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Organic Compounds Containing Nitrogen
Amines are the derivatives of ammonia prepared by the replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups.
Examples: (i) CH3-NH2
Structure of Amines
Preparation of Amines
Reduction of Nitro Compounds
Nitro compounds on reduction with hydrogen gas in the presence of finely divided nickel, palladium or platinum and also on reduction with metals in acidic medium give amines.
Alkyl halides or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which halogen atom is replaced by an amino (-NH2) group. The Process of cleavage of the C-X bond by ammonia molecule is known as ammonolysis.
The primary amine prepared behaves as a nucleophile and reacts with further alkyl halide to form secondary, tertiary amines, and finally quatemary ammonium salt
The free amine can be obtained from the ammoinium salt by treatment with a strong base
In this method, a mixture of primary, secondary and teritiary and also a quatemary ammonium salt. However a primary amine is prepared by taking large excess of ammonia
The order of reactivity of halides with amines is RI > RBr > RCl
Reduction of Nitriles
Nitriles in reducing with LiAlH4 or catalytic hydrogenation produce primary amines
Reduction of Amides
Amides on reducing with LiAlH4 yield amines
Gabriel pthalimide synthesis
Pthalimide on reacting with ethanolic solution of KOH forms potassium salt of pthalimide which on heating with alkyl halide followed by alkaline hydrolysis yields the corresponding primary amine.
Hoffmann bromamide degradation reaction
In this method, primary amines are prepared by treating an amide with bromine in an aqueous or ethanolic solution of NaOH.
The amine formed has one carbon atom less than the starting amide.
Physical Properties of Amines
Lower aliphatic amines are soluble in water because they can form a hydrogen bond with water. Solubility decreases with increase in molar mass of amines due to an increase in the size of the hydrophobic group.
Among the isomeric amines, primary and secondary amines have a high boiling point because they can form hydrogen bonds. Tertiary amines cannot form hydrogen bonds due to the absence of a hydrogen atom for hydrogen bond formation.
Hence, the order of boiling points of isomeric amines is Primary > Secondary > Tertiary
Chemical Properties of Amines
a) Basic character of amines
* Amines have an unshared pair of electrons on the nitrogen atom due to which they act as a Lewis base.
* The basic character of amines can be better understood in terms of their Kb and pKb values
R – NH2 + H2O ⇋ R -
* Greater Kh value or smaller pKh indicated that a base is strong
b) Comparision of basic strength of aliphatic amines and ammonia
Aliphatic amines are stronger bases than ammonia due to the +l effect of alkyl groups, leading to high electron on the nitrogen atom.
c) Comparison of basic strength of primary, secondary and tertiary amines
Tertiary amine > Secondary amine > Primary amine > NH3
Aniline on reacting with sulphuric acid forms anilinium hydrogen sulphate which on heating with sulphuric acid at 453-473K gives p-aminobenzene sulphonic acid as the major product.
Aniline does not undergo Friedel-Crafts reaction due to salt formation with Lewis acid aluminium chloride which is used as a catalyst. As a result, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.
Where R = Aryl group
X-ion = Cl- Br- HSO , Bf Etc.
ü Anion = chloride, hydrogensulphate, etc
ü Diazonium group =
Preparation of Diazonium Salts
Reactions involving displacement of Nitrogen
- Replacement by halide or cyanide ion: This reaction is called Sandmeyer reaction in which nucleophiles like Cl- ,Brand CN can be easily introduced in the benzene ring in the presence of Cu(I) ion.
- Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of Cu powder. This is referred as Gatterman reaction.
Iodobenzene is formed on treating diazium salt solution with potassium iodide
Hypophosphorus acid or ethanol are mild reducing agents and reduce diazonium salts to arenes and themselves get oxidized to phosphorus acid and ethanal respectively
Diazonium salt solution gets hydrolysed to phenol when the temperature is allowed to rise up to 283 K.
On heating diaozonium fluoroborate with aqueous sodium nitrate solution in the presence of copper, the diazonium group is replaced by – NO2 group
Reactions involving retention of diazo group coupling reactions
Importance of Diazonium Salts in Synthesis of Organic Compounds
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