Diazonium Salts and Their Related Compounds

1 The formation of a by the interaction of and a primary aromatic , 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

4 5 Reactions of the diazonium salts

Replacement reactions

C6H5N2+ HSO4- + H2O -——> C6H5OH + N2 + H2SO4 6 2. Replacement by hydrogen

Replacement of the -group by hydrogen offers a means of preparing substituted aromatic compounds in which the 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) 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 , naphthols, and aromatic , to form highly coloured azo-compounds, e.g., benzenediazonium chloride couples with 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 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