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2.5.4 Diazotisation and azo coupling
Chapter 2
[6, Ullmann, 2001, 19, Booth, 1988, 46, Ministerio de Medio Ambiente, 2003, 51, UBA, 2004]
See also Section 4.3.2.4 for environmental issues and treatment of waste streams from
diazotisation and azo coupling.
Diazotisation and coupling processes are important for the manufacture of APIs and represent
the essence of azo dye manufacture. Azo dyes are the predominant colourant family, accounting
for over 50 % of all commercial organic dyes. Diazotisation can also be followed by processes
such as hydrazine formation, Sandmeyer reactions and azo double bond reduction.
Diazo and coupling components can be halogenated and can contribute to an AOX load in waste
water streams. Often, azo coupling includes an immediate metallisation step involving heavy
metals to give metal complex dyes.
Chemical reaction
Diazotisation is the reaction of primary arylamines with nitrites, preferably with sodium nitrite,
in a usually aqueous mineral acid solution at around 0 °C, whereby the amine is converted into
the corresponding diazonium compound.
Weakly basic arylamines require a higher acid concentration (NO2 - surplus), since diazoamino
compounds Ar–N=N–HN–Ar may otherwise form. A further reason for using concentrated
acids (e.g. concentrated sulphuric acid) is the fact that diazonium compounds of weakly basic
arylamines are readily hydrolysable in dilute acids.
R
NH 2
+ NaNO 2 + HCl
- NaCl
- H2O Figure 2.12: Diazotisation and azo coupling
R
OFC_BREF Dezember 2005 43
N +
Cl -
N
+ R'-H
The azo coupling reaction is an electrophilic substitution reaction of the diazonium compound
with a coupling component R’H. In order to maintain an optimal reaction sequence, the pH must
be kept constant by adding alkalis or buffers.
Coupling components: phenols, naphthols and amines
Side reactions: formation of diazo amino compounds
decomposition of diazo salts to phenolic compounds
formation of isomers
processing of isomers contained in the starting material.
Operations
Figure 2.13 shows a typical sequence of operations for diazotisation and azo coupling in
dyestuff manufacturing.
Sodium nitrite is added in excess to a solution or suspension of the arylamine (diazo
component) in a diazotisation tank. The reaction is cooled to 0 °C by adding ice or by cooling
with brine. In a separate tank, the coupling component is dissolved in water and alkali. Both
solutions are clarified by filtering and added to the coupling vessel. The addition sequence
depends on the particular case, and the precise reaction conditions (pH, temperature) are
established by the addition of alkali or ice.
- HCl
R
N
N
R'