Amines Class 12 Notes

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Amines Class 12 Chemistry Notes

AMINES INTRODUCTION

            Amines are the organic derivatives of ammonia (NH3) in which one, two, or all three hydrogen atoms attached to nitrogen are replaced by equivalent number of same or different alkyl and/ or aryl groups.

Amines class 12 chemistry notes

Like ammonia, amines have pyramidal geometry and the nitrogen atom in amines, the nitrogen is attached to sp3 hybridized carbon of alkyl group and in aromatic amines to sp2 hybridized carbon of aryl group. H – N – H , C – N – H or C – N – C bond angle is less than 109ᵒ28’.

CLASSIFICATION OF AMINES

            Depending upon the number of hydrogen atoms replaced by alkyl or aryl groups attached to nitrogen atom in ammonia molecule, amines are classified as –

1) Primary (1ᵒ) Amines

The functional group present is – NH2 (Amino group)

e.g.     CH3 – NH2 Methylamine

2) Secondary (2ᵒ) Amines

The functional group present is – NH-  (Amino group)

e.g.     CH3 – NH– CH3 Dimethylamine

3) Tertiary (3ᵒ) Amines                

The functional group present is -N= (Tertiary nitrogen atom)

Trimethylamine

Secondary and tertiary amines are further classified as

Secondary and Tertiary amines of Amines Class 12 Notes are further classified into two groups i.e.

a) Simple / Symmetrical amines

In simple amines same alkyl or aryl groups are attached to the nitrogen.

b) Mixed / Unsymmetrical amines

In mixed amines different alkyl or aryl groups are attached to the nitrogen.

METHOD OF PREPARATION OF AMINES

1) By ammonolysis of alkyl halides

            When ethyl bromide is heated with alcoholic ammonia at 373K in sealed copper tube, it gives mixture of ethylamine, diethyl amine and triethyl amine along with tetraethyl ammonium bromide.

C2H5 – Br + NH3     Δ → C2H5 – NH2 + HBr

C2H5 – NH2 + C2H5 – Br    Δ  → (C2H5)2NH + HBr

(C2H5)2NH + C2H5 – Br    Δ  → (C2H5)3N + HBr

(C2H5)3 + C2H5 -Br    Δ  → (C2H5)4N+Br

Thus,

NH3     RX→ R – NH2    RX→ R2NH  RX→ R3  RX→ R4N+X

2) By reduction of Nitro Compounds

            Both aliphatic and aromatic primary amines can be prepared by the reduction of nitro compounds by either catalytically with H2 in the presence of Raney Ni, Pt or Pd or chemically with active metals.

e.g.       CH3NO2    +     3H2   Raney Ni/Pt→     CH3NH2     +     2H2O         Nitromethane                   Ethanol          Methylamine

preparation of amines By reduction of Nitro Compounds - Amines Class 12 Notes

3) By Gabriel phthalimide synthesis

            Phthalmide reacts with ethanolic potassium hydroxide to give potassium salt of phthalmide. In this step N – H proton is removed to give imide ion. It is then heated to give N – alkyl phthalimide, which on alkaline hydrolysis give a primary amine. Share these Amines Class 12 Notes of chemistry with your friends.

preparation of amines By Gabriel phthalimide synthesis - Amines Class 12 Notes - Chemistry Notes Info

4) By Reduction of Alkyl nitrites / cyanides

            Nitriles on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation produce primary amines.

   R – CN     — H2 / Ni & Na(Hg) / C2H5OH  →   R – CH2 – NH2

 Alkyl cyanide                                             1ᵒ Amine   

5) By reduction of amides

            Acid amides on reduction with lithium aluminium hydride give corresponding amines.

preparation of amines By reduction of amides

6) By Hoffmann bromamide degradation

            The conversion of amides into amines in the presence of bromine and alkali is known as Hoffmann degradation of amides.

R – CO – NH2 (Amides) +   Br2  +   4NaOH       →   R – NH2  (Amine) +  Na2CO3  +  2NaBr  +  2H2O

PHYSICAL PROPERTIES OF AMINES

1) Aliphatic amines with low molecular weight are colourless, gaseous compounds with fishy odour. High molecular weight aliphatic amines are solid.

2) Pure aromatic amines such as aniline are colourless liquid. Arylamines are toxic in nature.

3) Aliphatic amines are soluble in water. As the molar mass increase solubility decreases.

4) Amines are less polar than the corresponding alcohol but more polar than corresponding alkanes.

5) Amines are higher boiling points than corresponding alkanes but lower than corresponding alcohols or carboxylic acids. The order or boiling points of isomeric amines is 1ᵒ > 2ᵒ > 3ᵒ.

REACTIONS OF AMINES

            The lone pair of electron on nitrogen makes amines both basic and nucleophilic. They react with acids to form salts and react with nucleophiles in many reactions.

1) Basic nature of Amines

            Nitrogen atom of amines contains a lone pair of electron which can be donated. Thus, amines act as Lewis bases and are Lowery – Bronsted bases as they accept a proton.

Basic nature of amines

The parent amine is regenerated when alkyl ammonium halide is treated with NaOH.

R – NH3+X (Alkyl Ammonium Halide) + NaOH → R – NH2 (Amine)  + NaX  + H2O

Amines are weak bases. They dissolve in water and produce OH ions. Aqueous Solutions of amines turn the color of litmus paper from red to blue.

  R – NH2     +     H – O – H     ↔     R – NH3+    +     OH

Weak Base                                                           Strong base

            The equilibrium lies far to the left as OH is a stronger base than amines. The expressions for the equilibrium constant K, basicity constant Kb and pKb values are as follows.

Amines class XII notes - Amines Class 12 Notes - ChemistryNotesInfo

PKb = -logKb

Strong bases have high values of Kb and low values of PKb.

2) Action of nitrous acid

  • On primary (1ᵒ) amines

Except methylamine, primary amines react with nitrous acid, in cold condition to give alcohol and nitrogen gas.

  R – NH2  (1ᵒ amines) +   NaNO2  +  2HCl   273 – 278K→   R – OH  +  HCl  +  N2  ↑

  • On secondary (2ᵒ) amines

Secondary amines react with nitrous acid to give the N – nitrosoamines which are generally pale yellow oils.

 R2 – NH (2ᵒ amine) +  HNO2  —NaNO2 + dil. HCl (at 273 – 278K)

→ R2N – N = O  +  H2O

  • On tertiary (3ᵒ) amines

Tertiary amines react with nitrous acid to form water soluble nitrite salt. As visible change observed, it is said that there is no reaction.

     R3N (3ᵒ amine) + HNO2  NaNO2 / dil. HCl ( at 273 – 278K )

→    [ R3NH ]+NO2   (No change)

3) Acylation of amines

            Acylation is the replacement of a hydrogen atom of amino group by acyl group (R – CO). It is nucleophilic substitution reaction. Acetylation of primary / secondary amines with acetyl chloride or acetic anhydride gives corresponding amine.

Acylation of amines

4) Carbylamine reaction  

            Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines. This reaction is known as carbylamines reaction or isocyanides test.

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

5) Reaction with arylsulphonyl Chloride (Hinsberg’s test)

  • Primary amines when reacts with benzene Sulphonyl chloride ( Hinsberg’s reagent),

it yields N – ethylbenzene sulphonamide.

a)	Primary amines when reacts with benzene Sulphonyl chloride ( Hinsberg’s reagent),
it yields N – ethylbenzene sulphonamide.
  • Secondary amines reacts with benzene sulphonyl chloride, it yields N, N – dimethylbenzene sulphonamides.
b)	Secondary amines reacts with benzene sulphonyl chloride, it yields N, N – dimethylbenzene sulphonamides.
  • Tertiary amines do not react with benzene sulphonyl chloride.

6) Electrophilic aromatic substitution

  • Bromination

Reaction of aniline with bromine water at room temperature results in the formation of white precipitate of 2, 4, 6 – tribromoaniline.

bromination of aniline
  • Nitration

In direct nitration of aniline with concentrated nitric acid at 288K in the presence of sulphuric acid, a mixture of ortho, meta and para isomers of nitroaniline are obtained as product. Also, dark coloured tars are obtained due to oxidation.

Nitration of aniline
  • Sulphonation

Reaction of aniline with cold concentrated Sulphuric acid gives anilinium hydrogen sulphate. Heating anilinium hydrogen sulphate with Sulphuric acid at 453K to 473K gives p – aminobenzene sulfonic acid (sulphonic acid) as major product.

sulphonation of aniline

DIAZONIUM SALTS

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

The N2+ group is called diazonium group.

e.g.  C6H5N2+Cl IS named as benzenediazonium chloride.

The conversion of primary aliphatic or aromatic amines into diazonium salts is known as diazotization.

PREPARATION OF DIAZONIUM SALTS

            Diazonium salts are prepared by the reaction of nitrous acid in cold condition with alkyl/aryl primary amines (other than methylamine).

e.g.  

PREPARATION OF DIAZONIUM SALTS

REACTION OF DIAZONIUM SALTS

Reaction involving displacement of nitrogen (Diazonium group)

  • Replacement by -Cl , -Br and –CN

The reaction in which copper (1) salts are used to replace nitrogen in diazonium salt is called sandmeyer reaction

sandmeyer reaction
sand-meyer reaction

Yield in sandmeyer reaction is better than yield in Gattermann reaction.

  •  Replacement by – I

Ar – N+2X   +   KI       Δ  →    Ar – I   +   KX   +   N2    ↑

  • Replacement by – F

Ar – N+2X       HBr  →  Ar – N+2BF4     Δ  →    Ar – F   +   BF3   +   N2    ↑

  • Replacement by – H

Ar – N+2X  + H3PO2  +  H2    CuCl  →  ArH   +   H3PO3   +   HX   +   N2    ↑

  • Replacement by – OH

Ar – N+2X  + H2 Dil. H2SO4  →  Ar – OH    +   HX   +   N2    ↑

  • Replacement by – NO2

Ar – N+2X      HBF4  →  Ar – N+2BF4     NaNO2 / Cu &  Δ  →    Ar – NO2   +  NaBF4   +   N2    ↑

IMPORTANCE OF DIAZONIUM SALTS

1) Arene diazonium salts can be prepared from nearly all primary amines and are helpful in the synthesis of variety of organic compounds.

2) Arenes diazonium salts are used as useful intermediates to introduce various groups into aromatic ring. Such as – F, – Cl, -Br, – I, -CN, -NO2, -OH, -H, etc.

3) The halide or Cyano group can be easily introduced in aromatic ring through diazonium salts.

4) The compounds which cannot be prepared by direct electrophilic aromatic substitution can be prepared by replacement of diazo group.

5) Azo compounds obtained from diazonium salts are strongly coloured and are used as dyes. 

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