UV/vis spectrophotometric determination of slow equilibrated N(1)-H missing deprotonation constant of a pyrimidine and thiopyrimidine: The final situation of the four pKa values

Article abstract of DOI:10.1016/j.saa.2019.117867

The missing acidity constant of recently synthesized 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one (I) and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione (II) are investigated as a function of time at pH v

Full text of DOI:10.1016/j.saa.2019.117867

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UV/vis spectrophotometric determination of slow equilibrated
N(1)-H missing deprotonation constant of a pyrimidine and
thiopyrimidine: The final situation of the four pKa values
H. Kılıç
PII:
S1386-1425(19)31257-0
https://doi.org/10.1016/j.saa.2019.117867
SAA 117867
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 23 August 2019
Revised date:
26 November 2019
6 November 2019
Accepted
date:
2
Please cite this article as: H. Kılıç, UV/vis spectrophotometric determination of slow
equilibrated N(1)-H missing deprotonation constant of a pyrimidine and thiopyrimidine:
The final situation of the four pKa values, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy(2019), https://doi.org/10.1016/j.saa.2019.117867
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UV/vis spectrophotometric determination of slow equilibrated
N(1)-H missing deprotonation constant of a pyrimidine and
thiopyrimidine: The final situation of the four pK values
a
H. Kılıç
Department of Physical Chemistry, Faculty of Arts and Sciences, Marmara University,
Ziverbey 34722 Istanbul, Turkey
ABSTRACT
The
missing
acidity
constant
of
recently
synthesized
(I) and
5-benzoyl-1-
5-benzoyl-1-
(
(
methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one
methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione (II) are investigated as a
function of time at pH values from 1.0 to 3.20 for I and from 1.0 to 4.06 for II using UV/vis
spectroscopic analysis of their aged solutions, at a temperature of 25 ± 0.1 °C. In this study, a
novel time-dependent isosbestic point method was developed to confirm the presence of any
weak acid – base equilibrium and the detection of related equilibration time. The missing
acidity constants for each of I and II were determined in 5.0% v/v aqueous methanol.
Plausible acid base equilibrium mechanism for the missing acidity constant was presented for
each of the compounds based on the extensive experimental UV/vis data. In this context, all
the acid base equilibria for each of the regarding compounds were completely revised through
different pH values.
Keywords:
thiosemicarbazones; uv/vis. spectroscopy.
---------------------
acidity
constants;
pyrimidines;
thiopyrimidines;
semicarbazones;
-
*
Tel: (+90) 216 - 346 4553; fax: (+90) 216 - 347 8783.
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E-mail address: hkilic@marmara.edu.tr
. Introduction
The pyrimidine ring and its heterocyclic derivatives are well known bioactive compounds
1] that contains heteroatoms. Therefore it has a wide range of uses in various fields of
1
[
industry [2,3] including biological systems in our daily life [4,5]. Since heterocyclic
pyrimidine compounds have remarkable pharmacological activities such as antiviral [7],
analgesic [8], anti-microbial [8] and anticancer activities [9], and etc., there are many novel
applications of sulfur-containing pyrimidines in drug-delivery systems [6]. Moreover,
accurate determination of acidity constants, pK values of these compounds are important for
a
drug analysis.
Every weak acid and/or base has one or more protonable groups. Thus in each case one or
more definite acidity constants are expected to be obtained. Acidity constants correspond to
+
an equilibrium level in which one hydrogen ion, i.e. a proton (H ) is received or released. The
term of acidity constant or protonation constant is not only use for weak acid(s) but also is
used for weak base(s). Both acidic and basic groups in any molecule undergo ionization by
protonation or deprotonation to different extents depending on acidity or basicity of the
aqueous solution. As the majority of drugs are weak organic acids and/or bases, knowledge of
the protonation ↔ deprotonation constant or vice versa helps in understanding the ionic form
of the drug molecule that will take across a range of pH values. Therefore, an acidity constant
info is useful as a physicochemical indicator that showing the extent of dissociation of
ionizable groups in the medium with respect to pH, by which the changes in concentrations of
the components can be calculated. By using the pK value of an acid or base the extent of the
a
dissociation can be determined at a particular pH using the Henderson-Hasselbalch equation
via UV/vis spectroscopy [10-13] potentiometric titration [14] and conductometry [15]..
Molecules containing proton-binding or proton-releasing groups control the polarity of the
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medium and, consequently, the physicochemical properties of the respective aqueous solution,
thereby enabling interactions with target molecules. For this reason, acid dissociation
constant, pK
parameter that influence the pharmaceutical characteristics in choosing appropriate acidic or
basic reagents in drug discovery and development of new medicines. In this sense, pK value
a
value of an organic compound is one of the important physicochemical
a
of a drug molecule influences its solubility, hydrophilicity, lipophilicity, protein binding and
permeability. Additionally, pKa value of an organic compound also directly affects kinetic
(
pharmacokinetic) and thermodynamic (pharmacodynamic) properties of the related drug
compound in terms of absorption, distribution and metabolism. In the case of all
aforementioned importance of the calculation of acidity constant, computational programs
have been developed to screen and predict pK values with a high degree of accuracy [16].
a
Considering the aforementioned significance of the recently synthesized pyrimidine
derivatives 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one (I)
and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione (II), are
expected to present novel biological, pharmaceutical and industrial features. The structures of
the compounds are given in Figure 1.
O
N
X
Ph
Ph
6
1N
2
5
4
3
Me
Ph
N
I : X = O
II : X = S
Fig. 1. Pyrimidine compounds investigated in this study
As shown in Fig. 1, the pyrimidine compounds are analogs of each other containing both
semi- and thiosemicarbazone moiety at the same time. The numbered benzene ring containing
two nitrogen atoms shown in Fig. 1 is a pyrimidine ring. Thus, pyrimidine ring and all
derivatives of it are basic in character due to presence of nitrogen atoms. Therefore, the title
compounds undergo protonation in acidic solutions. So, pyrimidine compounds exist in their
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cationic forms, i.e., protonated amine cations in acidic solutions. Using fresh solutions of
these two compounds, acid-base properties were investigated by Kılıç and three pKa values
were found for each compound [13]. However, by looking at the structure of the two title
compounds four protonable nitrogen atoms are seen, indicating four pK values are expected
a
to be obtained for each compound. This difference clarifies that one acidity constant was
missing for each compound in the recent study using fresh solutions. The reason for this result
is slow balance of the equilibrium. Thus, the aim of this current study is to determine the
missing pK value of each compounds using their aged solutions against varying pH and time
a
with the related acid base equilibrium mechanism using UV/vis absorption spectroscopy.
Since UV/vis spectrophotometric method provides high precision and accuracy in order to
establish the dissociation constants of organic compounds, aged solutions of the compounds
were used in this current study. However, fresh solutions of the same compounds were used in
the recent study [13].
2
. Material and methods
2
.1. Synthesis of Pyrimidine Compounds
Compounds I and II were synthesized in our laboratory according to procedures described
elsewhere [17,18]. Briefly, for the synthesis of I, 0.28 g furan-2,3-dione and 0.18 g
methylphenyl semicarbazone (molar ratio 1:1) are refluxed in toluene for 45 minutes, yielding
0
.14 g (38%) of I. For the synthesis of II, 5 g furan-2,3-dione and 3.5 g methylphenyl
thiosemicarbazone are refluxed in 20 mL of toluene in a 100-mL round-bottom flask for 1 h,
affording 2.4 g (33%) of II. The compounds are filtered from the reaction mixtures and purified
by recrystallization from acetic acid. Their characteristic physical and chemical properties
including IR data have been reported previously [17,18]. Their purity is assessed by thin-layer
chromatography (TLC) using methanol/chloroform (1:9 v/v) and methanol/benzene (1:9 v/v) as
given in [19]. All other chemicals are obtained from Fluka as reagent-grade materials. Triple-
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distilled water is used for all aqueous solutions. Preparation of triple-distilled water is as
described recently [14]. Methanol is distilled before use. Stock solutions of the compounds are
prepared in an appropriate volume of distilled methanol.
2
.2. Instrumentation
IR spectra are recorded from KBr disks using a Shimadzu 8300 FT-IR spectrometer in the
-
1
range 4000–400 cm . UV/vis spectroscopy studies are carried out using a Shimadzu 1601 PC
UV/vis spectrophotometer with quartz cells (1.0-cm path length). The pH of the aqueous and
methanol solutions are measured using a digital pH meter (Hanna Instruments 8314, Italy; ±
0
4
.01 pH unit) and a combined pH electrode that is calibrated using standard aqueous buffers (pH
.00, 7.00, and 9.00) as described in previous report [20]. The measured pH values are not
+
corrected, and the symbol pH (defined as -log [H ]) is used in all cases. Glass electrodes in
solvents containing up to 95% alcohol show responses very similar to the theoretical response in
water. Of the organic solvents, methanol is the most similar to water in structure and properties.
Therefore, acidic dissociation in methanol takes place in a fashion analogous to that of in water
[
21]. An Eppendorf micro-pipette is used for precise volumetric measurement of the solutions.
The temperature of the solutions is kept constant at 25 ± 0.1 °C using a thermostat (WiseCircu,
Daihan Scientific Co., Ltd. Korea). A Sartorius A120 S analytical balance (sensitivity of ±
0
.0001 g) is used for weighing the chemicals.
2
.3. Measurements
Stock solutions of the compounds are prepared in methanol and stored in the dark to avoid
decomposition. Aqueous Britton-Robinson buffer (BRB) solutions [22] are prepared with the
-5
desired pH values for the experiments. Working solutions with concentrations of 1.641×10
-5
M for I and 2.044×10 M for II are prepared daily in 5.0% v/v aqueous methanol by adding 1
mL of the stock solution of I or II to a flat-bottomed volumetric flask (20 mL) containing
some BRB solution at 25 ± 0.1 °C and diluting it to 20 mL rapidly with the same solution.
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The experiments are conducted in BRB media over a pH range of 3.0 to 5.0 in increments of
1
.0 pH unit at regular time intervals. For more acidic solutions, 0.1 M HCl, 0.01 M HCl are
used. The ionic strength of the solution is constant with 0.10 M LiCl as the supporting
electrolyte.
Typically, experiments are initiated as soon as 1.0 mL of the stock solution of I or II is
mixed with the required buffer solution in a 20 mL volumetric flask at 25 ± 0.1 °C. After
preparation of the aqueous methanol working solution, the pH is first adjusted to the desired
pH by dotting with a relatively concentrated KOH or concentrated HCl on a thin glass rod and
then an appropriate volume of this solution is transferred immediately to the 1.0-cm quartz
cells and the corresponding spectrum is recorded. Experiments are initiated as soon as 1.0 mL
of the stock solution of I or II is mixed with the required buffer solution in a 20-mL
volumetric flask at 25 ± 0.1 °C. The absorption maxima are monitored by recording the
UV/vis spectra at regular time intervals until the maximum absorbance (A ) is reached. The
∞
absorbance is determined after baseline correction. The reference beam contained a blank of
the buffer containing the same amount of aqueous methanol as the solvent. The UV/vis
spectra are recorded from 600 to 200 nm for each compound at a medium scan rate (ca. 100
nm/minute) between pH 1.0 and 13.0.
2
.4. Determination of missing acidity constants
Since the compounds I and II are basic in character, they undergo protonation completely
in a strong acidic solution. Therefore, the both compounds undergo deprotonation on
increasing pH of the medium. So, the missing acidity constants for the protonated form of the
both compounds in their aged solutions were determined from their spectral behavior in buffer
solutions against varying pHs and time, at selected wavelengths, by using the Henderson-
Hasselbalch equation as described by Albert et al. [11]. For this purpose, the pH media of
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from 1.0 to 3.06 for I and of from 1.0 to 4.06 for II were scanned in BRB solutions for each
compound and the approximate position of each pK
a
value was determined by monitoring the
spectral changes over the pH range. The pK
a
values were calculated after the baseline
correction of the spectra.
3
. Results and discussion
The acid-base properties of the two compounds were investigated in their fresh solutions
from the pH-dependence of the absorbance values at selected wavelengths in aqueous
methanol [13]. From the obtained spectra the pH-dependence of the absorbance values of keto
and enol maxima at selected wavelengths were evaluated. By means of these evaluations three
acidity constants were obtained for each compound. Although the fourth one is expected to
occur for each compound it was not obtained in the recent study [13]. In other words, using
the pH dependence of the absorbance values of the keto and enol forms of both compounds
obtained in fresh solutions at different pHs, the fourth acidity constant cannot yet be
determined so far. During our previous study [23] in which we investigated hydration
behavior of the title compounds using their aged solutions, a clue about the loss acidity
constant for each compound was observed. Therefore, in this current study the acid base
behaviors of the title compounds are studied also as a function of time using their aged
solutions. The use of the aged solutions of the compounds provides an advantage as follows;
as the solution is kept at any pH level for a while (from 2 minutes to 360 minutes) hydration
of the related molecule occurs depending on pH and the passage of time. Thus, in addition to
the keto and its enolic form maxima obtained using fresh solutions in the recent study [13] a
new maximum, i.e. a hydration maximum is obtained in this current study. The examination
of the changes in the absorptions and their wavelengths of this new maximum with the other
keto and enol maxima have created a new hope in the determination of the lost acidity
constant of the compounds. The results related to the missing acidity constant obtained from
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the aged solutions of each compound in the pH range from 1.0 to 3.20 for I and from 1.0 to
4
.06 for II, are presented below separately.
The UV/Vis spectra of each compound are obtained in the aqueous methanol. The results
obtained from the aged solutions of compounds I and II are summarized below.
3
.1. Effect of pH and time on hydration property of I in aqueous solution
In the recent study [13], a keto maximum and its enolic form were obtained. However, in
addition to the keto and its enolic form in the recent study [13], a new well-separated
hydration maximum is obtained in this current study as function of time for I at pH values
from 1.0 to 3.20, above which the new maximum is not observed. As the aging time increased
from 1 minute to 20 minutes, the absorbance of the new maximum increases in pH 1.0
solution. Moreover, the absorbance of the new maximum increases and simultaneously the
keto maximum decreases by showing a time dependent isosbestic point depending on varying
aging time in the UV/vis spectra obtained at pHs from 1.0 to 3.20. A time dependent
isosbestic point is reported for the first time in this study. Increasing of the new maximum and
decreasing of the keto maximum indicates the presence of two absorbing species which are
interconvertible from carbonyl to enol for I in that pH range. It is just similar to the isosbestic
point which is seen in absorption vs. wavelength plots in UV/vis absorption spectroscopy. The
observed time dependent isosbestic point agrees well with the literatures [24,25] and thus it
confirms the presence of the missing acid-base equilibrium for I.
A bathochromic shift by 1.6 nm in the wavelength of the new hydration maximum of I is
observed on increasing the pH from 1.0 to 3.20 above which the new hydration maximum is
not observed. A hypsochromic shift by 16.1 nm in the wavelength of the keto maximum of I
is also observed in the same pH range. The absorbance of the new hydration maximum
decreased significantly, and the absorbance of the keto maximum increased steadily. From
both shifts, a fine isosbestic point at 249.7 nm (A = 0.795) is obtained, covering the pH range
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of from 1.0 to 3.20. Such decrease in the absorbance of the hydration maximum with
simultaneous increase in the keto form with pH clearly indicates that the two maxima are in
equilibrium [24,25] which established after 30 minutes of equilibration period. Thus, the
obtained single isosbestic point determined for I also confirms the presence of one
dissociation step, i.e. one equilibrium [24,25]. The related spectra in due to course of 30
minutes are given in Fig. 2 below. The related data are given in Table 1 below.
Figure 2 may be placed here
Table 1 may be placed here
3
.2. Effect of pH and time on hydration property of II in aqueous solution
Very fast new hydration maximum of the thioenol form of II is observed in 2 minutes after
start the experiment. Like I, in addition to this thioenol maximum a new well-separated
hydration maximum is obtained for also II in this current study. The new hydration maximum
is obtained as function of time at pH values 4.06 and below, above which the new maximum
is not observed. The absorbance of the new maximum and the thioenol maximum changed
continually depending on varying aging time in the UV/vis spectra obtained, in the range of
pH from 1.0 to 5.07. The related spectra for duration of 120 minutes are given in Fig. 3 below.
The related spectral characteristics are given also in Table 1 above.
Figure 3 may be placed here
A bathochromic shift by 15.6 nm in the wavelength of the new hydration maximum of II is
observed on increasing the pH from 1.0 to 4.06 above which the new hydration maximum is
not observed. A bathochromic shift by 2.8 nm in the wavelength of the thioenol maximum of
II is observed too in the same pH range. The absorbance values of both the new hydration and
of the thioenol maxima are decreased depending on varying aging time. Increasing one of
them and simultaneously decreasing the other one is never observed. Therefore, there is no
clear time dependent isosbestic point was obtained for II, covering the pH range from 1.0 to
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5
.07, despite 240 minutes waiting period. The reason for this can be explained as follows: On
increasing pH, the thioenol group, –SH of II undergoes deprotonation by producing sulfur
-
ions, -S which re-reacts with proton of the solvent water immediately to reform –SH.
+
Because, H ions accelerate the thioenolization (≡C–SH) of II, which in turn accelerates its
hydration. Thus, the continuous refreshing of the –SH form prevents its concentration from
falling. For this reason, the thioketo form of II is not seen in the spectra obtained, despite
using 240 minutes aged solutions at all pHs investigated. Although a clear time dependent
isosbestic point is not seen for II, this does not mean that there is no acid base equilibrium
between the new and thioenol maxima of II. As long as a time dependent isosbestic point is
observed for I, it is definitely expected to observe also for II by an analogical approach.
Fortunately, presence of the missing acid-base equilibrium for II is also proved from the pH
dependency of absorbance values obtained using aged solutions of 40 minutes as shown in
Fig. 4 in the section 3.4 below. As shown in Fig. 4, the obtained absorbance vs. pH curve for
II is an S-shaped indicating clearly to the presence of an acid-base equilibrium [11-13].
3
.3. Comparative interpretations of the findings obtained for I and II
The protonated cationic forms of I and II molecules are capable of binding water
molecules that is known as hydration. Thus, the hydration is too fast in pH 1.0 solution so that
each of I and II molecules has four types of binding.
I undergoes protonation and thus of hydration in its aged solutions at pHs below 3.20.
Similarly, II undergoes also protonation and thus of hydration in its aged solutions at pH
values 4.06 and below. Protonation, thus of hydration behavior, of each compound in pH 1.0
aqueous solution is displayed in Scheme 1 below.
Scheme 1 may be placed here
As shown by Ia→Ia' and IIa→IIa' in Scheme 1, both the keto and enol forms of I, and only
the thioenol form of II exist in their protonated forms in pH 1.0 aged solutions. Although I
1
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exists in comparable amounts of the keto and the enol forms in pH 1.0 aged solutions, II
exists wholly in the thioenol form (IIa→IIa') in the same medium. As displayed in Scheme 1,
each of Ia and IIa molecules has four protonated amine cations in pH 1.0 solutions. These
protonated forms can capable of forming hydrogen bonding as shown in red color in Scheme
1
1
with adjacent solvent or solute molecules. Thus, I and II undergo very fast hydration in pH
.0 solutions. It should be also noted that each protonated form corresponds to a private pK
a
value. Thus, there should be four acidity constants in total for each compound in pH 1.0
solutions. As clearly predictable, the number of protonated amine cations will decrease as the
pH increased and this will cause the observed hydrogen bonding formation to slow down. For
this reason, no significant hydration is observed at pH 3.20 and above for I and, above 4.06
for II. Consequently, the pH values of 3.20 for I and of 4.06 for II can be considered roughly
as limit upper pH values for hydration to be observed in the acidic pH region investigated.
Thus, the reason of stopping of the hydration at pHs 3.20 and below for I and at pHs 4.06
and below for II are investigated at a protonation - deprotonation mechanism level. The
possible equilibrium processes for I and II are schematically presented by the deprotonation
mechanism given in Scheme 2 below.
Scheme 2 may be placed here
As shown in Scheme 2 above, the reason for the observed stop is found to be the
+
deprotonation of the previously protonated nitrogen (-NH=) substituted to the ring nitrogen
at position 1 of both I (Ib') and II (IIb'). The aforementioned isosbestic point obtained for I
confirms this finding. Thus, this deprotonation causes to stop the observed hydrations of I and
II in the acidic solutions by forming a new intramolecular hydrogen bonding between the
deprotonated nitrogen −N= substituted to the ring nitrogen, =N< at position 1, and the
carbonyl oxygen at position 2 of I and II. The new hydrogen bonds are >C=O-H∙∙∙∙∙:N≡ by
1
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Ib→Ib' for I and >C=S-H∙∙∙∙∙:N≡, by IIb→IIb' for II. Thus, the deprotonation corresponds just
to one that we are looking for, i.e. to the missing pK value.
a
As it is seen in Scheme 2, it is plausible to show the stopping of the hydration, and thus of
protonation - deprotonation equilibrium of the related local amino form for I and II in acidic
medium. In this respect, Scheme 2 is faultless and very useful to understand the reason of the
stops occurring at pH values below 3.20 for I, and 4.06 and below for II. The related missing
pK value for the related deprotonation of each compound is investigated as given below.
a
3
.4. Determination of the missing acidity constant of the each compound of I and II
The compounds, I and II exist in their cationic protonated amine forms, mostly in aged
acidic solutions. Therefore, the obtained pK values are explained in this current study on the
a
basis of the relative contribution of acidic character of respective species. For each of the
compounds, the experiments are repeated twice in aged aqueous methanol solutions.
As shown in Scheme 1 and 2 above, the dissociation mechanisms are the same for the
both compounds. Every protonation – deprotonation equilibrium has its own acidity constant.
For this reason, the absorbance vs. pH curves in the related pH ranges are examined
carefully, and thus the value of the related missing pK value of each compound is
a
investigated at a selected wavelength by using the Henderson-Haselbalch equation as
described by Albert et al [11]. Using the preliminary findings of this examination, a
preliminary edition is presented in [26] in which the missing acidity constant for the each
compound could not be determined at the time of congress event.
The pH-dependence of the absorbance curves due to 30 minutes for I and 40 minutes for II
found at selected wavelengths in aqueous methanol are shown in Fig. 4. As shown in Fig. 4,
the absorbance vs. pH curves obtained both for I and II are an S-shaped indicating clearly the
existence of one dissociation step [11-13,27]. The isosbestic point reported in the section 3.1
for I above support the acid base equilibrium in the corresponding stage of the hydration
1
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[
24,25]. The mean missing pK
a
values calculated both for I and II are given in Table 1 above.
As shown in Table 1, newly calculated pK values of 2.881 for I and 3.684 for II in this study
a
can be considered roughly as a limit upper value for the hydration of the respective
compounds in the acidic media investigated. Because there is no hydration at pH values 3.06
and above for I and, above 4.06 for II in the acidic region. Thus, all the explanations and the
data given in Table 1 are in good agreement with Scheme 1 and 2.
Figure 4 may be placed here
The wavelengths selected are 245.2 nm for compound I and 247.4 nm for compound II. The
obtained inflection given by I in Fig. 4 is the missing acidity constant for which a pK value
a
2
of 2.881 (R =0.9993) is obtained at λ = 245.2 nm. On the other hand, the obtained inflection
given by II in Fig. 4 is the missing acidity constant for which a pK
a
value of 3.684
values with the
2
(
R =0.9981) was obtained at λ = 247.4 nm. The determined missed pK
a
related protonated amino form of I and II are displayed in Scheme 3 below.
Scheme 3 may be placed here
3
.5. Final situation of the four acidity constants for each compound
The preferred four acid dissociation mechanisms are fully revised based on the data
obtained in this current study, and the latest visual four acid – base equilibria of each
compound against varying pH are given in Scheme 4, below. The latest situations of all
acidity constants for both compounds are also given in Table 2 below.
Scheme 4 may be placed here
Table 2 may be placed here
As shown in Scheme 4, each of I and II must have theoretically four acidity constants. In the
recent study [13], the pK values related to ring nitrogen at position 1 of I and II were not
a
determined. However, the other three of them were determined in the recent study [13] and, the
pKa1 values were found to be 5.121 (λ = 267.8 nm) for I and 4.684 (λ = 256.6 nm) for II both of
1
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which were misattributed as the dissociation of one proton from the protonated nitrogen form (–
+
+
NH=) substituted to the ring nitrogen, (>N=) at position 1 of I and II, respectively. Because,
the pK value for the deprotonation mentioned earlier was obtained as 2.881 for I and 3.684 for
a
II in this current investigation. The determination of this misattribution is a great
accomplishment of this current study. Thus, the required correction and reinterpretation were
done as given in below.
Before going into reinterpretation of those, it should be reminded that there were no problems
with the obtained pKa2 and pKa3 values presented in the recent study [13]. The pKa2 value was
attributed in the recent study [13] to the dissociation of one proton from the protonated ring
+
nitrogen (–NH=) at position 3 of I and II. The pKa3 value in the recent study [13] was attributed
to the dissociation of the one proton from both the enol form of I and the thioenol form of II. No
experimental data was given for the fourth dissociation constant, relating it to ring nitrogen at
position 1 of both I and II in the recent study [13].
As for the reinterpretation, the missed acidity constant is fixed in this current study, in the
case of using aged solutions for the each compound. This current study showed that the pKa1
value obtained in the recent study [13] actually belongs to the unavailable fourth dissociation
constant in the recent study as relating it to ring nitrogen at position 1 of both I and II.
Unfortunately, it was misattributed as the dissociation of one proton from the protonated
+
+
nitrogen form (–NH=) substituted to the ring nitrogen, (>N=) at position 1 of I and II,
respectively. Finding this misattribution is the innovation of this current study. Therefore, it can
be seen clearly now from this current study that one of the four acidity constants are masked in
the recent study [13] for which fresh solutions of I and II were used. In the light of this current
study, it can thus be resulted that the inability in the determination of one of the four acidity
constants led to a misattribution and so on a misinterpretation of the acidity constants
determined for I and II in the recent study [13].
1
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4
. Conclusions
In this study, time-dependent isosbestic point method was improved for the first time to
confirm the presence of any weak acid – base equilibrium and the detection of the related
equilibration time. This is a great accomplishment of this study and will be a model for further
studies on weak acids and bases. Besides, the finding of the missing acidity constant is the
innovation of this study.
The pKa1 value of 5.121 (λ = 267.8 nm) for I and 4.684 (λ = 256.6 nm) for II obtained in
+
the recent study is actually not related to the deprotonation of the protonated nitrogen (-NH=)
substituted to the ring nitrogen at position 1. Rather, it is found in this study that it is to be
+
related to the deprotonation of the previously protonated form of the ring nitrogen (>N=) at
position 1 of the each compound. Thus, the required correction is done in this study and the
most accurate four acidic dissociation mechanisms for I and II are identified. The final
situation of the four pK values is displayed for each compound.
a
Acknowledgements
This work was supported by the Scientific Research Project Program of Marmara University
Project No: FEN-A-250608-0185). The author is grateful to Marmara University for their
(
financial support and the provision of laboratory facilities.
References
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Captions to figures
Table 1
The related UV/vis spectroscopic data and the mean missing pK values calculated both for I
a
and II in 95% v/v aqueous medium at 25 °C and constant ionic strength (0.10 M LiCl).
Table 2
The latest situation of all acidity constants and the related UV/vis spectroscopic data obtained
for I and II in 95% v/v aqueous medium at 25 °C and constant ionic strength (0.10 M LiCl).
a Obtained from the recent study [13].
Fig. 1. Pyrimidine compounds investigated in this study
Fig. 2. In due to course of 30 minutes time dependent UV-Vis absorption spectra of
-5
1
.641×10 M aged solutions of I obtained at different pHs in 95% v/v aqueous solutions.
Isosbestic point at 249.7 nm, A = 0.795.
Fig. 3. In due to course of 40 minutes time dependent UV-Vis absorption spectra of
-5
2
.044×10 M aged solutions of II obtained at different pHs in 95% v/v aqueous solutions.
Fig. 4. The pH-dependence of the absorbance curves due to 30 minutes for I and 40 minutes
for II found at selected wavelengths in 95% v/v aqueous solutions.
Scheme 1: The suggested all protonation mechanisms for I and II in pH 1.0 aqueous aged
solutions.
Scheme 2: The suggested deprotonation mechanisms of I and II causing to stop the
hydrations observed at pHs below 3.20 for I and, 4.06 and below for II.
Scheme 3: The determined missed pK
a
values with the related protonated amino form of I and
II.
Scheme 4: A fully revised the latest four acid – base equilibria of each compound against
varying pH.
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Table 1
Selected
wavelength missing
Isosbestic
The
2
pH
Range
Point
R
(nm)
pK
a
Wavelength Absorbans
(
nm)
(A)
0.795
-
1
1
.0 to 3.20
.0 to 4.06 II
I
249.7
-
I
II
245.2
247.4
2.881
3.684
0.9993
0.9981
I: 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one
II: 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione
Table 2
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I
Selected
Isosbestic
Points
(nm)
:
wavelength
pKa1
pKa2
pKa3
pKa4
Medium
(
nm)
5
-
2
2
3
2
2
2
3
2
45.2
67.8
37.4
70.0
47.4
56.6
01.2
99.6
249.7
346.2
251.5
322.5
-
2.881
-
-
-
b
e
n
z
o
y
l
Aqueous
methanol
I
a
a
-
5.121
-
-
a
a
-
-
7.929
-
(
v/v, 5%
a
a
-
-
-
11.130
methanol)
3.684
-
-
-
315.1
265.0
395.8
-
-
-
4.684
-
7.245
-
-
-
II
-
-
-
10.630
1
-
(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one.
II: 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione.
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Fig. 1
O
N
X
Ph
Ph
6
1N
2
5
4
3
Me
Ph
N
I : X = O
II : X = S
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Revised Fig. 2
The old Figure 2 shown below is deleted
2
3

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Revised Fig. 3
The old Figure 3 shown below is deleted
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Fig. 4
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Scheme 1
2
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Scheme 2
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Scheme 3
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Scheme 4
2
9