Spectrophotometric investigations of the role of the organic solvent on the acid dissociation constants of some azo dyes derived from 2-aminobenzothiazole

Article abstract of DOI:10.1016/j.molliq.2010.04.002

Proton dissociation constants (pK_a) of a set of hydroxyl azo dyes in various organic solvent + water mixtures have been determined. The results obtained were discussed in terms of the solvent characteristic. Effects of hydrogen-bonding interactions and solvent basicity on the ionization process have been also discussed. In a nutshell, the pK_a values of the investigated compounds were found to be largely depending on both the ratio and the nature of organic cosolvent. In addition, it was very important to know with which structures we are dealing in liquid solutions and how these structures influence physicochemical properties of individual tautomers and their acidity constants. Thus, theoretical calculations have been done in order to obtain an insight into structure features and physicochemical properties of the compounds under study.

Full text of DOI:10.1016/j.molliq.2010.04.002

Journal of Molecular Liquids 154 (2010) 52–57
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Spectrophotometric investigations of the role of the organic solvent on the acid
dissociation constants of some azo dyes derived from 2-aminobenzothiazole
⁎
Y.H. Ebead , H.M.A. Salman, M. Khodari, A.A. Ahmed
South Valley University, Faculty of Science, Chemistry Department, 83523 Qena, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2009
Received in revised form 30 March 2010
Accepted 2 April 2010
Available online 13 April 2010
Proton dissociation constants (pKa) of a set of hydroxyl azo dyes in various organic solvent+water mixtures
have been determined. The results obtained were discussed in terms of the solvent characteristic. Effects of
hydrogen-bonding interactions and solvent basicity on the ionization process have been also discussed. In a
nutshell, the pKa values of the investigated compounds were found to be largely depending on both the ratio
and the nature of organic cosolvent. In addition, it was very important to know with which structures we are
dealing in liquid solutions and how these structures influence physicochemical properties of individual
tautomers and their acidity constants. Thus, theoretical calculations have been done in order to obtain an
insight into structure features and physicochemical properties of the compounds under study.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Azo dyes
Medium effect
Acid dissociation constant
PM6 calculations
1. Introduction
aqueous buffer solutions containing varying proportions of organic
solvents of different polarities. Furthermore, we invoked semiempir-
During the last decades, tremendous numbers of papers have been
devoted to the determination of pKa values and tautomeric structure
of the ionisable molecules [1–9]. This is due to the knowledge of pKa
values provide a basis for understanding the chemical reactions
between the compound of interest and its pharmacological target. In
addition, they play a major role in the acid–base titration, complex
formation and many other analytical procedures. Furthermore, the
ionization constants are often used to help discover the structure of a
newly isolated substance [10]. In one hand, it is well known that
benzothiazole and its derivatives show a broad spectrum of biological
activities [11–13] and were investigated in azo push–pull systems
[14]. On the other hand, azo compounds surely constitute an
important group of compounds from cognitive and applicative points
of view. Consequently, they have significantly attracted the interest of
chemists and biologists globalwise [15–25]. Also, such compounds
have long been well known in the analytical chemistry as chromo-
genic agents and acid–base indicators [26,27]. Other important
applications of these substances include production of drugs in
chemotherapy [28,29]. Considering their significant biological activ-
ity, pharmacological and physicochemical properties, azo dyes are
extremely important.
ical PM6 and PM6 (COSMO) levels to predict the structures and
properties of all possible tautomers.
2. Experimental
2.1. Synthesis of the azo compounds 1–3
The azo compounds under investigation were synthesized accord-
ing to the well known standard diazotization procedure described in
literature [30]. To an ice cooled stirred solution of 2-aminebenzothia-
zol (0.01 mol) dissolved in H3PO4, a cold solution of NaNO2 (0.01 mol)
was added gradually with stirring for 20 min. To a solution of phenol,
resorcinol or β-naphtol (0.01 mol) in ethanol in the presence of
sodium acetate, a solution of diazotized amine was added dropwise
with vigorous stirring (0–5 °C) during 35 min. After the addition was
finished, stirring was continued for 1 hour and the obtained
precipitate was isolated by filtration and washed with water and
cold ethanol. The solid products were recrystallized from ethanol and
air-dried under reduced pressure over silica gel. The purity of the
prepared compounds was checked by elemental analysis and FT-IR
spectra (Scheme 1).
Owing to the high number of positive applications of benzothia-
zole and azo compounds in different fields, the present work is
focused on the determination of pKa values of compounds 1–3 in
2.2. Measurements
Stock solutions (1×10−3 mol dm−3) of the compounds were
prepared by dissolving a known mass of the solid in the required
volume of each solvent; more dilute solutions were obtained by
accurate dilution with the solvent. The pH control was achieved by
⁎
Corresponding author. Tel.: +20 108928365; fax: +20 96 5213383.
E-mail address: ebeadhassan88@yahoo.com (Y.H. Ebead).
0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2010.04.002

Y.H. Ebead et al. / Journal of Molecular Liquids 154 (2010) 52–57
53
Scheme 1. The compounds investigated with the number of atoms indicated: R=H (1), OH (2) and 3.
using modified universal buffer solutions [31]. To account for
3. Results and discussion
differences in acidity, basicity, dielectric constant, and ion activities
in partially aqueous media relative to pure aqueous solutions, where
the pH meter is standardized using aqueous buffers at 25 °C. The pH
values in the former media were corrected by using procedure
described by Douheret [32] (Eq. (1)), where the meter reading pH(R)
obtained in each water + organic solvent mixture differs by amount δ
3.1. Structure and physicochemical properties
The compounds under investigation can coexist in azo and
hydrazone tautomeric forms (Fig. 1). It is, therefore, important to
know with which structures we are dealing and how these structures
influence the features of individual tautomers. However, theoretical
calculations have been done in order to throw some light onto the
structure features and physicochemical properties of the compounds
under study. According to the results of our calculations given in
Table 1, the equilibrium constants (ln298 Ko) for compounds 2 and 3
were found to be so high that they should exist mainly as hydrazone
tautomers either in the gaseous phase or in solution. Contrary, PM6
method yields negative values of equilibrium constants of compound
1 indicating, without any doubt, that the azo tautomer should be
predominant. Inspection of the optimized molecular geometries
reveals that the azo and hydrazone tautomers are essentially planar
in compound 1. On the other hand, the main feature of the geometry
of compounds 2 and 3 is the planarity of the hydrazone and azo forms,
respectively (Fig. 1 and Table 1).
⁎
from the corrected reading pH .
*
pH
=
pHðRÞ−δ
ð1Þ
Values of δ for various aqueous + organic solvent mixtures were
determined as recommended by Douheret at the same ionic strength
[32,33]. The solutions were thermostated at 25 °C before measuring
their spectra. The electronic absorption spectra were recorded on a
Shimadzu 2401 PC spectrophotometer using 1 cm matched quartz
cells within the wavelength range of 340–650 nm. The pH measure-
ments were carried out using an Orion 501 digital ionalyzer accurate
to 0.01 pH unit. The FT-IR spectra of the compounds were recorded
on Nicolet FT-IR 560 spectrophotometer as KBr discs. All measure-
ments were carried out at 25 °C and temperature control was
achieved using an ultrathermostat of accuracy 0.05 °C.
The dipole moments listed in Table 1 apparently differ for the
tautomeric forms. Without going in details, calculations clearly
indicate that the hydrazone tautomer has the largest dipole moment
in the case of 1 and 2. However, the azo tautomer of compound 3 is
more polar than the hydrazone one. Moreover, the dipole moments
are increased with increasing the polarity of the medium.
2.3. Quantum chemical calculations
The fully geometry optimization of the isolated molecules 1–3
(Scheme 1) in gaseous phase and in different media were optimized
using the semiempirical PM6 and PM6 (COSMO) methods, respec-
tively [34]. From which some physicochemical and structural
parameter for each tautomer were estimated. The geometry results
were obtained from MOPAC 2007-free license on internet [35]. Heats
of formation, dipole moments and other parameters were extracted
directly from the data files following the geometry optimizations. All
calculations were done on a Pentium IV PC computer.
3.2. Electronic absorption spectra in universal buffer solution
The visible absorption spectra of compounds 1–3 in universal
buffer solutions containing different proportions of an organic solvent
(methanol, ethanol, acetone and DMF) show mainly two bands
(Fig. 2). The shorter wavelength band, appearing at 410–430 nm,
Fig. 1. The PM6-optimized structures of the tautomers of compounds 1–3 in ethanol (Scheme 1).

54
Y.H. Ebead et al. / Journal of Molecular Liquids 154 (2010) 52–57
Table 1
Structural and physicochemical features of tautomers of the investigated azo dyes (Scheme 1).
ob
Comp.
Computational level
Dihedral anglea
Azo
Dipole moment (D)
ln
298
K
Hydrazone
Azo
Hydrazone
1
PM6
179.99
179.95
−179.04
−179.05
−179.05
180.00
−178.62
−178.61
−178.64
−178.60
−179.98
179.72
−179.97
179.55
179.84
179.82
179.85
180.00
179.70
179.40
179.67
179.32
179.78
178.52
178.34
178.48
178.34
3.23
4.63
2.67
2.65
2.68
3.76
4.70
4.76
4.67
4.77
3.67
5.78
5.85
5.74
5.88
5.91
8.74
8.83
8.69
8.86
5.50
7.97
8.04
7.92
8.06
1.06
2.08
2.05
2.03
2.09
−13.00
−4.97
−6.37
−6.71
−6.31
11.26
5.98
5.92
6.01
5.94
16.28
9.22
PM6 (COSMO–C2H5OH)
PM6 (COSMO–CH3OH)
PM6 (COSMO–Acetone)
PM6 (COSMO–DMF)
PM6
PM6 (COSMO–C2H5OH)
PM6 (COSMO–CH3OH)
PM6 (COSMO–Acetone)
PM6 (COSMO–DMF)
PM6
2
3
PM6 (COSMO–C2H5OH)
PM6 (COSMO–CH3OH)
PM6 (COSMO–Acetone)
PM6 (COSMO–DMF)
179.82
179.73
179.84
9.00
9.30
8.92
a
Between C2N9N10C11
.
b
ln298 Ko =−δΔH/RT (δΔH denotes the difference in standard enthalpies of hydrazone and azo tautomers).
⁎
represents absorption by the neutral form, whereas the longer band
observed at 510–550 nm is attributed to the absorption by the ionized
species. On increasing the pH of the medium, the absorbance of the
former band decreases while the latter band increases. At high pH
values the shorter wavelength visible band disappears completely and
the longer one attains a more or less constant absorbance value. As a
consequence, a fine isosbestic point is achieved, indicating that two
species are in equilibrium of the type (Scheme 2).
Fig. 3 which represents the plots of the absorbance against pH
values shows a typical dissociation curves, confirming the establish-
ment of an acid–base equilibrium. The acid dissociation constants of
the compounds under study were determined using three different
spectrophotometric methods namely, the half-curve height, isosbestic
point and limiting absorbance methods [36,37]. Tables 2–4 report the
pKa values of the compounds investigated in different media. It is
widely clear from the data gathered in Tables 2–4 that the pKa values
of all compounds are largely dependent on both the nature and the
proportion of the organic cosolvent. In a broad sense, pKa values
HA
Hþ
+
A−
ð2Þ
Fig. 2. Electronic absorption spectra of 4.0×10−5 M solution of compound 1.(R=H) in water (1) + acetone (2) at different pH's: (A) w2 =16.53; pH (1) 7.00, (2) 7.20, (3) 7.30,
⁎
⁎
(4) 7.40, (5) 7.50, (6) 7.70, (7) 8.00, (8) 8.20, (9) 8.50, (10) 9.50. (B) w2 =25.35; pH (1) 7.10, (2) 7.25, (3) 7.40, (4) 7.55, (5) 7.70, (6) 8.10, (7) 8.50, (8) 9.60, (9) 10.00.
⁎
⁎
(C) w2 =34.56; pH (1) 7.10, (2) 7.20,(3) 7.30, (4) 7.40, (5) 7.50, (6) 7.60, (7) 8.00, (8) 8.50, (9) 9.50. (D) w2 =44.21; pH (1) 7.10, (2) 7.25, (3) 7.35, (4) 7.40, (5) 7.50, (6) 7.65,
(7) 8.15, (8) 8.50, (9) 9.55.

Y.H. Ebead et al. / Journal of Molecular Liquids 154 (2010) 52–57
55
Scheme 2.
increase with an increase of the organic cosolvent content in the
medium. This behavior can be interpreted on the basis as follows:
according to Coetzee and Ritchie [38], the acid dissociation constant in
aqueous medium (Ka1) is related to that in a partially aqueous
medium (Ka2) by the equation
where γ is the activity coefficient of the subscripted species in a
partially aqueous medium relative to that in a pure aqueous one, and 1
is pure water and 2 is water + solvent. It is known that the
electrostatic effect resulting from the charge in relative permittivity of
the medium operates on the activity coefficients of the charged
species [38], one can, therefore, expect that the increase in the amount
of the organic cosolvent in the medium will increase the activity
coefficient of both H+ and A− ions. According to the above Eq. (3), this
behavior will result in a decrease in the acid dissociation constant Ka
of the compound (i.e., high pKa value) on increasing the content of
the organic solvent. This finding concurs with the results cited in
Tables 2–4.
γH−ð1ÞγAð1Þ γHAð2Þ
Ka1 = K
ð3Þ
a2 γH−ð2ÞγAð2Þ γHAð1Þ
Interestingly, despite methanol and DMF have approximately
similar relative permittivity constants (32.6 and 36.7, respectively, at
25 °C), all the compounds are more acidic in water + DMF than
water + methanol at the same mole fraction (Tables 2–4). Moreover,
although ethanol and acetone have a comparable relative permittivity
constants also (24.3 and 20.7, respectively, at 25 °C), compounds 1
and 2 are more acidic in water + acetone than water + ethanol at the
same mole fraction of each is used. In general, pKa values for all
compounds increase with increasing ratio of organic cosolvent in the
medium; that is, the pKa values of a compound in water + organic
solvent are arranged according to the following sequence:
DMF b ethanol b methanol b acetone
This finding indicates that other solvent effects beside the
electrostatic one have a contribution to the ionization process of
the investigated compounds. This fact is further substantiated by the
nonlinear relations obtained by plotting of pKa against 1/ε of the
Fig. 3. Absorbance–pH* curves of 4.0×10−5 M solution of compound 2 (R=OH)
in water (1) + DMF (2), in ionized species at 518 nm, with w2 =9.21 (A), 28.96 (B),
38.81 (C), 48.75 (D).
Table 2
Table 3
pKa values for 4.0×10−5 M of compound 1 in different water (1) + organic solvent (2)
pKa values for 4.0×10−5 M of compound 2 in different water (1) + organic solvent (2)
mixtures at 25 °C.
mixtures at 25 °C.
100wa2
ε
pKa
SD
100wa2
ε
pKa
SD
Method 1
Method 2
Method 3
Mean value
Method 1
Method 2
Method 3
Mean value
Water (1) + methanol (2)
Water (1) + methanol (2)
16.53
25.35
34.56
44.21
73.72
71.03
67.88
64.27
7.60
7.60
7.65
7.70
7.65
7.70
7.70
7.70
7.50
7.60
7.60
7.60
7.58
7.63
7.65
7.66
0.08
16.53
25.35
34.56
44.21
73.72
71.03
67.88
64.27
10.60
10.50
10.60
10.70
10.40
10.50
10.60
10.60
10.50
10.60
10.60
10.60
10.50
10.53
10.60
10.63
0.10
0.06
0.05
0.06
0.06
0.00
0.06
Water (1) + ethanol (2)
Water (1) + ethanol (2)
16.59
25.42
34.65
44.30
74.46
72.00
69.08
65.54
7.50
7.50
7.65
7.70
7.60
7.65
7.70
7.60
7.40
7.45
7.60
7.80
7.50
7.53
7.65
7.70
0.10
0.10
0.05
0.10
16.59
25.42
34.65
44.30
74.46
72.00
69.08
65.54
10.50
10.55
10.60
10.60
10.50
10.60
10.60
10.70
10.60
10.60
10.70
10.70
10.53
10.58
10.63
10.66
0.06
0.03
0.06
0.06
Water (1) + acetone (2)
Water (1) + acetone (2)
16.53
25.35
34.56
44.21
74.92
72.87
70.25
66.99
7.50
7.50
7.50
7.60
7.50
7.70
7.90
7.80
7.45
7.60
7.50
7.60
7.48
7.60
7.63
7.67
0.03
0.10
0.23
0.11
16.53
25.35
34.56
44.21
74.92
72.87
70.25
66.99
10.40
10.60
10.50
10.60
10.50
10.40
10.50
10.60
10.50
10.50
10.70
10.70
10.47
10.50
10.60
1063
0.06
0.10
0.11
0.06
Water (1) + DMF (2)
Water (1) + DMF (2)
19.21
28.96
38.81
48.75
76.06
74.56
72.73
70.45
7.40
7.40
7.50
7.60
7.45
7.55
7.50
7.60
7.40
7.40
7.60
7.50
7.42
7.45
7.53
7.57
0.03
0.09
0.06
0.06
19.21
28.96
38.81
48.75
76.06
74.56
72.73
70.45
10.40
10.35
10.30
10.50
10.45
10.55
10.60
10.60
10.60
10.50
10.60
10.50
10.49
10.47
10.50
10.53
0.10
0.10
0.17
0.06
wa2 =mass fraction of component 2.
wa2 = mass fraction of component 2.

56
Y.H. Ebead et al. / Journal of Molecular Liquids 154 (2010) 52–57
Table 4
molecules. Since water molecules have a higher tendency to donate
hydrogen-bonds than other solvent molecules [42], the conjugated
base (A−) is expected to be less stabilized by the hydrogen-bonding
interaction with solvent molecules as the amount of the organic
cosolvent in the medium is increased. This will tend to increase the
pKa value of the compound, as Eq. (3) implies. It indicates also that the
difference in the stabilization of the ionic form by hydrogen-bond
donor solvent molecules plays an important role in the increase of the
pKa values as the amount of the organic cosolvent in the medium is
increased.
pKa values for 4.0×10−5 M of compound 3 in different water (1) + organic solvent (2)
mixtures at 25 °C.
100wa2
ε
pKa
SD
Method 1
Method 2
Method 3
Mean value
Water (1) + methanol (2)
16.53
25.35
34.56
44.21
73.72
71.03
67.88
64.27
9.50
9.55
9.60
9.65
9.45
9.50
9.60
9.60
–
–
–
–
9.48
9.53
9.60
9.60
0.04
0.04
0.00
0.04
Examination of the results depicted in Tables 2–4 reveals that the
pKa values in the presence of the poorer hydrogen-bond donor DMF
are less than those obtained in the presence of corresponding
amounts of the other solvents. This behavior can be ascribed to the
high basic character of DMF, which reflects itself in the construction of
a strong hydrogen-bond acceptor from the OH group of the
nonionized dye molecule and consequently promotes the ionization
process (i.e., low pKa).
If the dispersive forces, which possibly exist in the media used,
between the delocalized charge on the conjugated base of the dye (A
−) and the localized dispersion centers in near solvent molecules as
well as the proton–solvent interactions have important effects on the
ionization process of the compounds studied, one should expect that
by increasing the amount of the organic cosolvent both A− and H+
will be highly stabilized by DMF molecules (i.e., γ−A and γH+/γA−
decreases), since the effective density of dispersion centers in each of
the organic solvents used is higher than that of water [43]. Thus, in the
light of Eq. (3), the acid dissociation constant of the dyes studied
would increase (pKa decreases) with the increase in the amount of the
organic cosolvent in the medium. This is not the case obtained from
the results (Tables 2–4). Therefore, one can conclude that neither the
dispersive forces nor the proton-solvent interaction effects have an
effective contribution to the ionization process of the dyes investi-
gated. The values of pKa cited in Tables 2–4 show that the acidity of the
studied azo compounds decreases in the following order: 2b3b1.
Water (1) + ethanol (2)
16.59
25.42
34.65
44.30
74.46
72.00
69.08
65.54
9.60
9.63
9.65
9.70
9.60
9.65
9.70
9.70
–
–
–
–
9.60
9.64
9.68
9.70
0.00
0.01
0.04
0.00
Water (1) + acetone (2)
16.53
25.35
34.56
44.21
74.92
72.87
70.25
66.99
9.65
9.70
9.75
9.80
9.70
9.70
9.80
9.80
–
–
–
–
9.68
9.70
9.79
9.80
0.04
0.00
0.04
0.00
Water (1) + DMF (2)
19.21
28.96
38.81
48.75
76.06
74.56
72.73
70.45
9.30
9.35
9.45
9.50
9.30
9.30
9.40
9.45
–
–
–
–
9.30
9.33
9.43
9.43
0.00
0.04
0.04
0.04
wa2 = mass fraction of component 2.
medium (Fig. 4), according to the equation given by Denison and
Ramesy [39] and Gilkerson [40] which relates the variation of the pKa
of the acid with the relative permittivity of the medium ε. The relative
permittivity of water + organic solvent mixtures, ε, was obtained
using the following equation [41]
ε = ε1mf ðwÞ + ε2mf ðsÞ
ð4Þ
where ε is the relative of water + organic solvent mixtures, ε1 and ε2
are the relative permittivity of the water and organic solvent,
respectively, mf is the mole fraction, and the subscripts w and s
refer to water and organic solvent, respectively.
In general, effects such as hydrogen bonding, solvent basicity,
dispersive forces, and proton-solvent interactions play vital roles in
the ionization process of acids in the presence of organic solvents [38].
Thus, the observed increase in the pKa of the compounds as the
proportion of the organic cosolvent in the medium is increased can be
ascribed, in addition to the electrostatic effect, to the hydrogen-
bonding interaction between the conjugated base (A−) and solvent
4. Conclusion
The comprehensive spectrophotometric investigations of the
effect of both the nature and the proportion of the solvent on the
acidity constant have revealed that:
• The pKa values of all compounds are largely dependent on both the
nature and the proportion of the organic cosolvent.
• In general, the pKa values in the presence of a poorer hydrogen-bond
donor DMF are less than those obtained in the presence of
corresponding amounts of the other solvents.
• Both hydrogen-bonding, solvent basicity, dispersive forces, and
proton-solvent interactions beside the electrostatic play vital roles
in the ionization process.
Furthermore, PM6 calculations demonstrate that compounds 2
and 3 occur mainly as hydrazone forms in the gas phase and in
solution, whereas compound 1 in the same media exhibits an ability
to exist as azo form.
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Fig. 4. Variation of pKa of 4.0×10−5 M solution of compound 3 in water (1) + organic
solvent (2) with 102/ε of the medium at 25 °C: (A) methanol, (B) ethanol, (C) acetone,
(D) DMF.

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