Can be azo dyes obtained by grinding under solvent-free conditions?

Article abstract of DOI:10.1002/jccs.200900148

The solid-solid reactions of some electron-donors with sulfanilic acid in the presence of solid sodium nitrite afford azo dyes by self-catalyzed diazotization of sulfanilic acid (2) under solvent-free conditions with moderate yields. Also the reactions of some electron-donors with diazotization of o-nitroaniline (5), m-nitroaniline (6) and p-nitroaniline (7) in the presence of solid sodium nitrite catalyzed by p-toluenesulfonic acid (PTSA) afford azo dyes under solvent-free conditions in good yields. This new method totally avoids the use of acids, alkalies, and toxic and/or expensive solvents in diazotization and diazo coupling reactions.

Full text of DOI:10.1002/jccs.200900148

1018
Journal of the Chinese Chemical Society, 2009, 56, 1018-1027
Can be Azo Dyes Obtained by Grinding under Solvent-free Conditions?
Nader Noroozi-Pesyan,* Jabbar Khalafy and Zahra Malekpoor
Department of Chemistry, Faculty of Science, Urmia University, 57159, Urmia, Iran
The solid-solid reactions of some electron-donors with sulfanilic acid in the presence of solid sodium
nitrite afford azo dyes by self-catalyzed diazotization of sulfanilic acid (2) under solvent-free conditions
with moderate yields. Also the reactions of some electron-donors with diazotization of o-nitroaniline (5),
m-nitroaniline (6) and p-nitroaniline (7) in the presence of solid sodium nitrite catalyzed by p-toluenesul-
fonic acid (PTSA) afford azo dyes under solvent-free conditions in good yields. This new method totally
avoids the use of acids, alkalies, and toxic and/or expensive solvents in diazotization and diazo coupling
reactions.
Keywords: Azo dyes; Solvent-free; Self-catalyzed diazotization; Zwitterions; Sulfanilic acid;
p-Toluenesulfonic acid; Green chemistry.
INTRODUCTION
Azoic dyes are the most important group of all syn-
conditions of diazo-coupling by means of self-catalyzed
diazotization of zwitterionic aromatic amino acids or
diazotization catalyzed by p-toluenesulfonic acid (PTSA).
As part of our ongoing research program to explore bifunc-
tional catalytic properties, we herein describe a new pro-
cess for diazotization and diazo coupling reactions using
self-catalyzed diazotization of sulfanilic acid and acid
catalyzed diazotization of aniline derivatives by PTSA
toward the synthesis of azo dyes.
thetic dyes. These dyes possess a wide range of deep colors
(yellow, red, orange, violet, dark blue or green).1,2 In recent
decades, organic color chemistry has been undergoing very
exciting development as a result of the opportunities pre-
sented by dye applications in high technology fields: elec-
tronic devices, linear and non linear optics, reprography,
sensors and biomedical uses.3-6 Many azo compounds have
been applied as chromogenic reagents for the determina-
tion of several metal ions.7
RESULTS AND DISCUSSION
The formation of a diazotizing reagent starts with
protonation of nitrous acid under acidic conditions, and azo
coupling carried out at low temperature in the presence of
nucleophilic substrate increases with increasing basicity of
barbiturate, phenolate and/or other electron-donor deriva-
tives.1 The acid-base catalyzed processes are effective for
the near quantitative formation of the desired products. But
the main limitation of such processes is their environmental
incompatibility.8 In addition to solvent cases, the synthesis
of azo dyes in ionic liquids has been reported.9 The solid-
state diazotization of some electron-rich aromatics with
gaseous NO2,10,11 in the presence of ferric nitrate as an oxi-
dizing reagent,12 via resin supported diazonium salts,13 on
clays as catalyst,14 on silica-sulfuric acid15 and even in
supercritical carbon dioxide16 have been reported. How-
ever, there are no reactions reported under solvent-free
This paper describes the facile and novel route for the
synthesis of azo dyes by grinding under solvent-free condi-
tions (without using conventional acids or bases). The nov-
elty of this synthesis is the self-catalyzed diazotization of
an amino group on sulfanilic acid 2 via solid NaNO2. Di-
azonium salt of sulfanilic acid (2D) was first made without
using strong conventional inorganic acids (hydrochloric,
sulfuric acid, etc). The main reason for choosing 2 as diazo-
tizing agent is its zwitterionic character and ability to self-
catalyze by means of a sulfonic acid functional group under
solvent-free conditions (Schemes I and II).
Diazo coupling reaction of 2 with electron donors
(1a-g) using a grinding method under solvent-free condi-
tions afforded azo dyes (3a-g) in 25-70% yields (Scheme I
and Table 1). In this new process, grinding the three com-
ponent mixture, the electron donor (as a coupling agent),
* Corresponding author. Tel: +98-441-2780952; Fax: +98-441-2776707; E-mail: n.noroozi@mail.urmia.ac.ir or pesyan@gmail.com

Can be Azo Dyes Obtained by Grinding Conditions?
J. Chin. Chem. Soc., Vol. 56, No. 5, 2009 1019
sodium sulfanilate (2A) and nitrous acid in the presence of
NaNO2. In the next stage, an amino group of 2A initiated
the nucleophilic addition reaction with nitrous acid to pro-
duce intermediate 2B. Elimination of water from 2B gave
2C, then a second elimination of water from 2C produced
diazonium salt 2D. Finally, the nucleophilic coupling of
barbituric acid (1a) on 2D formed azo dye 3a.
Scheme I
In almost all the cases, the isolated yields of the prod-
ucts are outlined in Table 1. The yield of 3 was decreased
with increasing temperature. It appears that the formation
of diazonium salt 2D is stable at lower temperatures but de-
composes at higher temperatures. In the decomposition
process the water molecule is released making the reaction
mixture moist. This observation is good evidence for the
confirmation of the mechanism depicted in Scheme II.
1H NMR spectra of the crude reaction mixture of so-
dium 4-[(2,4,6-trioxohexahydropyrimidin-5-yl)diazenyl]-
benzenesulfonate (3a) showed two doublets at d 7.51 and
1
7.66 ppm (J = 8.7 Hz) and comparable with the H NMR
Scheme II
data of pure compound. Two amide protons and the high
deshielded NH/OH protons of 3a appeared at d 11.29,
11.49 and 14.13 ppm in the crude reaction mixture, respec-
tively (Fig. 1). The 1H NMR spectrum of crude 3a matches
solid NaNO2 and 2 formed a colored reaction mixture. The
residue NaNO2 was removed by extraction of the reaction
mixture with methanol.
Fig. 1. 1H NMR spectrum of the reaction mixture of 1a
and 2 as representative (crude products after ex-
traction with methanol including 3a): (a) and
spectrum of 3a after purification: (b) (300
MHz, DMSO-d6).
The mechanism of self-catalyzed diazotization and
diazo coupling reaction of sulfanilic acid with barbituric
acid in the presence of NaNO2 is depicted in Scheme II as
representative. Initially, sulfanilic acid 2 was converted to

1020 J. Chin. Chem. Soc., Vol. 56, No. 5, 2009
Noroozi-Pesyan et al.
the authentic sample of 3a. Two doublets at d 6.50 and 7.29
ppm (J = 8.4 Hz) are due to unreacted sulfanilic acid 2. Two
amide protons of unreacted barbituric acid 1 overlapped
with the peak of the one of the amide proton of 3 at d 11.49
ppm as a broad peak. These observations demonstrated the
formation of azo dyes under solvent-free conditions.
Diazo coupling of a-naphthol (1d) with diazonium
salt of 2 only proceeded on C-4 under solvent-free condi-
b-naphthol (1e) shows two broad singlet peaks at d 15.78
and 12.50 ppm with an integration ratio of 1.00:0.94, indi-
cating that two isomers are formed. Whereas, the reaction
of 1e with 2 only afforded sodium 4-[(2-hydroxynaphtha-
len-1-yl)diazenyl]benzenesulfonate (3e) in solution.17 1H
NMR spectra of the crude reaction mixture of phenol (1f)
show two broad singlet peaks at d 12.51 and 10.31 ppm. In-
terestingly, the azo dyes sodium 4-[(4-hydroxyphenyl)di-
azenyl]benzenesulfonate (3f) and sodium 4-[(2-hydroxy-
1
tions. H NMR spectra of the crude reaction mixture of

Can be Azo Dyes Obtained by Grinding Conditions?
J. Chin. Chem. Soc., Vol. 56, No. 5, 2009 1021
phenyl)diazenyl]benzenesulfonate (4f) obtained from the
reaction of 1f with 2 under solvent-free conditions indi-
cated that diazo coupling proceeded on both C-2 and C-4
carbon atoms on the phenyl ring (Scheme III). The integra-
tion ratio of the two deshielded protons in two isomeric
forms (diazotized at ortho- and para-position) was 1.00:
1.98, respectively. Instead, diazo coupling of phenol only
occurred on the C-4 carbon atom under solvent conditions
(Scheme III). This criterion is the advantage of solvent-free
to solution conditions. Azo compound sodium 4-[(2,4-di-
hydroxyphenyl)diazenyl]benzenesulfonate (Tropaeolin O)
(3g) showed two deshielded signals at d 12.47 and 9.14
ppm which correspond to two OH groups on the resorcinol
ring moiety (Scheme III, Table 2).
Scheme IV
Scheme III
NaNO2 by grinding under solvent-free conditions. These
reactions were carried out at 0 °C with good yields (50-
80%), and the reaction yields are summarized in Table 3. In
There are several tautomeric forms that exist for bar-
bituric and thiobarbituric acid azo dye derivatives.18,21 In-
terestingly, the 1H NMR and 13C NMR spectra of 3b showed
two distinct tautomeric forms II and V under solution con-
ditions, whereas 3a and 3c did not! (Scheme IV, Table 2,
Figs. 1 and 2). It is obvious from Fig. 2 [comparison of the
1H NMR spectra of 3b in crude reaction mixture (a) with its
pure case (b)] that there are two tautomeric forms of 3b.
FT-IR spectra of dye 3b showed two intense carbonyl
bonds at 1738-1665 cm-1 and a broad band for hydroxyl
group at 3428 cm-1. These suggest that the azo dye is pre-
dominantly in lactam-lactim-azo (keto-enol-azo (II)) and
lactim-hydrazone (keto-enol-hydrazone (V)) forms as op-
posed to the forms in the solid state. FT-IR spectra also
showed a bond at 3257 and 3428 cm-1, assigned to an imino
group (N–H) and hydroxyl group (O–H), respectively.
In continuation of this research, we examined the syn-
thesis of azo dyes in the presence of an electron donor, ani-
line derivative, p-toluenesulfonic acid (PTSA) and solid
Fig. 2. 1H NMR spectrum of the reaction mixture of 1b
and 2 (crude products after extraction with
methanol including two tautomers of 3b): (a)
and spectrum of 3b after purification: (b) (300
MHz, DMSO-d6).

1022 J. Chin. Chem. Soc., Vol. 56, No. 5, 2009
Noroozi-Pesyan et al.
Table 2. Spectroscopic data of azo dyes (3a-3g and 8a-8c- 10a-10e)
Azo
dyes
IR, 1H NMR, 13C NMR
3a
IR (KBr, cm-1): 3413.57 (s), 3257.20 (s), 3096.67 (s), 2831.50 (w), 1738.83 (s), 1712.20 (s),
1668.12 (s), 1513.53 (s), 1433.56 (s), 1263.18 (s), 1196.93 (s), 1042.98 (s).
1H NMR (DMSO-d6): 14.13 (s, 1H), 11.49 (s, 1H), 11.29 (s, 1H), 7.66 (d, J = 8.4 Hz, 2H), 7.51 (d, J
= 8.4 Hz, 2H).
13C NMR: 162.50, 160.45, 150.24, 145.59, 141.82, 127.53, 118.44, 116.39.
IR (KBr, cm-1): 3428.04 (s), 3257.76 (s), 3096.46 (s), 1738.76 (s), 1710.24 (s), 1665.54 (s), 1509.09
(s), 1433.80 (s), 1261.14 (s), 1203.30 (s), 818.91 (m), 618.76 (m).
1H NMR (DMSO-d6): 14.24 (s, 1H), 14.15 (s, 1H), 12.58 (s, 1H), 12.41 (s, 1H), 11.50 (s, 1H), 11.28
(s, 1H), 7.68 (d, J = 8.7 Hz, 2H), 7.67 (d, J = 8.7 Hz, 2H), 7.55 (d, J = 8.7 Hz, 2H), 7.52 (d, J = 8.7
Hz, 2H).
3ba
13C NMR: 177.88, 162.50, 160.45, 160.34, 158.86, 150.24, 146.03, 145.58, 141.81, 141.65, 127.56,
127.52, 119.33, 118.44, 116.83, 116.39.
3c
IR (KBr, cm-1): 2600-3600 (bs), 1731.13 (s), 1682.62 (s), 1647.57 (s), 1525.84 (s), 1450.42 (s),
1384.66 (s), 1201.24 (s), 1123.72 (s), 651.38 (s).
1H NMR (DMSO-d6): 14.16 (s, 1H), 7.65 (d, J = 8.7 Hz, 2H), 7.21 (d, J = 8.7 Hz, 2H), 2.60-4.60
(bs, H2O), 3.22 (s, 3H), 3.21 (s, 3H).
13C NMR: 160.82, 159.28, 151.04, 145.65, 141.89, 134.64, 127.53, 127.46, 121.92, 118.12, 116.56.
IR (KBr, cm-1): 3486.44 (s), 3382.78 (s), 1629.21 (m), 1599.33 (s), 1224.05 (s), 1175.85 (s),
1122.18 (s), 1033.70 (s), 769.97 (m), 567.18 (s).
3d
1H NMR (DMSO-d6): 10.20 (bs, 1H), 8.18 (d, 4H, J = 8.2 Hz, phenyl sulfonate), 7.31-8.59 (m, 6H,
naphthyl).
13C NMR: 160.0, 157.70, 146.30, 143.90, 129.50, 128.62, 127.90, 126.80, 126.60, 126.50, 124.0,
121.70, 115.50, 109.70.
3e
IR (KBr, cm-1): 3420.81 (bs), 1618.32 (m), 1597.51 (m), 1507.51 (s), 1210.38 (s), 1120.49 (s),
1032.85 (s), 831.90 (m), 759.72 (m), 698.09 (m), 644.58 (m).
1H NMR (DMSO-d6): 15.81 (bs, 1H), 12.50 (bs, 1H), 8.54 (d, 1H, J = 8.4 Hz), 7.95 (d, 1H, J = 9.6
Hz), 7.73-7.83 (m, 5H), 7.61 (t, 1H, J = 7.45 Hz), 6.90 (d, 1H, J = 9.3 Hz).
13C NMR: 170.29, 147.91, 145.13, 140.76, 133.17, 129.86, 129.63, 129.40, 128.32, 127.61, 126.45,
124.58, 121.88, 118.56.
3f
IR (KBr, cm-1): 3487.75 (bs), 3384.88 (s), 1627.31 (m), 1604 48 (s), 1223.65 (s), 1176.39 (s),
1123.50 (s), 1034.72 (s), 573.23 (s).
1H NMR (DMSO-d6): 12.51 (bs, 1H), 10.35 (bs, 1H), 7.73-7.78 (m, 7H), 6.90-6.93 (m, 2H).
13C NMR: 161.54, 152.26, 150.17, 145.67, 127.06, 125.34, 122.01, 116.41.
IR (KBr, cm-1): 3460.67 (bs), 3385.48 (s), 1624.73 (s), 1604.81 (s), 1177.06 (s), 1122.80 (s), 1035
12 (s), 570.96 (m).
3g
1H NMR (DMSO-d6): 12.47 (s, 1H, -OH), 9.14 (s, 1H, -OH), 7.74-7.81 (m, 4H), 7.67 (d, 1H, J =
8.7 Hz), 6.51 (dd, 1H, J = 2.4 Hz), 6.36 (d, 1H, J = 2.7 Hz).
13C NMR: 163.66, 157.03, 150.97, 149.07, 132.82, 130.54, 127.19, 121.60, 109.81, 103.44.
IR: 3447.85 (m), 3072.32 (s), 1747.31 (s), 1679.78 (s), 1643.32 (m), 1488.16 (s), 1328.62 (s),
1229.67 (s), 813.81 (s).
8a
8b
8c
9a
1H NMR: 15.18 (s, 1H), 11.70 (s, 1H), 11.48 (s, 1H), 8.261 (dd, J3 = 8.4 Hz, J4 = 1.2 Hz, 1H), 8.08
(d, J = 7.8 Hz, 1H), 7.88 (t, J = 7.5 Hz, 1H), 7.38 (td, J3 = 7.8 Hz, J4 = 1.2 Hz).
IR: 3633.07 (m), 3504.99 (m), 3137.79 (s), 1720 (s), 1700 (s), 1668.53 (s), 1606.06 (s), 1583.33 (s),
1467.49 (s), 1217.17 (s).
1H NMR: 15.23 (s, 1H), 12.76 (s, 1H), 12.59 (s, 1H), 8.27 (dd, J3 = 8.4 Hz, J4 = 0.9 Hz, 1H), 8.08
(d, J = 8.4 Hz, 1H), 7.90 (t, J = 7.2 Hz, 1H), 7.41 (td, J3 = 7.8 Hz, J4 = 1.2 Hz).
IR: 3440.00 (bm), 3088.92 (m), 2950.00 (w), 2925.00 (w), 1736.95 (s), 1680.80 (s), 1647.45 (s),
1491.56 (s), 1337.86 (s), 1230.70 (s), 747.16 (s).
1H NMR: 15.23 (s, 1H), 8.29 (d, J = 8.4 Hz, 1H), 8.10 (d, J = 8.1 Hz, 1H), 7.91 (t, J = 7.8 Hz, 1H),
7.41 (t, J = 8.1 Hz, 1H), 3.24 (s, 3H), 3.23 (s, 3H).
IR: 3194.63 (s), 3074.07 (s), 1755.13 (s), 1729.67 (s), 1665.42 (s), 1519.42 (s), 1355.94 (s), 1249.74
(s), 814.03 (s).
1H NMR: 13.92 (bs, 1H), 11.39 (bs, 2H), 8.44 (t, J = 2.1 Hz, 1H), 8.03 (d, J = 2.1 Hz, 1H), 8.00 (d,
J = 2.1 Hz, 1H), 7.71 (t, J = 8.4 Hz).

Can be Azo Dyes Obtained by Grinding Conditions?
J. Chin. Chem. Soc., Vol. 56, No. 5, 2009 1023
9b
IR: 3194.63 (s), 3074.07 (s), 1755.13 (s), 1729.67 (s), 1665.42 (s), 1519.42 (s), 1355.94 (s), 1249.74
(s), 814.03 (s).
1H NMR: 13.92 (bs, 1H), 11.39 (bs, 2H), 8.44 (t, J = 2.1 Hz, 1H), 8.03 (d, J = 2.1 Hz, 1H), 8.00 (d,
J = 2.1, 1H), 7.71 (t, J = 8.4 Hz).
9c
IR: 3436.00 (bs), 3100.00 (w), 2920.00 (w), 2960.00 (w), 1727.63 (s), 1677.71 (s), 1648.60 (s),
1516.46 (s), 1451.76 (s), 1352.64 (s), 1245.80 (s), 750.75 (s).
1H NMR: 14.023 (s, 1H), 8.50 (t, J = 2.1 Hz, 1H), 8.06 (m, J = 8.4 Hz, 2H), 7.73 (t, J = 8.4, 1H),
3.22 (s, 6H).
10a
IR: 3569.22 (s), 3460.47 (s), 3202.46 (s), 3064.62 (m), 1710.99 (s), 1697.96 (s), 1670.99 (m),
1611.43 (m), 1510.06 (s), 1446.78 (s), 1336.69 (s), 1246.96 (s).
1H NMR: 13.95 (bs, 1H), 11.65 (bs, 1H), 11.42 (s, 1H), 8.32 (d, J = 9.3 Hz, 2H), 7.79 (d, J = 9.3 Hz,
2H).
13C NMR: 162.00, 159.97, 150.20, 147.44, 144.29, 126.07, 121.29, 117.17.
IR: 3524.00 (s), 3434.03 (s), 3257.04 (s), 3088.14 (s), 1705.60 (s), 1664.05 (s), 1609.92 (s), 1494.20
(s), 1339.63 (s), 1240.07 (s).
10b
10c
10f
1H NMR: 14.03 (s, 1H), 12.69 (s, 1H), 12.53 (s, 1H), 8.32 (d, J = 9.3 Hz, 2H), 7.81 (d, J = 9.3 Hz,
2H).
IR: 3399.72 (bw), 3082.44 (m), 2943.78 (w), 2900.0 (w), 1730.66 (s), 1681.01 (s), 1643.49 (s),
1517.90 (s), 1455.18 (s), 1343.52 (s), 1248.28 (s), 746.58 (s).
1H NMR: 13.99 (s, 1H), 8.32 (d, J = 9.0 Hz, 2H), 7.83 (d, J = 9.0 Hz, 2H), 3.23 (s, 3H), 3.21 (s,
3H).
IR: 3413.45 (m), 1604.42 (m), 1585.47 (s), 1504.07 (s), 1336.12 (s), 1282.93 (m), 1139.09 (s),
1108.58 (w).
1H NMR: 10.56 (s, 1H), 8.35 (d, 2H, J = 8.4 Hz), 7.94 (d, 2H, J = 8.4 Hz), 7.84 (d, 2H, J = 8.4 Hz),
6.94 (d, 2H, J = 8.4 Hz).
13C NMR: 162.75, 155.98, 148.18, 145.82, 126.22, 125.43, 123.40, 116.65.
IR: 3467.92 (m), 1602.42 (m), 1526.61 (s), 1503.24 (s), 1342.03 (m), 1271.31 (m), 1140.41 (s).
1H NMRb: 8.71 (t, 1H, J = 1.8 Hz), 8.28-8.32 (m, 1H), 8.23 (t, 1H, J = 7.2 Hz), 7.95 (d, 2H, J = 8.7
Hz), 7.69, (t, 1H, J = 8.1 Hz), 6.99 (d, 2H, J = 8.7 Hz), 5.37 (bs, 1H, -OH).
13C NMRb: 159.17, 153.17, 146.81, 130.30, 129.86, 129.02, 125.63, 124.32, 116.76, 116.01.
10g
a 1H NMR and 13C NMR data of the mixture of two distinct tautomers.
b NMR solvent was CDCl3.
this new process, grinding a four component mixture, the
electron-donor (as a coupling agent), aniline derivative,
NaNO2 and PTSA formed a colored reaction mixture (see
experimental). Electron-donors (1a-c, 1f and 1g) coupled
with the diazotized ortho- (5), meta- (6) and para-nitro-
aniline (7) afforded corresponding azo dyes (8a-8c-10a-
10g) (Scheme V). The yields of azo dyes were increased
with increasing the electron-donor ability of the coupling
agent.
Scheme V
The proposed mechanism for acidic catalyzed and di-
azo coupling reaction of diazonium salt of p-nitroaniline 7
with barbituric acid 1a in the presence of solid NaNO2 is
depicted in Scheme VI as representative. Initially, PTSA
converted to sodium p-toluenesulfonate and nitrous acid in
the presence of solid NaNO2 by grinding. In the next stage,
an amino group of 7 initiated the nucleophilic addition re-
action with nitrous acid to produce intermediate 7A. Elimi-
nation of water from 7A gives 7B, then a second elimina-
tion of water molecules from 7B produces diazonium salt
7C. Finally, the nucleophilic attaching of 1a as coupling re-
agent to 7C forms azo dye, 10a as representative (Scheme
VI). Similar to the decomposition process in Scheme II, the
water molecule is released making the reaction mixture
moist. In comparison, the catalytic ability of PTSA is

1024 J. Chin. Chem. Soc., Vol. 56, No. 5, 2009
Noroozi-Pesyan et al.
greater than that of sulfanilic acid under solvent-free condi-
tions.
UV-visible spectra of azo dyes 3c and 3f are shown in
Figs. 3-5, respectively, that are recorded in water (a) and
DMSO (b) since azo dyes based on sulfanilic acid only are
soluble in DMSO and water. These spectra were recorded
over the range of l between 300 and 700 nm using two sol-
vents in concentrations (10-4-10-5 M), and the results of all
dyes are summarized in Table 4.
Scheme VI
It was observed that although in DMSO and water the
absorption spectra did not change significantly except for
dyes 3b, 3c, 3f and 3g. lmax values of dye 3c in water
shifted bathochromically in respect to the lmax in DMSO in
higher concentrations (Fig. 3). In contrast, lmax values of
dye 3g in DMSO shifted bathochromically with increasing
concentration. Interestingly, dye 3f, showed two lmax val-
ues in high concentration at 342 and 384 nm while in low
concentration showed only one lmax at 364 nm in DMSO
(Figs. 4 and 5). Dye 3b shifted hypsochromically in DMSO
with respect to water.
EXPERIMENTAL
Melting points were taken with a digital melting point

Can be Azo Dyes Obtained by Grinding Conditions?
J. Chin. Chem. Soc., Vol. 56, No. 5, 2009 1025
Compounds 1a-1c were synthesized in our laboratory based
on common and known methods.17 Compounds 1d-1g, 2,
5-7 and sodium nitrite were purchased from Merck and
used without purification.
Table 4. Absorption maxima of dyes 3a-3g in DMSO and water
Azo dyes
DMSO
Water
3a
3b
3c
3d
3e
3f
392
408
396
412
488
392
402
390a
480
486
352
428
A general preparative procedure is described below
for the preparation of sodium 4-[(2,4,6-trioxohexahydro-
pyrimidin-5-yl)diazenyl]benzenesulfonate (3a). All other
dyes (3b-3g) were prepared in a similar manner.
342,384b
400
3g
a The lmax is variable and depends on concentration.
b Two lmax observed in higher concentrations.
The mixtures of barbituric acid 1a (1.06 mmol), sul-
fanilic acid 2 (1.06 mmol) and sodium nitrite (1.06 mmol)
are mixed, ground in an agate mortar equipped in an ice-
bath and homogenized at 0 °C. After 10 minutes the ho-
mogenized mixture was perturbed yellow and reddish color
by grinding (Caution! most diazonium salt in the dry state
is very unstable and prone to heat or impact explosion22-24
and also some reactions of aryldiazonium salts come with a
serious warning of an explosion hazard25). The reaction
progression was monitored by thin layer chromatography
apparatus (Electrothermal) and were uncorrected. IR spec-
tra were determined in the region 4000-400 cm-1 on a
NEXUS 670 FT IR spectrometer by preparing KBr pellets.
The 1H NMR spectra were measured in DMSO-d6 at 300
MHz, using a Bruker 300 FT-NMR spectrometer referenc-
ing tetramethylsilane as internal standard. UV-visible spec-
tra were recorded on a T80 UV/VIS spectrophotometer.
Fig. 3. Absorption spectra of 3c in water (a) and
Fig. 4. Absorption spectra of 3f in water (a) and
DMSO (b).
DMSO (b).

1026 J. Chin. Chem. Soc., Vol. 56, No. 5, 2009
Noroozi-Pesyan et al.
electron-donors under solvent-free conditions with moder-
ate yields. Self-catalyzed diazotization of an amino group
in sulfanilic acid proceeded by means of its own sulfonic
acid functional group in the presence of solid sodium ni-
trite. Diazotization and diazo coupling of aniline deriva-
tives were carried out with acid catalyzing of PTSAin good
yield, successfully. This method totally avoids the use of
strong inorganic acids, alkalies or toxic solvents in diazo-
tization and diazo coupling reactions. The residue reactants
were removed by extraction with water and ethanol from
azo dyes that were catalyzed by PTSA, so this is the advan-
tage of this process. In comparison, the catalytic ability of
PTSA is greater than that of sulfanilic acid under solvent-
free conditions.
Fig. 5. Absorption spectra of dye 3f in water and in
DMSO (1.0 ´ 10-4 M).
ACKNOWLEDGEMENTS
(TLC) using the mixture of chloroform 60% and methanol
40% (v:v). The reaction mixture was extracted by methanol
(2 ´ 25 mL) twice to remove the sodium nitrite. After re-
moving the solvent the residue was eluted through plate
chromatography (HF254(type 60)) with the mixture of sol-
vents chloroform 60% and methanol 40% (v:v). Spectro-
scopic data and physical properties of obtained azo dyes
were identical with the authentic samples and are summa-
rized in Tables 1 and 2. All obtained azo dyes decomposed
up to 200 °C.
We thank the Urmia University Research Council for
financial support (Grant No. #15-9783). We also gratefully
acknowledge Prof. A. H. Dabbagh from Isfahan University
of Technology (IUT) for helping and some guidance.
Received January 13, 2009.
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