Photocatalytic cleavage of hydroxytriazenes: A solidstate synthesis of azo-dyes under sunlight irradiation

Article abstract of DOI:10.1007/s11164-012-0533-x

Sunlight-induced decomposition of hydroxytriazenes, and green photochemical synthesis of azo-dyes is described. Three substituted hydroxytriazenes, viz: 3-hydroxy-3-(2-methylphenyl)-1-(3-chloro-4-fluorophenyl)triazene (o-CFHT), 3-hydroxy-3-(3-methylphenyl)-1-(3-chloro-4-fluorophenyl)triazene (m-CFHT), and 3-hydroxy-3-(4-methylphenyl)-1-(3-chloro-4-fluorophenyl)triazene (p-CFHT) were co-crystallized with β-naphthol in equimolar ratio and exposed to sunlight for 9-10h. The reaction resulted in formation of azo-dyes which were identified by comparison with the products obtained by the conventional method (standard azodye), by use of HPLC. A probable mechanism has been suggested. This is first ecofriendly synthesis of azo-dyes using hydroxytriazenes. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11164-012-0533-x

Res Chem Intermed (2012) 38:2149–2153
DOI 10.1007/s11164-012-0533-x
Photocatalytic cleavage of hydroxytriazenes: a solid-
state synthesis of azo-dyes under sunlight irradiation
•
Prabhat Kumar Baroliya Anita Mehta
•
•
Rekha Dashora R. S. Chauhan A. K. Goswami
•
Received: 3 December 2011 / Accepted: 8 March 2012 / Published online: 25 March 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Sunlight-induced decomposition of hydroxytriazenes, and green pho-
tochemical synthesis of azo-dyes is described. Three substituted hydroxytriazenes,
viz: 3-hydroxy-3-(2-methylphenyl)-1-(3-chloro-4-fluorophenyl)triazene (o-CFHT),
3-hydroxy-3-(3-methylphenyl)-1-(3-chloro-4-fluorophenyl)triazene (m-CFHT), and
3-hydroxy-3-(4-methylphenyl)-1-(3-chloro-4-fluorophenyl)triazene (p-CFHT) were
co-crystallized with b-naphthol in equimolar ratio and exposed to sunlight for
9–10 h. The reaction resulted in formation of azo-dyes which were identified by
comparison with the products obtained by the conventional method (standard azo-
dye), by use of HPLC. A probable mechanism has been suggested. This is first eco-
friendly synthesis of azo-dyes using hydroxytriazenes.
Keywords Hydroxytriazene Á Solid-state photolysis Á Azo-dye
Introduction
During the last few years, application of solar energy to chemical synthesis has
acquired great importance [1–4]. Solid state photoreaction [3, 4] has also been
reported. It has also been reported that organic compounds participate in photoad-
dition [5, 6] and photodegradation [7, 8] reactions. In this work, therefore, we
investigated the use of three hydroxytriazenes, viz: 3-hydroxy-3-(2-methylphenyl)-1-
(3-chloro-4-fluorophenyl)triazene (o-CFHT), 3-hydroxy-3-(3-methylphenyl)-1-(3-
chloro-4-fluorophenyl)triazene (m-CFHT), and 3-hydroxy-3-(4-methylphenyl)-1-
(3-chloro-4-fluorophenyl)triazene (p-CFHT) as photo-labile reagents for solid-state
synthesis of azo-dyes.
P. K. Baroliya Á A. Mehta Á R. Dashora Á R. S. Chauhan Á A. K. Goswami (&)
Co-ordination Chemistry Lab., Department of Chemistry, Mohanlal Sukhadia University,
Udaipur 313001, Rajasthan, India
e-mail: akumargoswami@rediffmail.com
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P. K. Baroliya et al.
The hydroxytriazenes –N=N–N(OH)– were synthesized via a standard route
reported in the literature [9]. The compounds were duly characterized by IR and the
characteristic peaks were obtained, confirming the pure state of the compounds. In
this work PTLC did not give the desired quantities suitable for characterization, as
reported elsewhere [4], so high-performance liquid chromatography (HPLC) was
used for identification of the azo-dyes. HPLC is an excellent technique for
identification and quantification of azo-dyes [10, 11]. Separation was performed
with methanol–water 90:10 as mobile phase. Details are given in the ‘‘Experimen-
tal’’ section.
Experimental
Synthesis of hydroxytriazenes
All three hydroxytriazenes were synthesized by the standard method. In this method
o, m, and p nitrotoluenes (2 mmol) were reduced with Zn dust (4 g) in the presence of
NH4Cl (1.6 g) at 50–60 °C to obtain the respective phenyl hydroxylamines. The
diazotized product was obtained by adding sodium nitrite (2 mmol) in small portions
to 3-chloro-4-fluoroaniline (2 mmol) dissolved in a mixture of HCl (5 ml) and water
(10 ml) at 0–5 °C and with constant magnetic stirring. The diazonium salts were
coupled with the phenyl hydroxylamine at 0–5 °C under constant stirring with
occasional addition of sodium acetate solution to maintain the pH close to 5. After
coupling, the resulting product was purified by recrystallization from absolute
alcohol. The compounds were identified by recording their M.P. and IR spectra. Each
of the hydroxytriazenes gave all four characteristic bands reported elsewhere [12,
13]. The compounds gave bands for mO–H (3,480 cm-1), mN–H (3,190 cm-1), dN–H
(1,520 cm-1), and dN–OH (1,070 cm-1) confirming the presence of the hydroxytria-
zene group. The M.P. of o-CFHT, m-CFHT, and p-CFHT were 118, 166, and 168 °C,
respectively. The compounds synthesized were thus in pure state and suitable for
further photolytic synthesis.
Synthesis of standard dye
The standard dyes were synthesized by the conventional chemical route for
comparison purposes and for identification of the photochemical product. 3-Chloro-
4-fluoroaniline (2.9 g) was dissolved in mixture of 6 ml HCl and 10 ml distilled
water. To this, a solution of 1.4 g sodium nitrite in 10 ml water was added dropwise
at 0–5 °C and the mixture was stirred for 1 h at 0–5 °C. Separately, b-naphthol
(2.8 g) was dissolved in 10 ml 10 % NaOH solution and cooled to 0–5 °C. To this
solution, the solution of the diazotized compound, prepared as described above, was
added dropwise with vigorous stirring. The mixture was stirred for an additional
30 min in an ice-bath. The precipitate obtained was isolated by filtration, initially
purified by washing with water, and further purified by recrystallization from
absolute alcohol. A dark brown red solid was obtained in a yield of approximately
80 %. Its structure was verified on the basis of IR absorption bands occurring at
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Photocatalytic cleavage of hydroxytriazenes
2151
mO–H (3,350 cm-1), mN=N (1,490 cm-1), and mC–O (1,200 cm-1), confirming
formation of the dye.
Photolysis of a mixture of hydroxytriazene and b-naphthol
Mixed co-crystals were obtained by mixing of 1 mmol of each CFHT with 1 mmol -
naphthol in acetone followed by dissolution and evaporation. The mixed crystals
were simple polycrystalline mixtures of the two components. They were finely
ground in a mortar, spread over Petri dishes, and directly exposed to sunlight. After
9–10 h, the reaction mixtures gradually changed to a deep brown–red colour. The
photoreactions were monitored by thin-layer chromatography using a 1:10 mixture
of ethyl acetate and toluene as mobile phase. For this purpose three mixtures were
photolysed; details are given in Table 1.
HPLC measurements
Apparatus, reagents, and solutions
HPLC was conducted with Merck–Hitachi equipment consisting of a pump (model
L2130), autosampler (L2200), and a UV detector (L2400). Separation was
accomplished on a 250 mm 9 4 mm (5 lm) reversed-phase C18 column (LiChro-
CART) protected by a guard column of same material. Stock solutions of the
standard synthetic dyes and photolytic samples were prepared at a concentration of
4 mg ml-1. All solutions were prepared in HPLC-grade methanol. Filtration was
required before injection into the HPLC.
Method
The objective was to develop flexible HPLC method for separation of the azo-dyes
from the three photolysed samples. A 20-min isocratic method was chosen for
sufficient separation of azo-dyes from the mixtures. The mobile phase was 90:10 (v/
v) methanol–water containing trifluoroacetic acid. The flow rate of the mobile phase
was 1 ml min-1 and the column temperature was kept at 25 °C by use of an AC
chamber. The volume of each sample injected was 20 lL; detection of each dye was
by absorption at 350 nm.
Table 1 Details of the photolysed mixtures
No.
Coupling reagent
Initial colour
Final colour
Mixture code
1
2
3
b-Naphthol
b-Naphthol
b-Naphthol
o-CFHT
m-CFHT
p-CFHT
Milky white
Yellow
Coffee colour
Dark brown
CCM-o
CCM-m
CCM-p
Light brown
Coffee colour
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P. K. Baroliya et al.
Fig. 1 HPLC chromatograms obtained from standard and photolysed mixtures
Results and discussion
In this work, the sunlight-induced photolytic addition reaction of three CFHT and b-
naphthol in the mixed crystalline state was studied. HPLC results as described in
Fig. 1 revealed the RT of CCM-o, CCM-m, and CCM-p samples corresponded to
the RT of the standard azo-dyes. Thus HPLC confirmed formation of the azo-dyes
via the photolytic route.
In the crystalline state the photoreactions proceeded at the interface of the two
crystallites. Absorption of sunlight led to the observation of two primary
photoprocesses:
1
N–N bond cleavage, with production of aryldiazenyl and N-phenylhydroxy-
laminyl radicals; and
2
free radical diazocoupling of the aryldiazenyl radical with b-naphthol yielding
the azodye.
It is proposed that the aryldiazenyl radical attacks the more active site of the
naphthol ring at the position ortho to the phenolic group giving adducts which, in
turn, lose a hydrogen radical to afford the corresponding azo-dyes. The proposed
mechanism of overall conversion is depicted in Fig. 2.
Conclusion
In this investigation, preparation of azo-dyes from hydroxytriazenes by use of solar
radiation was attempted. This is first report of the use of hydroxytriazenes for
photolysis. The photo labile triazene moiety can further be used for organic
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Photocatalytic cleavage of hydroxytriazenes
2153
OH
Cl
Cl
F
N
N
HO
hν
F
N
N
N
+
N
R
HO
Cl
Cl
HO
F
H
+
N
N
N
F
N
R=o,m,p-CH3
Fig. 2 Proposed mechanism of conversion
synthesis via the aryldiazenyl free radical. The reaction involves trapping of
sunlight for chemical synthesis and is thus a green route for chemical synthesis.
Acknowledgment Authors are thankful to Professor K. G. Ramawat, Department of Botany, MLS
University Udaipur, for helping in the recording the HPLC of all the compounds reported in this work.
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