INTRODUCTION
Azo dyes are by far the largest group of disperse dyes and constitute about
60-70% of all commercial dyes in both number and amount produced. They are characterized
by the typical nitrogen to nitrogen (-N = N-) bonds; which is the most common
chromophore of azo dyes. They are fairly cheap, easy to apply and have a wide
range of colours for both synthetic and natural fibres. The azo group is attached
to two radicals (organic residues) of which at least one but, more usually,
both are aromatic (carboxylic or heterocyclic). They exist in the trans form
with a bond angle of ca. 120° and the nitrogen atoms are Sp2 hybridized
(Venkataranman, 1970; Abrahart, 1977;
Otutu, 2008). The range of shades that could be obtained
from azo dyes includes yellows, reds, oranges, violets, navy blues and blacks
but green shades are limited. The depth of shades is usually influenced by the
number of azo groups contained in the dye structure. Thus, the more the number
of azo groups in the dye structure, the greater the depth of shade but the duller
the brightness (Waring and Hallas, 1990; Otutu
et al., 2008; Venkataranman, 1970).
The 2-Methoxy-5-nitroaniline is an aromatic base and could be a useful source
of diazonium ions for dye synthesis. This intermediate has been used in the
preparation of some organic compounds such as (2-Methoxy-5-nitrophenyl) piperidin-2-ylme
thylamine by heating under reflux a mixture of 2-bromomethyl piperidine hydrobromide
and 2-methoxy-5-nitroaniline for 17 h.
Another compound that has been prepared from 2-methoxy-5-nitroaniline is 2-[(2-Methoxy-5-nitrophenyl
amino) methylpiperidin-1-y1] acetic acid ethyl ester. This was done by heating
a mixture of 2-methoxy-5-nitrophenyl piperidin-2-yl-methylamine, ethyl bromoacetate
and triethylamine in dry ethanol under reflux for 4 h (El-Kholy
et al., 1998). Hence, in this present study, we report the synthesis
and spectral properties of some monoazo disperse dyes of general structure I
derived from 2-Methoxy-5-nitroaniline. Also the solvatochromic behaviour of
the dyes in various solvents was evaluated (R is given Table 2).
MATERIALS AND METHODS
Materials: The 2-Methoxy-5-nitroaniline, 1-hydroxynaphthalene, 2-hydroxynaphthalene,
N-phenylnaphthylamine, 1,3-diaminobenzene, 1,3-dihydroxybenzene and 3-aminophenol
were purchased from Aldrich chemical company and Fluka chemical company and
used without further purification. All other chemicals used in the synthesis
and characterization were of laboratory reagent grade.
Chemical and instrumental analysis: The proton nuclear magnetic resonance
('H NMR) spectra were obtained with a mercury-200BB spectrometer equipped with
an Oxford wide bore magnet, sun 3/160 MHz based computer with an array processor
and GE Omega 6.0 software for solutions in a deuterated chloroform as solvent.
The chemical shifts were reported in ppm using tetramethylsilane (TMS) as the
internal reference.
Melting points were determined using the Thumbnail melting point instrument.
Fourier Transform Infrared (FTIR) spectra were recorded on a Nicolet Averser
330 series spectrophotometer. The UV-visible spectra were recorded in 1 cm quartz
cells on a Genesys 10s VL 200 series spectrophotometer. Dye purity was assessed
by Thin Layer Chromatography (TLC) using Whatman 250 m silica gel 60AMK 6F plates
as the stationary phase and ether/acetone (5:1 by volume) mixture as developing
solvent. Characterisation data are shown in Table 1 and 2.
Method 1: Synthesis of 2-methoxy-5-nitrophenyl azo-4-hydroxynaphthalene:
The 2-Methoxy-5-nitroaniline (4 g, 0.024 mole) was diazotized in 6 mL of concentrated
sulphuric acid and 50 mL of water by adding 10 mL of sodium nitrite solution
(0.02 mole) dropwise at a temperature of 0-5°C. After 30 min the diazotisation
was complete which was verified by using a solution of 4-(N, N-dimethylamino)
benzaldehyde which generates colour if undiazotized aromatic amine is still
present. The excess nitrous acid was destroyed by adding (0.15 g, 0.003 mole)
urea solution.
The prepared diazonium salt solution was added slowly with vigorous stirring
to 1-hydroxynaphthalene (3.7 g, 0.03 mole) dissolved in 60 mL of 2 M sodium
hydroxide for 5 min. The reaction mixture was further stirred for 1.5 h and
the precipitated product was filtered off, washed with water and dried. The
crude product was then recrystallized from ethanol-methanol mixture to give
2-methoxy-5-nitrophenylazo-4-hydroxynaphthalene (78% yield). Rf (ether/acetone
5:1) = 0.85. This procedure was also used to synthesise dyes 2b, 2e and 2f.
Method 2: Synthesis of 2-methoxy-5-nitrophenylazo-4-(N-phenylnaphthylamine):
The 2-Methoxy-5-nitroaniline (4 g, 0.24 mole) was mixed with 40 mL water and
5 mL of concentrated sulphuric acid. (98%) was added and stirred. The mixture
was cooled to 0-5°C.
| Table 1: |
FTIR and 'H NMR data of dyes 2a-f |
 |
| Table 2: |
Yield and melting point of the synthesised dyes 2a-f |
 |
Sodium nitrite (3.7 g, 0.03 mole) dissolved in 5 mL of water at a temperature
of 0-5°C was added dropwise with stirring to the mixture for 20 min. The
resulting diazonium salt solution was added slowly to a solution of N-phenylnaphthylamine
(5.2 g, 0.024 mole) with vigorous stirring for 5 min. After further stirring
for 2 h, the crude product was collected by filtration, washed with water and
dried. Recrystallisation from methanol gave brown solid crystals of dye 2c (4.6
g, 58% yield) Rf (ether/acetone 5:1) = 0.75. This procedure was also
used to synthesise dye 2d.
RESULTS AND DISCUSSION
Absorption spectra: The details of the visible absorption spectra of
the dyes are summarized in Table 3. The synthesized dyes developed
a colour ranging from yellow (λmax 436 nm) to orange (λmax
520 nm) in ethanol.
| Table 3: |
Absorption spectral properties of the synthesised dyes 2a-f |
 |
| s: Shoulder |
It is well known that λmax values tend to be related to the
strength of the electronic power in the benzenoid system (Karci,
2005). Since, the electronic transition in these compounds involves a general
migration of electron density from the donor group towards the azo group, the
greatest effect in terms of longer wavelength is achieved by placing the substituent
in the positions ortho or para to the azo group for effective conjugation (Griffiths,
1976; Shirai et al., 1998).
Off all the dyes, dye 2c gave the highest λmax value in all
the solvents. This result can be attributed to the longer wavelength effect
exerted by the N-phenyl group and the naphthalene moiety which has an electron
donating property. Half-band widths of the absorption band in DMF were determined
(Table 3).
In addition to the effect on λmax, substituents also cause
a change in the half-band width values (Δv½). The value
of Δv½ is a convenient criterion for the evaluation of
the hue brightness of dyes; dyes with low values of Δv½
show bright hues while those with high values of Δv½
show dull hues. Thus, dyes 2a, 2e and 2f showed brighter hues compared to the
others. Dye 2f gave the narrowest half-band width and the brightest hue in solvent
while dye 2c with half-band width of 75000 cm-1 gave the dullest
hue in solvent.
In the case of tinctorial strength, from the molar extinction coefficient (εmax),
the dyes 2b and 2c seem to be more intensely absorbing than dyes 2a, 2d, 2e
and 2f. However, it is the oscillator strength (f), rather than εmax,
that gives a true measure of tinctorial strength since it expresses the area
under the absorption curve (Eq. 1). Thus, the dye with a high
value but narrow absorption curve (i.e., low Δv½) could
be tinctorially weaker than the dye which, although it has a lower εmax
value, has a broader absorption curve (Gordon and Gregory,
1987).
Indeed, as shown in Table 2, dye 2f with high molar extinction
coefficient but narrow absorption curve (i.e., low Δv½) is tinctorially
weaker than dye 2d which although has lower molar extinction coefficient, has
broader absorption curve.
Keto-enol tautomerism: Keto-enol tautomerism is not only of utmost importance
to the dyestuff manufacturer but is also important in other areas of Chemistry.
Keto-enol tautomers not only have different colours, they also have different
tinctorial strengths (and hence economics) and different properties e.g., light
fastness (Karci, 2005). Azo dyes 2a-b and 2e-f can exist
as a mixture of several tautomeric forms as shown in scheme 1.
 |
| Scheme 1 |
The infrared spectra of dyes 2e and 2f showed a broad hydroxyl band at 3441-3459
cm-1 and C-O band at 1072-1113 cm-1. This suggests that
these compounds are predominantly in the azo-enol form as opposed to the hydrazone-keto-form
(ketohydrazone form). On the other hand, the infrared spectra of dyes 2c and
2d showed intense NH2 bands at 3418-3260 cm-1. This suggests
that these compounds are predominantly in the amino-azo form, since the amino-azo
colorants show no evidence of tautomeric behavior (Hallas
and Renfrew, 1996). This is true because the imino grouping as shown by
the equilibrium equations of 2c and 2d is very unstable. The infrared spectra
of dyes 2a and 2b showed intense carbonyl bands at 1620-1680 cm-1.
This suggests that these compounds exist exclusively in the hydrazone-keto form
as opposed to the azo-enol form, in the solid state (Hallas
and Towns, 1997).
'H NMR spectra of dyes 2e and 2f showed no O-H peak at 12.72-13.50 ppm and
no N-H peak at 16.00-16.55 ppm. This suggests that these compounds exist predominantly
as a single tautomeric form in deuterated chloroform. However, the 'H NMR spectra
of dyes 2a and 2b showed an O-H peak at 8.55-9.00 and an N-H peak at 9.20-9.40
ppm. This result suggests that dyes 2a and 2b were present as a mixture of tautomeric
forms in deuterated chloroform.
Effect of solvent: The visible absorption spectra of the dyes did not
correlate with the polarity of solvent. Each of the dyes gave a single dominant
absorption peak without a shoulder in all the solvents employed with the exception
of dyes 2b (in N, N-methylformamide (DMF) and chloroform (CHCl3),
2c (in ethanol (ETOH) and DMF) and 2e (in DMF). The reason for this is probably
that dyes 2a, 2d and 2f were present predominantly in a single tautomeric form
in each of the solvents. As dyes 2b, 2c and 2e gave a maximum absorption peak
with a shoulder in DMF, CHCl3 and ETOH, it suggests that these dyes
were present in more than one tautomeric form.
It was observed that although in ETOH, chloroform and N; N-dimethylformamide,
the absorption spectra did not change at all for dye 2b. The λmax
of dye 2e and 2f shifted hypsochromically in chloroform. For example, for dye
2e, λmax was 380 nm in chloroform, 430 nm in ethanol and 469
nm in N, N-dimethylformamide. On the other hand, it was also observed that λmax
values of dye 2c were shifted bathochromically with respect to the λmax
in ethanol, N, N-dimethylformamide and chloroform, respectively. Whereas, the
λmax value of dye 2a shifted hypsochromically in N, N-dimethylformamide.
For example, for dye 2c, λmax was 520 nm in ethanol, 526 nm
in N,N-dimethylformamide and 530 nm in chloroform.
CONCLUSIONS
A series of monoazo disperse dyes were prepared from 2-methoxy-5-nitroaniline
by diazotization and coupling reactions between it and aryloxy and arylamine
couplers and their absorption spectra were investigated.
The dyes developed the colour of yellow to orange. Also the half-band width
of the dyes were evaluated and it was found that dye 2a, 2e and 2f showed narrower,
half-band widths than dyes 2b, 2c and 2d and hence the former have brighter
hues than the later. However, dye 2f with narrow absorption curve was tinctorially
weaker than dye 2d which although has lower molar extinction coefficient, has
broader absorption curve. The solvent influence on the wavelength of maximum
absorption spectra was studied. It was observed that the absorption spectra
of dye 2b showed the same λmax in all the solvents and dyes
2a, 2c, 2d, 2e and 2f did not show any correlation with the polarity of the
solvents. It was also observed that dyes 2a and 2b may exist as a mixture of
tautomeric forms. Dyes 2a-f can be applied to polyester and/or polyamide fibres
as disperse dyes.
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