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
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
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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
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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.
CR NH2
O
RNH2
KOH / Br2
See the number of carbon atom get reduced by one
From Hoffman degradation method
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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.
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 + 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)
By Catalytic Hydrogenation
The catalytic hydrogenation of nitro group takes place smoothly in presence of finely divided nickel or platinum catalyst
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.