Determination of pKa values for (E)-2-hydroxy-5-(aryldiazenyl) benzaldehydes in dimethyl sulfoxide: Cyclic voltammetry and density functional theory calculations

Article abstract of DOI:10.1002/jccs.201800472

The (E)-2-hydroxy-5-(aryldiazenyl) benzaldehydes (azo dyes 1–4) were synthesized in high purity. As they are insoluble in water, the usual analytical methods cannot be utilized to determine their pK_a values. Cyclic voltammetry was experimentall

Full text of DOI:10.1002/jccs.201800472

Received: 14 December 2018
DOI: 10.1002/jccs.201800472
Revised: 10 May 2019
Accepted: 11 May 2019
A R T I C L E
Determination of pK values for (E)-2-hydroxy-5-(aryldiazenyl)
a
benzaldehydes in dimethyl sulfoxide: Cyclic voltammetry and
density functional theory calculations
1
2
2
1,3
Forouzan Zonouzi | Alireza Shayesteh | Taher Alizadeh | Hamid Dezhampanah
|
1,3
2,4
Bahram Ghalami-Choobar
| Afsaneh Zonouzi
1
Department of Chemistry, University
The (E)-2-hydroxy-5-(aryldiazenyl) benzaldehydes (azo dyes 1–4) were synthe-
sized in high purity. As they are insoluble in water, the usual analytical methods
campus 2, University of Guilan, Rasht, Iran
2
School of Chemistry, College of Science,
University of Tehran, Tehran, Iran
cannot be utilized to determine their pK values. Cyclic voltammetry was experi-
a
3
Department of Chemistry, Faculty of
mentally used to determine their pKa values in DMSO solvent. In addition, compu-
Science, University of Guilan, Rasht, Iran
tational methods and a conductor-like screening model (COSMO) were used to
4
Pharmaceutical and Cosmetic Research
ꢀ
ꢀ
Center (PCRC), University of Tehran,
Tehran, Iran
calculate the solvent effect. ΔU
, ΔG
, Kexchange, and pK
a
values were
exchange
exchange
estimated for the azo dyes being studied using the BP86, TPSS, B3LYP, PBE0,
TPSSh, and PW6B95 density functionals in def2-TZVP basis sets. The obtained
mean absolute deviations (MADs) indicate that the results of BP86, PBE0, and
PW6B95 functionals are in good agreement with experimental values.
Correspondence
Bahram Ghalami-Choobar, Department of
Chemistry, Faculty of Science, University of
Guilan, Rasht, Iran.
Email: b-ghalami@guilan.ac.ir
K E Y W O R D S
Funding information
azo dyes, cyclic voltammetry, DFT calculations, dimethyl sulfoxide, pK
a
Iranian National Science Foundation (INSF)
1
| INTRODUCTION
compounds are insoluble in water, and conventional experi-
mental procedures cannot be used to determine their pKa
values. In order to evaluate the theoretical results, we also
used cyclic voltammetry (CV) in dimethyl sulfoxide
(DMSO) to experimentally report the related pKa values for
the first time (Table 1).
Knowledge of dissociation constants and proton transfer
reactions for organic compounds is critical in many fields
including biology and chemistry, as well as industrial pro-
cesses.
industrial compounds that have been used for many years
in applications including but not limited to coloring pro-
cesses in textile industry, data storage, inkjet printing,
dye-sensitized solar cells, sensors, and pharmaceuti-
cals. In 1889, Tummeley reported the synthesis of aryl azo
salicylaldehydes. Since then, these compounds have been
[
1,2]
Azo dyes are an important class of synthetic
[
3]
In addition, the measurement of pK may not be easy and
a
would turn into a significant experimental challenge for
some compounds, including azo dyes. Thus, experiments
should be performed carefully under standard conditions.
Furthermore, theoretical calculations provide a proper way
[
4]
[5]
[6]
[7]
[8]
[
9]
to gain a first idea about pK values. However, developing
a
[
10]
used to obtain a wide variety of more complex dyes.
In
computational methods to determine the pK values is of
a
this project, (E)-2-hydroxy-5-(aryldiazenyl) benzaldehydes
particular interest. The goal of these methods is to improve
the reliability of the computational approach through com-
parison with the available experimental data and to predict
1
–4 (Scheme 1) are considered model representative azo
dyes for pK calculations. These dyes were synthesized in
a
their pure form using a double-solvent catalyzed procedure,
which was recently reported by our team ; thus, these azo
unknown values. In continuation of our quest for the theoret-
[11]
[12–16]
ical calculation of pK values,
here, we performed
a
©
2019 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2019;1–5.
http://www.jccs.wiley-vch.de
1

2
ZONOUZI ET AL.
conventional three-electrode system, including a glassy car-
bon electrode (working electrode), platinum rod (counter
electrode,) and Ag/Ag + electrode (reference electrode), was
used for the electrochemical experiment. Dry DMSO was
used for the preparation of solutions of the dyes under study
and benzoquinone (BQ). Tetra butyl ammonium bromide
(TAB) was used as a supporting electrolyte.
SCHEME 1 The structure of the azo dyes under study
2.3 | Computational details
TABLE 1 The structure, name, and experimental pK
a
values of
In this section, pKa values of compounds 1–4 using density
compounds 1–4 obtained from cyclic voltammetry (CV) in DMSO
functional theory (DFT) were calculated. All calculations
Comp.
Name
pK
a
(CV)
[17]
were performed using the ORCA 3.0.3 program,
and the
1
(E)-2-hydroxy-5-(phenyldiazenyl)
benzaldehyde
8.62
8.29
8.33
8.95
solvent effects were included by the conductor-like screen-
[18]
ing model (COSMO).
The structures were optimized in
[
19,20]
[21]
2
3
4
(E)-5-([4-chlorophenyl]
diazenyl)-2-hydroxybenzaldehyde
the DMSO solvent using the BP86,
TPSS,
density
Vibrational
[22]
[23]
[24]
[25]
B3LYP,
functionals with the def2-TZVP basis sets.
frequency calculations and thermochemical analyses were
PBE0,
TPSSh,
and PW6B95
[
26]
(E)-5-([4-bromophenyl]
diazenyl)-2-hydroxybenzaldehyde
[
19,20]
(E)-5-([4-ethylphenyl]
diazenyl)-2-hydroxybenzaldehyde
carried out using the BP86 functional.
Single-point
energies were obtained for all six density functionals with
the def2-TZVPP basis sets.
density functional calculations to complement experimen-
tally measured pK values. This approach is hopefully gen-
3 | RESULTS AND DISCUSSION
a
eral to many organic compounds, especially azo dyes, which
are usually insoluble in water, and their hue color intensity
are highly pH dependent.
The azo dye compounds under study are important precur-
sors in the dyeing industry, and having precise information
about their pK values will play a crucial role in color con-
a
trol and stability. Although measuring the pK seems to be
easy, azo dyes 1–4 are not soluble in water; therefore, we
tried using CV in DMSO, followed by theoretical calcula-
a
2
2
| METHOD
.1 | Synthesis of azo dyes 1–4
tions, to measure and estimate the pK values.
a
The azo dyes under study were obtained in pure form via
[
11]
our recently reported double-solvent catalyzed method.
3
.1 | Cyclic voltammetry
Briefly, 8 mL of HCl (37%) was added dropwise to a stirred
solution of aniline derivatives maintained at 0 C, and 20 mL
ꢀ
Figure 1 shows the electrochemical behavior of BQ in the
absence and presence of azo dyes 1–4 in DMSO. As can be
seen, new reduction peaks appear at more positive potentials
(−0.04, −0.03, −0.03, and −0.05 V for 1–4, respectively)
than the peak potential due to the first reduction of BQ alone
(about −0.2 V). Based on the depicted data, the pK values
of 1–4 were estimated to be 8.62, 8.29, 8.33, and 8.95
respectively (Figure 1).
of NaNO solution (4 g in 20 mL of H O) was then added.
2
2
Separately, 0.05 mol of salicylaldehyde was dissolved in
0 mL of petroleum ether, a regioselective catalyst based on
1
zinc (Zn/SBA-15), and then, 5 mL of 10% aq NaOH were
added. The new mixture was added to the first aqueous solu-
tion. After 45 min of vigorous stirring, a yellow to brown
organic precipitate can be separated easily, which contains
azo dyes 1–4 that can be purified by aqueous washing and
are obtained by solvent evaporation.
a
The structure of the compounds under study contains a
phenol group; phenolic compounds are organic acids with-
out a carboxylic moiety. The acidity of such compounds
change on substitution. For example, pK of phenol in water
a
2
.2 | Electrochemical measurements
is 0.89, while for picric acid (2,4,6-trinitro phenol), it is
[
27]
CV experiments were carried out via a computerized hand-
held potentiostat/galvanostat (Palm Instruments BV, the
Netherlands) interfaced with a laptop. Analytical procedure
was controlled by Drop Sens PC software, Drop view. A
0.25.
On the other hand, the pK values also depend on
a
the solvent, that is, pK of picric acid in DMSO has been
a
[
27]
reported to be −1.0 by CV.
For phenolic compounds
1–4, deprotonating is made easy as a result of electron-

ZONOUZI ET AL.
3
[
29]
aprotic solvent.
Based on their report, an advantage of
using exchange reactions for the calculation of pK values is
a
that their computational errors are usually small due to error
cancellation; therefore, the following deprotonation reaction
of azo dye molecules in DMSO can be considered for the
simple calculation of pK values (Scheme 2).
a
ꢀ
The ΔG values of the above reaction, that is, the proton
exchange reaction, were calculated at 298.2 K using various
density functionals, and the equilibrium constants were
obtained:
ꢀ−
ꢁ
ΔG0
RT
Kexchange = exp
:
ð1Þ
FIGURE 1 Cyclic voltammetry curves of BQ in the absence
(
[
BQ) and presence of various dyes 1–4; scan rate: 50 mV/s;
BQ] = 5 × 10 M, [dye] = 5 × 10 M, [TAB] = 1 × 10
[
29]
−3
−3
−2
In the next step, as pointed out by Fu,
using the
M
autoionization constant of DMSO, that is, 10− , the Ka
35
values in DMSO were calculated as:
withdrawing groups (CHO, N -Ar). The presence of intra-
molecular H-bonding (proven by FTIR and H NMR spec-
2
Ka = 10−35Kexchange:
ð2Þ
1
tra) leads to distance between the proton and oxygen in the
hydroxyl group; meanwhile, that proton becomes less avail-
able for the solvent. The range of experimental and calcu-
lated pKa at about 8 seems logical. The presented CV
In order to complement the results obtained from CV
ꢀ
ꢀ
measurements, ΔU
, ΔG
, Kexchange, and pK
exchange
exchange
a
values were calculated using the BP86, TPSS, B3LYP,
PBE0, TPSSh, and PW6B95 density functionals with the
def2-TZVP basis sets. The results of our calculations are
summarized for azo dyes 1–4 in Table 2. It is obvious that
the results obtained from PW6B95, BP86, and PBE0 func-
tionals agree better (mean absolute deviation [MAD] = 0.26,
approach may be generalized for the pK determination of
a
organic azo compounds that are not soluble in water in order
to provide valuable data for color control in dyeing
processes.
0
.28, and 0.35, respectively) with the experimental pKa
3
.2 | Theoretical calculations
values for azo dyes 1–4, which were estimated by (CV) to
Theoretical calculations had been proven to be significant
be 8.62, 8.29, 8.33, and 8.95, respectively (Table 3).
[28]
complementary tools for pK estimation.
There are some
a
reports on computational methods for compounds that are
[12–16]
soluble in water or other protic solvents.
Following
our quest for the pK estimation for organic compounds,
a
here, we tried to calculate the pK values of compounds 1–4
a
using DFT methods. For this aim, the following thermody-
namic cycle was considered (Figure 2).
Fu and coworkers predicted the principles of absolute
pK of some small organic compounds in DMSO as an
a
SCHEME 2 Reaction of azo dyes in DMSO used in DFT
FIGURE 2 The thermodynamic cycle
calculations

4
ZONOUZI ET AL.
ꢀ
TABLE 2 Theoretical values for
ꢀ
Comp.
def2-TZVPP
BP86
ΔU
ðkJ=molÞ
ΔG
ðkJ=molÞ
K
exchange
ꢀ
ꢀ
exchange
exchange
ΔU
, ΔG
, and Kexchange in
exchange
exchange
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
6
7
7
6
7
6
6
7
7
6
7
6
6
7
7
5
7
6
6
7
7
5
7
6
1
−150.669
−158.015
−155.136
−148.856
−155.726
−152.660
−152.198
−159.237
−156.944
−150.507
−157.549
−153.851
−149.859
−156.876
−154.160
−147.856
−155.043
−151.557
−148.634
−155.606
−153.399
−147.140
−153.886
−150.607
−150.778
−158.125
−155.245
−155.245
−155.245
−155.245
−152.098
−159.136
−156.843
−150.407
−157.449
−153.750
−150.155
−157.171
−154.456
−148.152
−155.339
−151.853
−149.832
−156.805
−154.597
−148.339
−155.084
−151.805
2.600 × 10
5.036 × 10
1.576 × 10
1.252 × 10
2.000 × 10
5.807 × 10
4.429 × 10
7.575 × 10
3.003 × 10
2.239 × 10
3.834 × 10
8.624 × 10
2.023 × 10
3.428 × 10
1.147 × 10
9.014 × 10
1.637 × 10
4.011 × 10
1.776 × 10
2.957 × 10
1.214 × 10
9.720 × 10
1.477 × 10
3.935 × 10
DMSO solvent for compounds 1–4,
calculated using various density
functionals with the def2-TZVPP
basis sets
TPSS
B3LYP
PBE0
TPSSh
PW6B95
BP86
2
3
4
TPSS
B3LYP
PBE0
TPSSh
PW6B95
BP86
TPSS
B3LYP
PBE0
TPSSh
PW6B95
BP86
TPSS
B3LYP
PBE0
TPSSh
PW6B95
TABLE 3 Theoretical values for pK
for compounds 1–4 in comparison with
experimental values obtained from cyclic
voltammetry (CV) in DMSO and
corresponding MADs
a
a
pK (cal.)
Comp.
BP86
8.58
8.35
8.69
8.75
0.28
TPSS
7.30
7.12
7.46
7.53
1.19
B3LYP
7.80
PBE0
8.90
8.65
9.05
9.01
0.35
TPSSh
7.70
PW6B95
8.24
pK
a
(exp.) CV
8.62
1
2
7.52
7.42
8.06
8.29
3
7.94
7.79
8.40
8.33
4
7.92
7.83
8.41
8.95
MAD
0.74
0.85
0.26
—
4
| CONCLUSIONS
order to complement the results obtained from CV measure-
ꢀ
ꢀ
ments, ΔU
, ΔG
, Kexchange, and pKa values
exchange
exchange
In this work, first (E)-2-hydroxy-5-(aryldiazenyl) benzalde-
hydes 1–4 were synthesized in high purity. These com-
pounds are soluble in DMSO rather than protic solvents or
were calculated using DFT with various density functionals
using the def2-TZVPP basis sets. Then, we calculated the
pK values of azo dyes 1–4 for the first time and used the
a
water. Thus, their pK values were measured by CV in
COSMO to apply the solvent effect. The structures were
a
ꢀ
ꢀ
DMSO and are being reported for the first time. This method
optimized in the DMSO, and the ΔU
, ΔGexchange,
exchange
could be generalized for pK measurement for organic azo
a
Kexchange, and pK values were estimated for azo dyes being
a
compounds that are insoluble in water, which will provide
valuable data for color control in the dyeing processes. In
studied using the BP86, TPSS, B3LYP, PBE0, TPSSh, and
PW6B95 density functionals with the def2-TZVP basis sets.

ZONOUZI ET AL.
5
The results obtained from PW6B95, BP86, and PBE0 func-
tionals are in good agreement (MAD = 0.26, 0.28, and 0.35,
[11] F. Zonouzi, A. Rahmani, H. Dezhampanah, B. Ghalami-Choobar,
A. Zonouzi, Chem. Pap., In Press. https://doi.org/10.1007/
s11696-019-00790-1.
respectively) with the experimental pK values for azo dyes
a
[12] B. Ghalami-Choobar, H. Dezhampanah, P. Nikparsa, A. Ghiami-
Shomami, Int. J. Quant. Chem. 2012, 112, 2275.
1–4, which were estimated by (CV) to be 8.62, 8.29, 8.33,
and 8.95, respectively. We hope this approach can be gener-
alized for organic compounds that are not soluble in water or
protic solvents and would be helpful for industrial coloring
procedure design and control.
[13] B. Ghalami-Choobar, A. Ghiami-Shomami, P. Nikparsa, J. Theor.
Comput. Chem. 2012, 11, 283.
[14] B. Ghalami-Choobar, A. Ghiami-Shomami, Comput. Theor.
Chem. 2015, 1054, 71.
[15] B. Ghalami-Choobar, A. Ghiami-Shomami, S. Asadzadeh-
Khanghah, Chem. Phys. Lett. 2018, 706, 426.
ACKNOWLEDGMENTS
[
16] B. Ghalami-Choobar, A. Ghiami-Shomami, M. Fereidoonzadeh,
J. Chin. Chem. Soc. 2019, 66, 150-156.
The authors thank the Universities of Guilan and Tehran.
We also thank the Iranian National Science Foundation
[
[
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How to cite this article: Zonouzi F, Shayesteh A,
Alizadeh T, Dezhampanah H, Ghalami-Choobar B,
Zonouzi A. Determination of pK values for (E)-2-
a
hydroxy-5-(aryldiazenyl) benzaldehydes in dimethyl
sulfoxide: Cyclic voltammetry and density functional
theory calculations. J Chin Chem Soc. 2019;1–5.
https://doi.org/10.1002/jccs.201800472
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