CHE 211 Organic Chemistry I
Exp12 Preparation of Azo Dyes
Fall 2006 Last Revised 1/17/2008 Page 1 of 3
Relevant Chapters in Zubrick: None Applicable
Introduction
Using dyes to improve the appearance of plain fabrics is not new. According to history books, the first recorded
use of dyes was in China over 4600 years ago, where artisans used plant extracts as dyes. In the early 1500s,
the French, Dutch, and Germans developed a dye industry based upon the cultivation of plants. However, the
use of synthetic dyes is relatively new, arising as a serendipitous accident by an 18-year-old research assistant
named William Henry Perkin. In a misguided attempt to synthesize quinine by the oxidation of allyltoluidine,
Perkin produced a red-brown solid, which was definitely not quinine. He repeated the experiment, using aniline,
an aromatic amine that was structurally less complex than allyltoluidine. The aniline he used was isolated from
tar, and actually contained a mixture of aniline, o-toluidine, and p-toluidine.
CH
3
NHCH
2
CH CH
2
NH
2
allyltoluidine aniline
NH
2
CH
3
NH
2
CH
3
o-toluidine p-toluidine
Figure 1 – The components of the “aniline” isolated by Perkins.
This time he isolated black crystals. However, when he dissolved the black crystals in ethanol, the solution
formed a beautiful purple color, which we know as aniline purple or mauve. This was the first recorded
preparation of a synthetic dye. This fortuitous accident made Perkin a rich and famous man.
N
N
CH
3
N
H
CH
3
NH
2
CH
3
Figure 2 – The chemical structure of aniline purple (mauve).
Other chemists were also interested in using aniline as a precursor to dyes. A brewing chemist named Peter
Griess discovered that aniline could be converted to a diazonium salt, which (if it didn’t explode) could be coupled
to other aromatic compounds to form azo dyes in a variety of colors. Azo dyes have the general structure shown
here, where G is H, OH, NR
2
, or other electron-donating group and Z can be a variety of different functional
groups:
NZ N G
Figure 3 – The structure of a generic azo dye.
Virtually all azo dyes are synthesized through the process of diazotization and coupling. Diazotization involves
treating arom atic am ines with nitrous acid to yield diazonium salts. These salts are very versatile intermediates
that can be used to prepare a wide variety of aromatic substitution products in which the amino group is replaced
by some other substituent, such as F, Cl, Br, I, OH, H, and CN. However, when the diazonium salt is reacted with
