NEET Chemistry Notes Organic Compounds Containing Nitrogen – Amines
Amines are the derivatives of ammonia in which one, two or all the three hydrogen atoms are replaced by alkyl or aryl groups. Amines are classified as primary, secondary or tertiary according to the replacement of one, two or three hydrogen atoms from ammonia respectively.
Structure of Amines
Nitrogen in amines is sp3- hybridised. The sp3– hybridised orbitals of nitrogen’- overlap with orbitals of hydrogen or carbon depending upon the composition of amines. The fourth hybridised orbital of nitrogen in all amines has an unshared pair of electrons. Due to the presence of lone pair of electrons, the bond angle C—N—E (where, E is C or H) is less than 109.5° and shape is pyramidal.
Preparation of Amines
Several methods can be used to prepare aliphatic amines, out of which some are specific for 1°, 2°, 3° amines which gives some mixtures of all the three.
- By the Reduction of Nitro Compounds, Nitriles and Amides
Reduction of nitriles to primary amines by using Na and alcohol is called Mendius reaction.
All the aboye reactions are used in the preparation of primary amines.
- By Ammonolysis of Alkyl Halides (Hofmann’s Method)
When alkyl halide is heated with alcoholic solution of ammonia in a sealed tube at about 393 K, a mixture of amines is obtained. This reaction is called Hofmann’s ammonolysis method.
- Gabriel Phthalimide Synthesis
This reaction is used for the preparation of 1° amine (aliphatic as well as aromatic amines having electron withdrawing group at o-and p-positions, e.g. —NO2 group). Except those containing electron withdrawing group at o- and p-positions, e.g. —NO2 group cannot be synthesised by this method because ArX does not undergo SN reaction with anion formed by phthalimide.
Aromatic primary amines cannot be. prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.
- Hofmann Bromamide Degradation Reaction
Amides on reaction with Br2 in an aqueous or ethanolic solution of NaOH give 1° amine with one C atom less than that present in the amide.
- Schmidt Reaction
Carboxylic acid, when warmed with hydrazoic acid (N3H) in the presence of concentrated sulphuric acid, gives a high yield of primary amines.
Instead of N3H and cone. H2S04, a mixture of NaN3 and conc. H2S04 can also be used.