Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 19. Amines.
Azo dyes, sought after for their almost endless number of shades, are used in coloring products ranging from purses and wallets to children's toys. These dyes can be manufactured through the diazotization process, which begins by converting an aromatic amine to a diazonium compound. The diazonium compound then reacts with a coupling component to produce the dyes. All azo dyes possess a -N=N- group, but the color produced is determined by the substituent groups. The dyes are difficult to biodegrade, but can be reduced to aromatic amines through reductive cleavage of the azo bond. The aromatic amines formed include such substances as 4-aminodiphenyl, 3,3-dimethoxylbenzidine, and 2,4,5-trimethylaniline (all of which can be viewed on Chemfinder.
The aromatic amines formed by this degradation have recently attracted attention due to their possibly carcinogenic properties. Use of azo dyes has been linked to development of cancer, although it is often difficult to assess causation, since many everyday substances are known carcinogens.
In addition to their carcinogenic properties, the Danish Environmental Protection Agency, which has been monitoring these dyes for quite some time, has found the aromatic amines to be harmful to the environment. Azo dyestuffs and pigments resist biodegradation under aerobic conditions, so they are difficult to remove from an ecosystem. Some substances formed upon degradation are extremely toxic, particularly chlorinated aromatic amines. The Danish EPA has proposed sulfonated azo-dyestuffs as an alternative, which appear to be less harmful to the environment as well as less carcinogenic.
The increased risk associated with using azo dyes has caused these dyes to
come under fire worldwide. The German authorities in particular have
monitored usage of these dyes very closely. Because of possible harmful
effects associated with azo dyes, the European Commission has recently
passed directive 76/769/EEC, which
the usage of certain azo dyes. This ban restricts the use of these dyes
in any textile or leather product which may come in "direct or prolonged"
contact with the skin or mouth. This directive not only addresses the
types of dyes that are unsuitable, but also the products in which it is
prohibited to use azo dyes. The proposed restrictions will affect all
member states of the European Commission and will be enforced beginning
July 1, 2000. Similar restrictions on azo dyes are already in place in
Germany, the Netherlands, and Austria. China Chemical News provides a
complete list of specific aromatic amines that are to be avoided by
restricting the use of azo dyes.
Pertinent Text References
Chapter 18. Aldehydes and Ketones.
Chapter 19. Amines.
Explain why common azo dyes often have sulfonate (-SO3-) or
carboxylate (-COO-) substituents on the molecule.
These groups promote solubility in water and help bind the dye to the surfaces of common fibers such as cotton and wool.
One of the most important features of azo dyes is that they
are synthesized by diazo coupling. Draw the azo linkage structure which
is the result of diazo coupling.
Question 3: Using Chemfinder, investigate the structures of 4-aminodiphenyl, 3,3-dimethoxybenzidine, and 2,4,5-trimethylaniline, three different amines that are formed from the decomposition of azo dyes.
Give a general example of the carcinogenic structure the ban
is designed to prohibit.
Aromatic amines Ph-NH2
Question 5: Do you believe the benefits of using a possibly carcinogenic substance that is both cheap and effective, outweigh the risk involved?
Question 6: What are some of the difficulties in assessing the carcinogenic properties of azo dyes? Propose some ways to overcome these difficulties.