Amines 3

The diazonium salts have the general formula –R N X2 where R stands for an aryl group and X ion may be Cl– Br–, HSO4–, BF4- , etc. They are named by suffixing diazonium to the name of the parent hydrocarbon from which they are formed, followed by the name of anion such as chloride, hydrogensulphate, etc. The N2 - group is called diazonium group. For example, – C H N Cl 6 5 2  is named as benzenediazonium chloride and C6H5N2 +HSO4 – is known as benzenediazonium hydrogensulphate.

Primary aliphatic amines form highly unstable alkyldiazonium salts. They decompose to form alcohol and quantitative evolution of nitrogen gas.

Primary aromatic amines form arenediazonium salts which are stable for a short time in solution at low temperatures (273-278 K). The stability of arenediazonium ion is explained on the basis of resonance.

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.

Diazonium group being a very good leaving group, is substituted by other groups such as Cl– , Br–,I–, CN– and OH– which displace nitrogen from the aromatic ring. The nitrogen formed escapes from the reaction mixture as a gas.

From the above reactions, 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. Thus, the replacement of diazo group by other groups is helpful in preparing those substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene.