Diazonium Salts and Their Related Compounds
1 The formation of a diazonium compound by the interaction of sodium nitrite and a primary aromatic amine, in ice-cold solution, is known as diazotization. This reaction was discovered by Griess in 1858. + - ArNH2 + HNO2 + HCl —> ArN2 + Cl + H2O
+ - Ar N N: X
The diazonium salts are very important synthetic reagents, being the starting-point in the preparation of various aromatic compounds, dyes and drugs.
2 Why does diazonium salt be stable?
The stability of diazonium salts is attributed to resonance
.. + + + ...... N N: N N: N N..H +
3 The mechanism of diazotisation
the rate-determining step is nitrosation of the free amine to a primary nitrosamine
4 5 Reactions of the diazonium salts
Replacement reactions
C6H5N2+ HSO4- + H2O -——> C6H5OH + N2 + H2SO4 6 2. Replacement by hydrogen
Replacement of the diazo-group by hydrogen offers a means of preparing substituted aromatic compounds in which the substituents are in positions, which they would not take up by direct substitution by means of hypophosphorous acid. Also, cuprous salts catalyze this reaction
ArN2Cl- + H2P(O)OH →HCl + Ar—N = N—PH(O)OH —— > ArH+N2 + ClPH(O)OH 3. Replacement by halogen, (i) Sandmeyer reaction 8 m-Chloronitrobenzene
NH2 + - N N Cl Cl
NaNO2 CuCl/HCl
HCl NO2 NO2 NO2 p-Bromotoluene
NH 2 + - N N HSO4 Br
NaNO2 CuBr/HBr
H2SO4
CH3 CH3 CH3 ii) Gattermann reaction (1890) 9
NH2 + - N N Br Br CH3 CH3 CH3 NaNO2 Cu
HBr lodo-compounds
N NCl I
KI KCl N2 Fluoro10 -compounds may be prepared by the Schiemann reaction. Coupling reactions 12
Diazonium salts readily undergo coupling reactions with phenols, naphthols, and aromatic amines, to form highly coloured azo-compounds, e.g., benzenediazonium chloride couples with phenol in faintly alkaline solution to form p- hydroxyazobenzene, and with dimethylaniline in faintly acid solution to form p-dimethylaminoazobenzene.
C6H5OH .. + C6H5NMe2 ...... N N OH N N: N N N(CH ) - - + 3 2 HO Cl H A detailed study of the coupling reaction has shown that the 13 mechanism is electrophilic substitution, the electrophile being the diazonium cation and the substrate being the free amine or the phenoxide ion:
+ .. + .. N N: N N: + N(CH3)2 I (II) + .. .. + -H ...... N N N(CH3)2 N N N(CH3)2 H The phenoxide ion also undergoes the resonance effect in a similar14 manner
OH
N N N N H
O H O H H
OH
N N 15 Since (II) is the "reactive' contributing resonating structure, any factor that increases its contribution will increase the reactivity of that diazonium cation. Electron withdrawing groups would favor (I) and electron-releasing groups would favor (II). This has been found to be so in practice, e.g., the p- nitrobenzenediazonium cation is more than 10000 times as reactive as the p-methoxy ion under the same conditions. Coupling16 with benzene substrates occurs preferentially in the p-position to the hydroxyl or amino-group, but if this is blocked, then o-coupling occurs, e.g., p-cresol gives the o-azo- compound
- HO + - H C OH Ar N N: Cl + H3C OH 3
N N Ar