Effect of Medium on the Acidity Constant of Some New Arylazopyrazolopyrimidine Derivatives

Article abstract of DOI:10.1021/je020194t

The acid dissociation constants of some new 8-[(aryl)azo]-2,4-dimethylpyrazolo[1,5-a]pyrimidine-7(6H)-one derivatives were determined in aqueous-organic solvent mixtures (methanol, ethanol, acetone, and dimethylformamide (DMF)). Methanol and DMF had approximately similar relative permittivity constants (32.6 and 36.7, respectively, at 25°C), and all the compounds were more acidic in water + DMF than in water-methanol although the same mole fraction of each was used. The increase in the acid dissociation constants (pK_a) of the compounds as the proportion of the organic cosolvent in the medium was increased could be ascribed, in addition to the electrostatic effect, to the hydrogen-bonding interaction between the conjugate base (A-) and solvent molecules. The pK_a values in the presence of the poorer hydrogen bond donor DMF were less than those obtained in the presence of corresponding amounts of the other solvents.

Full text of DOI:10.1021/je020194t

652
J . Chem. Eng. Data 2003, 48, 652-656
Effect of Med iu m on th e Acid ity Con sta n t of Som e New
Ar yla zop yr a zolop yr im id in e Der iva tives
Elh a m M. Abd -Alla h ,† Na sr M. Ra geh ,*,‡ a n d Ha ssa n M. A. Sa lm a n ‡
Department of Chemistry, Faculty of Science, El-Minia University, El-Minia, Egypt, and Department of
Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt
The acid dissociation constants of some new 8-[(aryl)azo]-2,4-dimethylpyrazolo[1,5-a]pyrimidine-7(6H)-
one derivatives were determined in aqueous-organic solvent mixtures. The organic solvents are methanol,
ethanol, acetone, and dimethylformamide. The results obtained were discussed in terms of the solvent
characteristics. The ionization constants of the title dyes depend largely on both the ratio and the nature
of the organic cosolvent. Hydrogen-bonding interactions of the conjugate base with solvent molecules as
well as the solvent basicity contribute the major effects on the ionization process. It has been concluded
that the pKa values increase by increasing the amount of organic cosolvent in the medium. The effect of
molecular structure of the azo compound on the pKa value is discussed.
In tr od u ction
stirring a cold solution of sodium nitrite (1.6 g, 0.023 mol
in 20 mL of H2O). The resulting diazonium salt was added
slowly with stirring to a cold solution of 3-aminopyrazolin-
5-one (1.98 g, 0.02 mol) in 40 mL of (1:1) water/ethanol (the
latter compound was prepared12 by the reaction of ethyl
cyanoacetate with hydrazine hydrate and sodium ethoxide).
After completion of the addition, stirring was continued for
30 min; then a solution of sodium acetate (20 g in 60 mL
of H2O) was added until complete precipitation of the dye.
The precipitated solid was filtered, washed with cold water,
and recrystallized from ethanol to give the corresponding
arylazopyrazolinone derivative.
P r epa r a tion of Ar yla zopyr a zolopyr im id in e Der iva -
tives.13,14 Each of the resulting azo dyes from the above
step was refluxed for 4 h with acetylacetone in the (1:1)
molar ratio in 30 mL of acetic acid. The reaction mixture
was then cooled to room temperature, whereby a solid
compound was precipitated. The precipitated solid was
filtered, washed with cold ethanol, and recrystallized from
ethanol to give the corresponding arylazopyrazolopyrimi-
dine derivative. The purity of the compounds was checked
by elemental analyses and IR spectra (cf. Table 1).
The knowledge of the pKa is considered to be of interest
for organic and inorganic compounds because it has a
significant role in many chemical reactions. Therefore,
numerous works have been devoted to the determination
of pKa values of the different azo compounds.1-5 The
widespread application of the azo compounds as dyes;
acid-base, redox, and metallochrome indicators; or histo-
logical stains has attracted many studies of their acid-
base properties.6-9 In continuation of our studies on the
acid-base properties of azo compounds,1,6,10 we have
investigated the medium effect on the ionization constants
of some arylazopyrazolopyrimidine derivatives by the study
of the electronic spectra of the compounds in aqueous buffer
solutions containing varying proportions of organic solvents
of different polarities, such as methanol, ethanol, acetone,
and dimethylformamide (DMF). The pKa values have been
determined and discussed in terms of solvent characteris-
tics. The arylazopyrazolopyrimidine derivatives under
investigation have the structures given below.
Stock solutions (10-3 mol dm-3) of the compounds were
prepared by dissolving a known mass of the solid in the
required volume of the solvent. The pH control was
achieved by using the modified universal buffer solution.15
To account for differences in acidity, basicity, dielectric
constant, and ion activities for a water + organic solvent
mixture relative to pure water, where the pH meter is
standardized using aqueous buffers at 25 °C, the pH values
in the former media were corrected by using the procedure
described by Douheret16 (eq 1), where the meter reading
pH(R) obtained in each water-organic solvent mixture
differs by an amount δ from the corrected reading pH*.
Exp er im en ta l Section
The preparation of arylazopyrazolopyrimidine deriva-
tives includes the following two steps.
P r epa r a tion of Ar yla zopyr a zolin on e Der iva tives.
3-Amino-4-(arylazo)pyrazolin-5-one derivatives were pre-
pared similarly to the literature method.11 The appropriate
amine (aniline, p-toludine, p-anisidine, p-chloroaniline, and
p-nitroaniline, 0.02 mol) was dissolved in dilute HCl (20
mL, 3 N), and the resulting solution was cooled in an ice
bath to 5 °C. To this solution was added dropwise with
pH* ) pH(R) - δ
(1)
Values of δ for various aqueous-organic solvent mixtures
were determined as recommended by Douheret16,17 at the
same ionic strength. The solutions were thermostated at
25 °C before measuring their spectra. The absorption
spectra were recorded on a Shimadzu 2401PC spectropho-
† El-Minia University.
‡ South Valley University.
* Corresponding author.
10.1021/je020194t CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/11/2003

J ournal of Chemical and Engineering Data, Vol. 48, No. 3, 2003 653
Ta ble 1. An a lytica l Da ta , Color s, Meltin g P oin ts, Yield , a n d IR Sp ectr a l Ba n d s of Com p ou n d s
mp yield analyses/% [calcd (found)]
°C
relevant IR bands/cm-1
ν(NH + CH) ν(CdO) ν(NdN)
3150-2920 b 1640 vs 1440 s
compd emp form. form. wt
color
%
C
H
N
Cl
I
C
14H13N5O
267.13 reddish 220 88 62.89 (63.20) 4.90 (4.82) 26.22 (26.08)
orange
II
III
IV
C
C
C
15H15N5O
15H15N5O2
14H12N5OCl 301.63 red
281.15 yellow
297.15 yellow
215 83 64.02 (64.31) 5.38 (5.58) 24.90 (24.84)
210 83 60.58 (60.84) 5.08 (5.02) 23.57 (23.46)
250 77 55.70 (55.92) 4.01 (3.95) 23.22 (23.00)
3200-2915 b 1642 vs 1440 s
3140-2900 b 1630 vs 1430 s
3200-2922 b 1642 s 1437 m
11.77
(11.62)
V
C
14H12N6O3 312.13 red
260 79 53.82 (54.10) 3.88 (3.73) 26.92 (26.84)
3210-2920 b 1645 s 1435 m
F igu r e 1. Absorption spectra of 2.0 × 10-5 M compound V (p-NO2) in water (1) + methanol (2) at different pH’s: (A) W2 ) 8.49 methanol;
pH* ) (1) 4.17, (2) 5.15, (3) 5.84, (4) 6.53, (5) 6.86, (6) 7.11, (7) 7.30, (8) 7.48, (9) 7.66, (10) 7.90, (11) 8.12, (12) 8.59, and (13) 9.02. (B)
W2 ) 16.53 methanol; pH* ) (1) 4.36, (2) 5.81, (3) 6.08, (4) 6.39, (5) 6.78, (6) 6.89, (7) 6.93, (8) 7.10, (9) 7.52, (10) 7.68, (11) 7.87, (12) 8.10,
(13) 8.32, (14) 9.05, and (15) 10.39. (C) W2 ) 25.35 methanol; pH* ) (1) 5.49, (2) 6.33, (3) 6.67, (4) 7.05, (5) 7.21, (6) 7.39, (7) 7.83, (8) 7.99,
(9) 8.15, (10) 8.36, (11) 8.89, (12) 9.56, and (13) 10.06. (D) W2 ) 34.56 methanol; pH* ) (1) 5.83, (2) 6.57, (3) 7.16, (4) 7.31, (5) 7.68, (6)
8.09, (7) 8.32, (8) 8.56, (9) 8.70, (10) 9.01, (11) 9.41, and (12) 10.03.
tometer, containing a thermoelectrically temperature-
controlled cell holder, within the wavelength range of 280
to 600 nm using 1 cm matched quartz cells. The pH
measurements were carried out using an Orion 501 digital
ionalyzer accurate to (0.01 pH unit. All measurements
were carried out at 25 °C, and temperature control was
achieved using an ultrathermostat of accuracy (0.05 °C.
visible absorption spectra of 8-[p-substituted phenylazo]-
2,4-dimethylpyrazolo[1,5-a]pyrimidine-7(6H)-one deriva-
tives in universal buffer solutions containing different
proportions of an organic solvent (methanol, ethanol,
acetone, and DMF) show mainly two bands (Figure 1). The
shorter wavelength band, appearing at low pH* values
(pH* < 6.0), represents absorption by the nonionized
species, whereas the longer wavelength band, observed at
higher pH* values (pH* > 8.8), is due to the absorption by
ionized species. On increasing the pH of the medium, the
absorbance of the former band decreases while that of the
latter band increases, where a fine isosbestic point is
achieved, denoting the existence of an equilibrium of the
type
Resu lts a n d Discu ssion
The compounds can exist in the following tautomeric
forms:
HA h H+ + A-
(2)
The acid dissociation constants, pKa’s, of the compounds
are determined from the variation of the absorbance with
pH, making use of three different spectrophotometric
methods, namely, the half-curve height, isosbestic point,
It was found that they exist in the keto form in solids
state while in solution the enol form predominates.1,14 The

654 J ournal of Chemical and Engineering Data, Vol. 48, No. 3, 2003
Ta ble 2. p Ka Va lu es for (2 × 10-5 m ol d m -3) of
8-[(P h en yl)a zo]-2,4-d im eth ylp yr a zolo[1,5-a ]p yr im id in e-
7(6H)-on e, (p -H), in Wa ter (1) + Or ga n ic Solven t (2)
Mixtu r es a t 25 °C
Ta ble 4. p Ka Va lu es for (2 × 10-5 m ol d m -3) of
8-[(4-Meth ylp h en yl)a zo]-2,4-d im eth ylp yr a zolo[1,5-a ]-
p yr im id in e-7(6H)-on e, (p -CH3), in Wa ter (1) + Or ga n ic
Solven t (2) Mixtu r es a t 25 °C
pKa
pKa
100w2 method 1 method 2 method 3 mean value
Water (1) + Methanol (2)
SD
100w2 method 1 method 2 method 3 mean value
Water (1) + Methanol (2)
SD
8.49
16.53
25.35
34.56
7.60
7.65
7.75
7.90
7.65
7.70
7.85
7.90
7.65
7.75
7.80
7.85
7.63
7.70
7.80
7.88
(0.03
(0.04
(0.04
(0.03
8.49
16.53
25.35
34.56
7.40
7.6
7.45
7.50
7.65
7.80
7.50
7.55
7.60
7.85
7.45
7.55
7.63
7.80
(0.04
(0.04
(0.03
(0.03
7.65
7.57
Water (1) + Ethanol (2)
Water (1) + Ethanol (2)
8.52
16.59
25.42
34.65
7.55
7.75
7.90
8.05
7.70
7.80
7.95
8.10
7.65
7.80
7.85
8.10
7.63
7.78
7.90
8.08
(0.06
(0.03
(0.04
(0.03
8.52
16.59
25.42
34.65
7.80
7.93
8.14
8.30
7.85
7.98
8.20
8.40
7.75
7.88
8.14
8.35
7.80
7.93
8.16
8.35
(0.04
(0.04
(0.03
(0.04
Water (1) + Acetone (2)
Water (1) + Acetone (2)
8.49
16.53
25.35
34.56
8.00
8.05
8.25
8.40
7.90
7.95
8.30
8.40
7.80
8.00
8.35
8.45
7.90
8.00
8.30
8.42
(0.08
(0.04
(0.04
(0.03
8.49
16.53
25.35
34.56
7.90
8.10
8.15
8.38
8.00
7.90
8.22
8.45
7.9
7.93
8.03
8.18
8.40
(0.047
(0.057
(0.029
(0.033
8.10
8.18
8.38
Water (1) + DMF (2)
Water (1) + DMF (2)
9.67
19.21
28.96
38.81
7.65
7.65
7.70
7.75
7.60
7.65
7.70
7.70
7.55
7.60
7.65
7.80
7.60
7.63
7.68
7.75
(0.04
(0.03
(0.03
(0.04
9.67
19.21
28.96
38.81
7.72
7.73
7.88
7.92
7.62
7.83
7.83
7.87
7.67
7.78
7.88
7.87
7.67
7.78
7.86
7.89
(0.041
(0.041
(0.024
(0.024
Ta ble 3. p Ka Va lu es for (2 × 10-5 m ol d m -3) of
Ta ble 5. p Ka Va lu es for (2 × 10-5 m ol d m -3) of
8-[(4-Meth oxyp h en yl)a zo]-2,4-d im eth ylp yr a zolo[1,5-a ]-
p yr im id in e-7(6H)-on e, (p -OCH3), in Wa ter (1) + Or ga n ic
Solven t (2) Mixtu r es a t 25 °C
8-[(4-Ch lor op h en yl)a zo]-2,4-d im eth ylp yr a zolo[1,5-a ]-
p yr im id in e-7(6H)-on e, (p-Cl), in Wa ter (1) + Or ga n ic
Solven t (2) Mixtu r es a t 25 °C
pKa
pKa
100w2 method 1 method 2 method 3 mean value
Water (1) + Methnol (2)
SD
100w2 method 1 method 2 method 3 mean value
Water (1) + Methanol (2)
SD
8.49
16.53
25.35
34.56
7.75
7.80
7.86
7.94
7.80
7.90
7.96
7.97
7.80
7.80
7.96
7.94
7.78
7.83
7.91
7.95
(0.03
(0.05
(0.05
(0.02
8.49
16.53
25.35
34.56
7.10
7.40
7.50
7.60
7.15
7.35
7.54
7.55
7.20
7.40
7.45
7.55
7.15
7.38
7.50
7.57
(0.04
(0.03
(0.04
(0.03
Water (1) + Ethanol (2)
Water (1) + Ethanol (2)
8.52
16.59
25.42
34.65
7.55
7.60
7.89
7.9
7.60
7.65
7.84
7.95
7.60
7.70
7.94
8.00
7.58
7.65
7.89
7.95
(0.03
(0.04
(0.04
(0.03
8.52
16.59
25.42
34.65
7.40
7.50
7.65
7.74
7.30
7.55
7.60
7.65
7.35
7.50
7.55
7.64
7.35
7.52
7.60
7.68
(0.04
(0.03
(0.04
(0.05
Water (1) + Acetone (2)
Water (1) + Acetone (2)
8.49
16.53
25.35
34.56
7.77
7.89
8.10
8.30
7.82
7.84
8.05
8.41
7.72
7.94
8.15
8.35
7.77
7.89
8.10
8.35
( 0.03
( 0.04
( 0.04
( 0.05
8.49
16.53
25.35
34.56
7.62
7.74
7.85
7.96
7.67
7.65
7.95
8.02
7.72
7.70
7.95
8.02
7.67
7.70
7.92
8.00
(0.04
(0.04
(0.05
(0.03
Water (1) + DMF (2)
Water (1) + DMF (2)
9.67
19.21
28.96
38.81
7.52
7.68
7.68
7.70
7.50
7.63
7.63
7.67
7.48
7.58
7.60
7.70
7.50
7.63
7.64
7.69
(0.02
(0.04
(0.03
(0.02
9.67
19.21
28.96
38.81
7.25
7.30
7.38
7.40
7.30
7.35
7.30
7.40
7.35
7.33
7.38
7.37
7.30
7.33
7.35
7.39
(0.04
(0.02
(0.04
(0.02
and limiting absorbance.18,19 The experimental error in
determined pKa values was checked by using the least-
squares method. The results obtained are given in Tables
2-6. Examination of the results shown in Tables 2-6
reveals that pKa values of all compounds are largely
dependent upon both the nature and the proportion of the
organic cosolvent. In general, pKa values increase with an
increase of the organic cosolvent content in the medium.
This can be explained as follows. According to Coetzee and
Ritchie,20 the acid dissociation constant in aqueous medium
(Ka1) is related to that in a partially aqueous medium (Ka2)
by the equation
aqueous one. As the electrostatic effect operates on the
activity coefficients of any charged species,20 one can 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 eq 3, this will result in a
decrease in the acid dissociation constant Ka (i.e., high pKa
value) on increasing the organic cosolvent content in the
medium, which is consistent with the results reported in
Tables 2-6.
However, methanol and DMF have approximately simi-
lar relative permittivity constants (32.6 and 36.7, respec-
tively, at 25 °C), and all the compounds are more acidic in
water + DMF than in water + methanol, despite the fact
that the same mole fraction of each is used (cf. Tables 2-6).
Moreover, although ethanol and acetone also have compa-
rable relative permittivity constants (24.3 and 20.7, re-
spectively, at 25 °C), all the compounds are more acidic in
Ka1 ) Ka2(γH+γA-/γHA
)
(3)
where γ is the activity coefficient of the subscripted species
in a partially aqueous medium relative to that in a pure

J ournal of Chemical and Engineering Data, Vol. 48, No. 3, 2003 655
Ta ble 6. p Ka Va lu es for (2 × 10-5 m ol d m -3) of
8-[(4-Nitr op h en yl)a zo]-2,4-d im eth ylp yr a zolo[1,5-a ]-
p yr im id in e-7(6H)-on e, (p -NO2), in Wa ter (1) + Or ga n ic
Solven t (2) Mixtu r es a t 25 °C
If dispersive forces, which possibly exist in the media
used, between the delocalized charge on the conjugate base
of the dye (A-) and the localized dispersion centers in near
solvent molecules as well as the proton-solvent interac-
tions have important effects on the ionization process of
the compounds, 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., γA- and γH+/γA-
decrease), since the effective density of dispersion centers
in each of the organic solvents used is higher than that of
water.22 Thus, in 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 (cf.
Tables 2-6). 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 hydroxyazopyrazolopyrimidine dyes.
pKa
100w2 method 1 method 2 method 3 mean value
Water (1) + Methanol (2)
SD
8.49
16.53
25.35
34.56
7.00
7.10
7.18
7.20
7.10
7.13
7.13
7.18
7.05
7.13
7.18
7.22
7.05
7.12
7.16
7.20
(0.04
(0.02
(0.03
(0.02
Water (1) + Ethanol (2)
8.52
16.59
25.42
34.65
7.10
7.25
7.36
7.44
7.20
7.15
7.28
7.40
7.15
7.30
7.26
7.40
7.15
7.23
7.30
7.41
(0.04
(0.04
(0.04
(0.02
Water (1) + Acetone (2)
8.49
16.53
25.35
34.56
7.12
7.24
7.35
7.46
7.12
7.29
7.30
7.51
7.15
7.24
7.39
7.54
7.13
7.26
7.35
7.50
(0.02
(0.03
(0.04
(0.03
Effect of Molecu la r Str u ctu r e
The values of pKa, reported in Table 2-6, show that the
acidity of the studied azo compounds decreases in the
following order: p-NO2 > p-Cl > p-H > p-OCH3 > p-CH3.
This trend is in accordance with the increase in the electron
donor ability of the substituent, which reflects itself in an
increase of the electronic density on the OH group oxygen
atom and consequently increases the intramolecular H-
bond strength and retards ionization.
Water (1) + DMF (2)
9.67
19.21
28.96
38.81
6.80
6.73
6.88
6.93
6.70
6.88
6.93
6.93
6.80
6.88
6.88
6.87
6.77
6.83
6.90
6.91
(0.05
(0.07
(0.03
(0.03
water + ethanol than in water + acetone, where the same
mole fraction of each is used. In general, pKa values for all
compounds increase with increasing ratio of organic cosol-
vent in the medium; that is, the pKa values of a compound
in water + organic solvent are arranged according to the
following sequence: DMF < ethanol < methanol < acetone
(Tables 2-6). This behavior indicates that other solvent
effects beside the electrostatic one contribute to the ioniza-
tion process of the investigated compounds.
Liter a tu r e Cited
(1) Ibrahim, S. A.; Rageh, N. M.; Mohamed, A. A.; Ebead, Y. H.
Medium effects on theacid dissociation constants of some hetero
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1525-1533.
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.20 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 conjugate base (A-) and solvent
molecules. Since water molecules have a higher tendency
to donate hydrogen bonds than other solvent molecules,21
the conjugate base (A-) is expected to be less stabilized by
hydrogen-bonding interaction with solvent molecules as the
amount of the organic cosolvent in the medium is increased
(i.e. γA- increases). 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 in the pKa values as the amount of the organic
cosolvent in the medium is increased. Another factor to be
considered, in this context, is the higher stabilization of
the intramolecular H-bonding within the conjugate base,
A-, as the amount of the organic cosolvent of the medium
is increased, which in turn retards the ionization; that is,
pKa increases.
(3) Simoncic, B.; Span, J . A study of dye-surfactant interactions (part
3) Thermodynamic of the association of C.I. acid orange 7 and
cetylpyridinum chloride in aqueous solution. Dyes Pigm. 2000,
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process (i.e., low pKa).

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Received for review October 16, 2002. Accepted February 27, 2003.
J E020194T