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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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
It is one of the most important of all the amines.
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
Tin and Hydrochloric Acid
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
From Nitro Alkanes
Other reducing agents Sn/Conc. HCl, H2/Ni
The order of reactivity of halides with amines is RI > RBr >RCl.
From Acid Amide
See number of carbon atoms remains same after reduction
From Hoffman degradation method
See the number of carbon atom get reduced by one
Other reducing agents:
H2 / Ni 140oC
Na / ethanol
Further substitution give mixture of products.
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.
Basic strength depends on the availability of lone pair e- on N and solubility in water (stability of the conjugate cation).
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
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.
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, –CH3increase basic strength
Electron withdrawing groups : NO2, –SO3, –COOH, –X decrease it.
RNH2 + HCl RNH3+Cl- (white fume)
Such method will give mixture of products.
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
Classification of Amines
Characteristic group of primary Amine
Characteristic group of Secondary Amine
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.