Amines
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Amines. Introduction. Amines are formed by replacing one or more hydrogen atoms of ammonia (NH 3 ) with alkyl groups. In nature , they occur among proteins, vitamins, alkaloids and hormones. Synthetic examples include polymers, dyestuffs and drugs .

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Amines

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Amines

Amines


Introduction

Introduction

  • Amines are formed by replacing one or more hydrogen atoms of ammonia (NH3) with alkyl groups.

  • In nature, they occur among proteins, vitamins, alkaloids and hormones. Synthetic examples include polymers, dyestuffs and drugs.

  • Two biologically active compounds, namely adrenaline and ephedrine, both containing secondary amino group, are used to increase blood pressure.


Amines

N

H3C

CH3

H3C

Structure of Amines

Nitrogen orbitals in amines are sp3 hybridized also the geometry of amines is pyramidal.

Every of the three sp3 hybridized orbitals of nitrogen are related with orbitals of hydrogen or carbon depending ahead the composition of the amines


Amines

Aniline

It is one of the most important of all the amines.


Manufacturing process

MANUFACTURING PROCESS

1 By Reduction of Nitrobenzene

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2. By Reduction from Nitro Alkanes

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3. By Reduction from Acid amide

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4. By Hofmann degradation method

5. From Acid Nitrile

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6. From Halo Alkane


Amines

Tin and Hydrochloric Acid

Nitro Benzene

Reduction with Iron scrap with Hydrochloric acid is preffered because FeCl2 formed in reaction get hydrolysed and release HCl hence small amount of HCl is required to initiate the reaction

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Amines

From Nitro Alkanes

Other reducing agents Sn/Conc. HCl, H2/Ni

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Ammonolysis of alkyl halides

Ammonolysis of alkyl halides

  • The process of cleavage of the C–X bond from an alkyl or benzyl halides by ammonia molecule is known as ammonolysis.

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


Amines

From Acid Amide

See number of carbon atoms remains same after reduction

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Amines

From Hoffman degradation method

See the number of carbon atom get reduced by one

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Amines

  • From acid nitrile

Other reducing agents:

H2 / Ni 140oC

Na / ethanol

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Amines

From Haloalkane

Further substitution give mixture of products.

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Amines

Physical Properties

  • With “fishy”smell.

    Soluble in water due to H-bond formation.

    Aromatic amine are liquid with high boiling point with a characteristic fishy smell and insoluble in water.


Amines

Chemical Properties

Basic character

Basic strength depends on the availability of lone pair e- on N and solubility in water (stability of the conjugate cation).


Amines

  • Basic character (cont’d)

  • Inductive effect:

  • Due to the alkyl group (e-donating group)

  • 3o > 2o > 1o > NH3 BUT

Ease of solvation of protonated amine:

Due to the formation of H-bond between N-H and H2ONH3 > 1o > 2o > 3o

Overall effect:2o 1o > 3o >NH3


Amines

  • Basic character (cont’d)

    Comparing ammonia, CH3NH2 & (CH3)2NH, basic strength increased as more alkyl groups attached to the N atom. Alkyl group is e-donating group which makes the lone pair of e- of N more available to co-ordinate with a proton.


P kb values of amines in aqueous phase

pKb Values of Amines in Aqueous Phase


Amines

  • Basic character (cont’d)

    Aromatic amines are less basic strength than aliphatic amines. This is because the lone pair e- of N can overlap with the pi-molecular orbital of the benzene ring. It is thus less available to co-ordinate with a proton.


Amines

Resonance Structure of Aniline

Electron releasing groups:–OCH3, –CH3increase basic strength

Electron withdrawing groups : NO2, –SO3, –COOH, –X decrease it.


Amines

  • Basic character (cont’d)

  • For acid amide, it is even less basic strength than aromatic amine. This is because the negative inductive effect of the C=O which makes the lone pair e- on N less available. Moreover, lone pair e- on N can ovrelap with p-orbitals of C.


Carbylamine reaction

Carbylamine reaction

  • Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances.


Amines

  • Test for amine

RNH2 + HCl  RNH3+Cl- (white fume)


Amines

  • Alkylation (nucleophilic substitution of RX)

Such method will give mixture of products.


Amines

Acylation (ammonolysis of acid derivative)


Amines

  • Benzoylation


Amines

  • Reaction With HONO


Amines

  • Diazonium coupling


Amines

By Catalytic Hydrogenation

The catalytic hydrogenation of nitro group takes place smoothly in presence of finely divided nickel or platinum catalyst


Amines

Classification of Amines

Characteristic group of primary Amine

Primary Amines


Amines

Characteristic group of Secondary Amine

Secondary Amines


Amines

Tertiary Amines


Diazonium salts

DIAZONIUM SALTS

The diazonium salts have the general formula–RN2+X-

where R standsforan aryl group and–X ion may be Cl– Br,–

HSO4−, BF4− , etc.

The stability of arenediazonium ion is explained on the

basis of resonance.


Physical properties of diazoniun salts

Physical Properties of DiazoniunSalts

  • Benzenediazonium chloride is a colourless crystalline solid.

  • It is readily soluble in water and is stable in cold but reacts with water when warmed.

  • It decomposes easily in the dry state. Benzenediazoniumfluoroborateis water insoluble and stable at room temperature.


Importance of diazonium salts in synthesis of aromatic compounds

Importance of Diazonium Salts in Synthesis of Aromatic Compounds

  • It is clear that the diazonium salts are very good intermediates for the introduction of –F, –Cl, –Br, –I, –CN, –OH,–NO2 groups into the aromatic ring.

  • Aryl fluorides and iodides cannot be prepared by direct halogenation.

  • The cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene but cyanobenzene can be easily obtained from diazonium salt.


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