answer, and multiple-choice questions.

The question-answer section of the chapter covers a wide range of topics, from the classification of amines and their physical properties to their reactions with acids, alkyl halides, and carbonyl compounds. It also includes questions on the biological importance of amines and their applications in various industries.

The mind map provides a visual representation of the key topics covered in the chapter, allowing students to understand the connections between different concepts and ideas. The mind map covers the classification of amines, their nomenclature, and the various methods of preparation.

NCERT Solution / Notes Class 12 Chemistry Chapter 13 Amines with Mind Map PDF Download

Introduction

Amines are the derivatives of ammonia prepared by the replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups.

Examples: (i) CH₃-NH₂

Structure of Amines

• In amines, the nitrogen atom is trivalent and has an unshared pair of electrons. Hence the nitrogen orbitals are sp³ hybridised with pyramidal geometry.
• The three sp³ hybrid orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending on the nature of the amines.
• The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. It is due to the presence of unshared pair of electrons, the angle C-N-E is less than 109.5°.

For example: The bond angle for trimethyl amine is 108°.

Classification of Amines

Nomenclature of Amines by IUPAC System

Common Names of Amines

An aliphatic amine is named by prefixing alkyl group to amine, i.e. alkylamine as 1 word.

We are considering primary amines.

In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is added to the before the name of alkyl group.

In case of mixed amine, alkyl group are written in alphabetical order.

IUPAC System

In IUPAC system, amines are named as alkanamines, derived by replacement by ‘e’ of alkane by the word amine.

For Example: – Methanamine (CH₃NH₂), Ethan amine (C₂H5NH₂).

For naming higher member hydrocarbon, longest chain containing amino group is selected. C atom to which amino group is attached is given to lower number.

In case, more than one amino group is present at different positions in the parent chain, their positions are specified by giving numbers to carbon atoms bearing –NH2 groups and suitable prefix such as di, tri etc. is attached to the amine. The letter ’e’ of the suffix of the hydrocarbon part is retained.

For Example:- (Ethane 1,2 diamine) H₂N-CH₂ – CH₂ – NH₂

Each alkyl group bonded to the N atom is named as N-alkyl group.

Aryl Amines

In aryl amines, -NH2 group is directly attached to the benzene ring.

C₆H₅NH₂ is the simplest example of aryl amine. It is also known as aniline.

Naming aryl amines according to IUPAC system, suffix ‘e’ of arene is replaced by ‘amine’.

For Example: – C₆H₅ – NH₂ is named as benzenamine.

Preparation of Amines

Reduction of Nitro Compounds

Nitro compounds on reduction with hydrogen gas in the presence of finely divided nickel, palladium or platinum and also on reduction with metals in acidic medium give amines.

Ammonolysis

Alkyl halides or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which halogen atom is replaced by an amino (-NH₂) group. The process of cleavage of the C-X bond by ammonia molecule is known as ammonolysis.

The primary amine prepared behaves as a nucleophile and reacts with further alkyl halide to form secondary, tertiary amines, and finally quaternary ammonium salt.

The free amine can be obtained from the ammonium salt by treatment with a strong base.

In this method, a mixture of primary, secondary and tertiary and also a quaternary ammonium salt. However a primary amine is prepared by taking large excess of ammonia.

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

Reduction of Nitriles

Nitriles on reducing with LiAlH₄ or catalytic hydrogenation produce primary amines.

Reduction of Amides

Amides on reducing with LiAlH₄ yield amines.

Gabriel pthalimide synthesis

Pthalimide on reacting with ethanolic solution of KOH forms potassium salt of pthalimide which on heating with alkyl halide followed by alkaline hydrolysis yields the corresponding primary amine.

Hoffmann bromamide degradation reaction

In this method, primary amines are prepared by treating an amide with bromine in an aqueous or ethanolic solution of NaOH.

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

Physical Properties of Amines

• Solubility

Lower aliphatic amines are soluble in water because they can form a hydrogen bond with water. Solubility decreases with increase in molar mass of amines due to an increase in the size of the hydrophobic group.

• Boiling points

Among the isomeric amines, primary and secondary amines have a high boiling point because they can form hydrogen bonds.

Tertiary amines cannot form hydrogen bonds due to the absence of a hydrogen atom for hydrogen bond formation.

Hence, the order of boiling points of isomeric amines is Primary > Secondary > Tertiary

Chemical Properties of Amines

1. Basic character of amines
   • Amines have an unshared pair of electrons on the nitrogen atom due to which they act as a Lewis base.
   • The basic character of amines can be better understood in terms of their K_b and pK_b values.

• Greater K_b value or smaller pK_b indicates that a base  is strong.

2. Comparison of basic strength of aliphatic amines and ammonia

Aliphatic amines are stronger bases than ammonia due to the +I effect of alkyl groups, leading to high electron density on the nitrogen atom.

3. Comparison of basic strength of primary, secondary and tertiary amines
   • The order of basicity of amines in the gaseous phase follows the expected order on the basis of the +I effect:

• In aqueous solution, tertiary amines are less basic than either primary or secondary amines. This can be explained on the basis of the following factors:

Tertiary amine > Secondary amine > Primary amine > NH₃

• Solvation effect
  • Greater the stability of the substituted ammonium cation formed, stronger is the corresponding amine as a base.
  • The tertiary ammonium ion is less hydrated than the secondary ammonium ion, which is less hydrated than the primary amine. Thus, tertiary amines have less tendency to form ammonium ion and consequently are least basic.
  • On the basis of the solvation effect, the order of basicity of aliphatic amines should be: Primary amine > Secondary amine > Tertiary amine NH₃

• Steric factor
  • As the crowding of the alkyl group increases from primary to tertiary, amine hindrance to hydrogen bonding increases which eventually decreases the basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.

• When the alkyl group is small like CH₃, there is no steric hindrance to hydrogen bonding. In this  case, the order of basicity in aqueous medium is:

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N> NH₃

• When the alkyl group is the ethyl group, the group order of basicity in the aqueous medium is (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂> NH₃

• Comparison of basic strength of aryl amines and alkanamines
  • Generally, aryl amines are considerably less basic than alkyl amines.

Example: Ethyl amine is more basic than aniline.

• In aniline, the –NH₂ group is directly attached to the benzene ring. Hence, the unshared pair of electrons on nitrogen is less available for protonation because of resonance.

• In the above resonating structures, there is a positive charge on the nitrogen atom making the lone pair less available for protonation. Hence, aniline is less basic than ethyl amine which has no resonating structures.
• Less basicity of aniline can also be explained by comparing the relative stability of aniline and anilinium ion obtained by accepting a proton.
• Greater the number of resonating structures, greater is the stability of that species.
• Aniline is a resonance hybrid of five resonating structures, whereas anilinium ion has only two resonating structures.

• Thus, aniline has less tendency to accept a proton to form the anilinium ion.

• Effect of substituent on the basic character of amines
  • An electron-donating or electron-releasing group (EDG) increases basic strength.
  • An electron-withdrawing group (EWG) decreases basic strength.

The reaction of Amines

Acylation reaction

• Aliphatic and aromatic primary and secondary amines (which contain replaceable hydrogen atoms) react with acid chlorides, anhydrides and esters to form substituted amide.
• The process of introducing an acyl group (R–CO–) into the molecule is called acylation.
• The reaction is carried out in the presence of a stronger base than the amine, such as pyridine, which removes HCl formed and shifts the equilibrium to the product side.

Important Note: Because tertiary amines do not contain a replaceable hydrogen atom, they do not undergo acylation.

   Carbylamine reaction

• On heating aliphatic and aromatic primary amines with chloroform and ethanolic KOH they form isocyanides or carbylamines which have foul odour.
• Secondary and tertiary amines do not show this reaction.
• This reaction is used as a test for primary amines.

   Reaction with Nitrous acid

• Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts. Being unstable diazonium salts liberate nitrogen gas quantitatively which is used in the estimation of amino acids and proteins.

• Aromatic amines on treating with nitrous acid at low temperatures to form diazonium salts which are used in the synthesis of a variety of aromatic compounds.

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

Reaction with arylsulphonyl chloride

Hinsberg’s reagent or benzenesulphonyl chloride (C₆H₅SO₂Cl) reacts with primary amines and secondary amines to form sulphonamides.

• Primary amine reacts with benzenesulphonyl chloride to form N-ethylbenzenesulphonyl amide.

The hydrogen bonded to nitrogen is strongly acidic due to the presence of strong electron withdrawing sulphonyl group and is hence soluble in alkali.

• With secondary amine,N,N-diethyl-benzenesulphonamide is formed.

• Tertiary amines do not react with benzenesulphonyl chloride.

Electrophilic substitution

Ortho- and para-positions to the -NH₂ group become centres of high electron density. So -NH₂ group is ortho and para directing and a powerful activating group.

1. Bromination

• Aniline reacts with bromine water at room temperature to give a white precipitate of 2, 4,6- tribromoaniline.

Due to the high reactivity of aromatic amines, electrophilic substitution takes place at ortho- and para- positions.

For preparing monosubstituted aniline derivative, the -NH₂ group is protected by acetylation with acetic anhydride then carrying out the desired substitution followed by the hydrolysis of the substituted amide to the substituted amine.

2. Nitration

• Nitric acid is a nitrating agent plus a good oxidising agent. So direct oxidation of aromatic amines is not useful since it gives tarry oxidation products along with some nitro derivatives.
• In strong acidic medium, aniline is protonated to form the anilinium ion which is meta directing. Hence besides the ortho and para derivatives, significant amount of meta derivative is also formed.

• However if we protect the –NH₂ group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative derivative can be prepared as the major product.

3. Sulphonation

Aniline on reacting with sulphuric acid forms anilinium hydrogen sulphate which on heating with sulphuric acid at 453-473K gives p-aminobenzene sulphonic acid as the major product.

Aniline does not undergo Friedel-Crafts reaction due to salt formation with Lewis acid aluminium chloride which is used as a catalyst. As a result, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.

Diazonium Salts

• Diazonium salts have the general formula

• A suffix diazonium is added to the parent hydrocarbon from which they are formed followed by the name of the anion.
  • Anion = chloride, hydrogensulphate, etc.
  • Diazonium group = N⁺₂
• Primary aliphatic amines form highly unstable alkyldiazonium salts whereas primary aromatic amines form arenediazonium salts which are stable for a short time in a solution at low temperatures.

• The stability of arenediazonium ion is explained on the basis of resonance.

Preparation of Diazonium Salts

• Benzenediazonium chloride is prepared by the action of aniline with nitrous acid at 273-278K.
• The conversion of primary aromatic amines into diazonium salts is known as diazotisation.

Physical Properties

• Benzenediazonium chloride is a colourless crystalline solid.
• It is readily soluble in water and is stable in cold but reacts with warm water.
• It decomposes easily in the dry state.
• Benzenediazonium fluoborate is water insoluble and stable at room temperature.

Chemical Reactions

Reactions involving displacement of Nitrogen

• Replacement by halide or cyanide ion:

This reaction is called Sandmeyer reaction in which nucleophiles like Cl^–,Br^– and CN^– can be easily introduced in the benzene ring in the presence of Cu(I) ion.

Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of Cu powder. This is referred as Gatterman reaction.

• Replacement by iodide ion:

Iodobenzene is formed on treating diazonium salt solution with potassium iodide.

• Replacement by fluoride ion:

Arenediazonium chloride on treating with fluoboric acid gives a precipitate of arene diazonium fluoroborate which on heating decomposes to give aryl fluoride.

• Replacement by H:

Hypophosphorus acid or ethanol are mild reducing agents and reduce diazonium salts to arenes and themselves get oxidised to phosphorus acid and ethanal respectively.

• Replacement by hydroxyl group:

Diazonium salt solution gets hydrolysed to phenol when the temperature is allowed to rise up to 283K.

• Replacement by –NO₂ group:

On heating diazonium fluoroborate with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by –NO₂ group.

Reactions involving retention of diazo group coupling reactions

• Benzene diazonium chloride reacts with phenol in which the phenol at its para position is coupled with the diazonium salt to form orange colour dye called p-hydroxyazobenzene.
• The reaction of diazonium salt with aniline gives yellow dye p-aminoazobenzene.
• The reaction is known as coupling reaction and it is an example of electrophilic substitution reaction.

Importance of Diazonium Salts in Synthesis of Organic  Compounds

• Diazonium salts are very good intermediates for introducing –F, –Cl, –Br, –I, –CN, –OH,–NO₂ groups into the aromatic ring.
• Direct halogenation method cannot be used for preparing aryl fluorides and iodides.
• Cyanobenzene can be easily prepared from diazonium salt.
• Thus the replacement of diazo group by other groups is useful in preparing substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene.