The 'coupling' of diazonium salts with phenols yields azo compounds, containing the azo group, -N=N-. For example, an alkaline solution of phenol (the coupling agent) reacts with benzene diazonium chloride (an electrophile) to form a yellow azo-dye. Coupled with naphthalen-2-ol a bright red precipitate is formed.
These particular dyes are of little practical value owing to their only slight solubility in water. However, azo-dyes containing one or more sulphonic acid groups are much more soluble and of considerable commercial importance in the dyestuffs industry.
In azo compounds the -N=N- group is part of an extended delocalised electron system involving the aromatic rings, called a chromophore. The quantised molecular electronic energy levels are closer together in such delocalised systems and light from the visible region of the electromagnetic spectrum is absorbed (DE) when electrons are promoted from lower to higher levels. The azo compound then appears a colour corresponding to the unabsorbed visible light. Also, groups such as -OH and -NH2 are often attached to chromophores (also forming part of the extended delocalised electron system, so changing the DE values) to modify the colours of the molecules. Click here to read more about colour.
Colour results from the absorption of photons in the visible region of the spectrum of electromagnetic radiation (400 to 750 nm). If a substance absorbs visible (white) light of all colours, it appears black; if no visible light is absorbed, the substance appears white (or colourless). Colourless substances usually absorb infra-red and the ultra-violet radiation. For example, ozone in the stratosphere absorbs ultra-violet radiation emitted by the Sun. Carbon dioxide in the troposphere absorbs infra-red re-emitted from the Earth's surface. Both of these gases appear colourless.
Complementary colours are two colours that, when combined, produce white or nearly white light. When only one colour is absorbed, the substance has the complementary colour. For example, the manganate(VII) ion absorbs green (typical wavelength 524 nm) and appears purple (typical wavelength 430 nm).
|Colour and typical|
wavelength (nm) absorbed
observed by eye (nm)
Complementary colours are shown in the 'colour wheel' below:
Azo-dyes bind to fabrics in different ways. For cotton, many are insoluble and become trapped in the fibres. Others, called direct dyes, become attached to the fibres by hydrogen bonding and instantaneous dipole-induced dipole intermolecular bonding. Because intermolecular bonding is much weaker than covalent bonding, the dye molecules must be long and straight so that they can align closely with the cellulose fibres of cotton giving more opportunities for intermolecular attractions. The structure of the dye Direct Blue 1 is illustrated below:
Both aliphatic and aromatic primary amines can form diazonium salts. They do this by reacting with Nitric(III) acid (nitrous acid), HNO2, at a temperature of 0 - 5 °C.
Owing to the instability of nitric(III) acid, it is always generated during a reaction, usually by the action of dilute sulphuric(VI) acid or hydrochloric acid on sodium nitrate(III) (sodium nitrite), NaNO2. The acid used to generate nitric(III) acid provides the anion of the diazonium salt.
R-NH2 + NaNO2 + 2HCl ® R-N+ºN: Cl- + 2NaCl + 2H2O
Ph-NH2 + NaNO2 + 2HCl ® Ph-N+ºN: Cl- + 2NaCl + 2H2O
However, alkyl diazonium salts are extremely unstable and always decompose to evolve the colourless unreactive nitrogen gas, amongst other products.
The diazonium cations of aromatic diazonium salts are somewhat more stable than their aliphatic counterparts. With phenylamine the benzene diazonium ion is formed:
C6H5 - NH2 + HNO2 + H+ ® C6H5 - N+ºN: + 2H2O
Although benzene diazonium salts can be isolated in the crystalline form, they are usually retained in solution and used immediately as they decompose on standing even in the cold. In the solid state the salts are explosive and can be easily detonated by a slight shock or on mild warming.
A structure for benzene diazonium chloride is shown below:
The greater stability of the benzene diazonium cation compared with the alkyl diazonium cation is attributed to the diazo group being part of the delocalised system with the benzene ring and so the way in which the positive charge is distributed about the ring.
In practice, a solution of a benzene diazonium salt is added to an alkaline solution of a phenol (or aromatic amine, such as phenylamine). The benzene diazonium cation behaves as an electrophile, but it is a weak electrophile and so the aromatic ring which it attacks must have attached to it an activating group such as -OH or -NH2. An electrophilic substitution reaction occurs to form an azo-dye.