- General formula of diazonium salts is RN2+X- where R = aryl group and X- ion can be Cl-, Br-,HSO4- and BF4-.
- Diazonium salts 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 salt.
- For example, C 6H5 N2+ Cl- is named as benzenediazonium chloride and C6H5N2 +HSO4 – is known as benzenediazonium hydrogensulphate.
- Primary aliphatic amines form highly unstable alkyldiazonium salts. 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.
Preparation of Diazonium Salts
- Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K.
- Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid.
- The conversion of primary aromatic amines into diazonium salts is known as diazotisation. Due to its instability, the diazonium salt is not generally stored and is used immediately after its preparation.
C 6H5NH2 + NaNO2 + 2HCl à C 6H5N2+Cl- + NaCl +2H2O
(273 – 278K)
Properties of Diazonium Salts
- 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.
There are 2 reactions into with diazonium salts can be divided:-
- Reactions involving displacement of nitrogen
- Reactions involving retention of diazo group.
Reactions involving displacement of nitrogen
- 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.
- Replacement by halide or cyanide ion:
- The Cl–, Br –and CN– nucleophiles can easily be introduced in the benzene ring in the presence of Cu (I) ion.
- This reaction is called Sandmeyer reaction.
- 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 copper powder. This is referred as Gatterman reaction.
- The yield in Sandmeyer reaction is found to be better than Gatterman reaction.
- Replacement by iodide ion:
- Iodine is not easily introduced into the benzene ring directly, but, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed.
ArN2+Cl- + KI à Ar I + KCl + N2
- Replacement by fluoride ion:
- When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.
ArN2+Cl- + HBF4 à Ar – N2+BF4- à Ar-F + BF3 +N2
- Replacement by H:
- Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and they get oxidised to phosphorous acid and ethanal, respectively.
ArN2+Cl- + H3PO4 + H2O à ArH + N2 + H3PO3 + HCl
ArN2+Cl- + CH3CH2OH à ArH + N2 + CH3CHO + HCl
- Replacement by hydroxyl group:
- If the temperature of the diazonium salt solution is allowed to raise upto 283 K, the salt gets hydrolysed to phenol.
ArN2Cl- + H2O à ArOH + N2 + HCl
- Replacement by –NO2 group:
- When diazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by –NO2 group
Reactions involving retention of diazo group coupling reactions:-
- The azo products obtained have an extended conjugate system having both the aromatic rings joined through the –N=N– bond.
- These compounds are often coloured and are used as dyes. Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form
- This type of reaction is known as coupling reaction.
- Similarly the reaction of diazonium salt with aniline yields
p-aminoazobenzene. This is an example of electrophilic substitution reaction.