Question -
Answer -
(i) Carbylamine reaction (ii) Diazotisation
(iii) Hofmann’s bromamide reaction (iv) Coupling reaction
(v) Ammonolysis (vi) Acetylation
(vii) Gabriel phthalimide synthesis.
Answer
(i) Carbylaminereaction
Carbylamine reaction is used as a test for theidentification of primary amines. When aliphatic and aromatic primary aminesare heated with chloroform and ethanolic potassium hydroxide, carbylamines (orisocyanides) are formed. These carbylamines have very unpleasant odours.Secondary and tertiary amines do not respond to this test.
For example,
(ii) Diazotisation
Aromatic primary amines react with nitrousacid (prepared in situ from NaNO2 and a mineralacid such as HCl) at low temperatures (273-278 K) to form diazonium salts. Thisconversion of aromatic primary amines into diazonium salts is known asdiazotization.
For example, on treatment with NaNO2 andHCl at 273−278 K, aniline produces benzenediazonium chloride, with NaCl and H2O asby-products.
(iii) Hoffmannbromamide reaction
When an amide is treated with bromine in an aqueous orethanolic solution of sodium hydroxide, a primary amine with one carbon atomless than the original amide is produced. This degradation reaction is known asHoffmann bromamide reaction. This reaction involves the migration of an alkylor aryl group from the carbonyl carbon atom of the amide to the nitrogen atom.
For example,
(iv) Couplingreaction
The reaction of joining two aromatic rings through the−N=N−bond is known as coupling reaction. Arenediazonium salts such as benzenediazonium salts react with phenol or aromatic amines to form coloured azocompounds.
It can be observed that, the para-positions of phenol andaniline are coupled with the diazonium salt. This reaction proceeds throughelectrophilic substitution.
(v) Ammonolysis
When an alkyl or benzyl halide is allowed to reactwith an ethanolic solution of ammonia, it undergoes nucleophilic substitutionreaction in which the halogen atom is replaced by an amino (−NH2)group. This process of cleavage of the carbon-halogen bond is known asammonolysis.
When this substituted ammonium salt is treated with astrong base such as sodium hydroxide, amine is obtained.
Though primary amine is produced as the major product,this process produces a mixture of primary, secondary and tertiary amines, andalso a quaternary ammonium salt as shown.
(vi) Acetylation
Acetylation (or ethanoylation) is the process ofintroducing an acetyl group into a molecule.
Aliphatic and aromatic primary and secondaryamines undergo acetylation reaction by nucleophilic substitution when treatedwith acid chlorides, anhydrides or esters. This reaction involves the replacementof the hydrogen atom of −NH2 or > NH groupby the acetyl group, which in turn leads to the production of amides. To shiftthe equilibrium to the right hand side, the HCl formed during the reaction isremoved as soon as it is formed. This reaction is carried out in the presenceof a base (such as pyridine) which is stronger than the amine.
When amines react with benzoyl chloride, the reaction isalso known as benzoylation.
For example,
(vii) Gabrielphthalimide synthesis
Gabriel phthalimide synthesis is a very useful method forthe preparation of aliphatic primary amines. It involves the treatment ofphthalimide with ethanolic potassium hydroxide to form potassium salt ofphthalimide. This salt is further heated with alkyl halide, followed by alkalinehydrolysis to yield the corresponding primary amine.
