Amines

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.

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

N

CH3

H3C

H3C

It is one of the most

important of all the

amines.

Aniline

MANUFACTURING PROCESS

1 By Reduction of Nitrobenzene

2. By Reduction from Nitro Alkanes

3. By Reduction from Acid amide

4. By Hofmann degradation method

5. From Acid Nitrile

6. From Halo Alkane

CLICK

CLICK

CLICK

CLICK

CLICK

CLICK

Nitro Benze

ne

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

Tin and Hydrochloric Acid

BACK

RNO2

LiAlH4 / dry etherRNH2

From Nitro Alkanes

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

BACK

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.

From Acid Amide

See number of carbon

atoms remains

same after reduction BACK

CR NH2

O

RNH2

KOH / Br2

See the number of carbon atom get reduced by one

From Hoffman degradation method

BACK

RCNLiAlH4

dry etherRCH2NH2

From acid nitrile

Other reducing agents:H2 / Ni 140oCNa / ethanol

BACK

RX + NH3 RNH3 X RNH2

OH+-

-

From Haloalkane

Further substitution give mixture of products.

BACK

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.

Physical Properties

NH3 + H2O NH4 + OH+ -

RNH2 + H2O RNH3 + OH+ -

]RNH[

]OH][RNH[K

2

3b

Chemical PropertiesBasic

character

Basic strength depends on the

availability of lone pair e- on N and

solubility in water (stability of the

conjugate cation).

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

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.

pKb Values of Amines in Aqueous Phase

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.

Resonance Structure of Aniline

Electron releasing groups:–OCH3, –CH3 increase basic strengthElectron withdrawing groups : NO2, –SO3, –COOH, –X decrease it.

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

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

RNH2 + HCl RNH3+Cl- (white fume)

Test for amine

RNH2 + RX R2NH

R2NH + RX R3N

R3N + RX R4N X+ -

NH2

+ RX

NRH

Alkylation (nucleophilic substitution of RX)

Such method will give mixture of products.

RNH2 +

RNH2 +

RNH2 +

RNH2 +

CR'

O

OR"

CR'

O

Cl

CR'

O

OH

CR'

O

O C R"

O

CR'

O

N H

R

CR'

O

N H

R

CR'

O

N H

R

CR'

O

N H

R

Acylation (ammonolysis of acid derivative)

RNH2 + C

O

Cl C

O

N R

H

C

O

Cl C

O

NH2 + N

H

Benzoylation

RNH2 + HNO2ROH + N2 + H2O

NH2 NaNO2 / HCl

< 5 Co

N N+

Reaction With HONO

N N+

+

N

CH3

CH3

N NN

CH3 CH3

Diazonium coupling

By Catalytic Hydrogenation

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

Classification of Amines

Primary Amines

Characteristic group of

primary Amine

Secondary

Amines

Characteristic group of Secondary

Amine

Tertiary Amines

DIAZONIUM SALTSThe diazonium salts have the general formula–RN2 +X-

where R standsfor an aryl group and–X ion may be Cl– Br,–HSO4

− , BF4− , etc.

The stability of arenediazonium ion is explained on thebasis of resonance.

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. Benzenediazonium fluoroborate is water insoluble and stable at room temperature.

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.