J. Iran. Chem. Soc., Vol. 8, No. 2, June 2011, pp. 502-512. JOURNAL OF THE
Iranian Chemical Society
A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium M. Jankulovskaa,*, I. Spirevskaa and V. Dimovab Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts Cyril and Methodius University, Arhimedova 5, MK-1001 Skopje, Republic of Macedonia b Faculty of Technology and Metalurgy, Sts Cyril and Methodius University, P. O. Box 580, MK-1001 Skopje, Republic of Macedonia a
(Received 17 July 2010, Accepted 7 October 2010) The spectral behavior of some benzaldehyde hydrazones was examined via the UV spectroscopic technique at room temperature and in the pH region between 1 and 7 in aqueous perchloric acid medium. The acid-base equilibrium was characterized qualitatively and quantitatively. It was found that the protonation reaction took place. The spectra of the solution of benzaldehyde hydrazones at different pHs were studied and utilized for the determination of ionization constants of the protonated forms of hydrazones (pKBH+). The pKBH+ values were determined numerically and graphically from the absorbance values of the experimental and reconstructed spectra by characteristic vector analysis. In order to obtain thermodynamic pKBH+ values, measurements were performed at ionic strengths of 0.1, 0.25 and 0.5 M (NaClO4). There was a good agreement between the obtained pKBH+ values of the investigated hydrazones and those of similar classes of compounds. The site of protonation of the investigated hydrazones was also studied, too. The proton affinities for the different nitrogen atoms of the hydrazone molecule were calculated using AM1 and PM3 semiempirical methods. It was demonstrated that the protonation occurred at the imino nitrogen atom of hydrazone molecule. The effect of the chemical structure on the ionization constants was also examined. Keywords: UV spectroscopy, Protonation, Thermodynamic ionization constants, Benzaldehyde hydrazones, Semiempirical methods
INTRODUCTION The hydrazones are well-known class of organic compounds with a wide spectrum of biological activity [1]. The remarkable biological activity of these compounds is a corollary of having an azometine proton (-NH-N=CH-). It is shown that the hydrazones possess varied biological activities including: anticonvulsant, antidepressant, analgesic, antiinflammatory, antiplatelet, antimicrobial, antimalarial, antitumoral, antiviral, vasodilator and antituberculosis [2-11]. On the other hand, hydrazones are widely used in synthetic *Corresponding author. E-mail:
[email protected]
chemistry for the preparation of other compounds, and in analytical chemistry for the identification of carbonyl compounds [12]. Additionally, they are used in industry as plasticizers, polymer stabilizers, antioxidants and polymerization initiators [13]. Because of their physiological activity, they are used as herbicides, insecticides and plant growth stimulants [14]. Furthermore, the hydrazones are used as spectrophotometric reagents because of their selectivity for metal ions. So, they act as multidentate ligands, especially with transition metals forming colored chelates [15]. The ionization constants of organic compounds like hydrazones play a fundamental role in many analytical
Jankulovska et al.
procedures such as acid-base titration, solvent extraction, complex formation and ion transport [16]. It has been shown that the acid-base properties affect the toxicity and pharmaceutical characteristics of organic acids and bases [16]. Moreover, the biological activity of hydrazones depends on the ionic forms in which they exist in the solution. Therefore, we decided to investigate the behaviour of five newly synthesized benzaldehyde hydrazones in acid media (pH from 1 to 7) and determine the ionization constants of the protonated forms which exist in the solution. To do this, there are different methods like potentiometric titration, spectrophotometry and capillary electrophoresis and so on. It is known that spectrophotometric methods are highly sensitive and suitable for the study of chemical equilibrium in the solution [17]. Therefore, spectrophotometry was chosen for the determination of pKBH+ values of the investigated hydrazones. Furthermore, we intended to investigate the site of protonation of the investigated hydrazones. For this purpose, different theoretical and experimental methods could be adopted. In our work, we used the theoretical semiempirical methods, AM1 [18] and PM3 [19]. The benzaldehyde hydrazones experimented in this study share the general structural formula as follows:
O R
C NH
N
CH
H1 H2 H3 H4 H5
H CH3 OCH3 Cl OH
These hydrazones were synthesized in our laboratory and their structures were confirmed by a variety of methods such as: UV, IR, 1H NMR, 13C NMR and the elemental analysis (the results are in preparation for publication).
EXPERIMENTAL Preparation and the Stability of the Solutions Stock solutions (c = 1.00 × 10-3 M) of the investigated hydrazones were prepared by dissolving the required amounts of the substances (H1-H5) in 96% ethanol. These solutions were stable over a long period of time, under ordinary conditions. The concentration of the investigated hydrazones in the test solutions was 3 × 10-5 M. They were prepared by
transferring 0.75 cm3 of the stock solution of each hydrazone into a 25 cm3 measuring flask, and after adding appropriate amounts of sodium perchlorate (1.0 M) and perchloric acid (0.5 M), the measuring flask was diluted up to the mark with deionized water. Sodium perchlorate was used to maintain constant ionic strength (0.1, 0.25 and 0.5 M), while perchloric acid was used to adjust the required pH values of the solutions. After each pH adjustment, the solution was transferred into a quartz cell and the UV absorption spectra were recorded. The stability of the working solutions was followed with UV spectra recorded for 48 h in neutral media (pH = 6.6). The obtained results showed that there was no significant change in the position of the absorption bands in the spectra of all hydrazones after 24 h, but their intensity decreased insignificantly. More significant changes, however, were observed in the solutions after 48 h. These changes probably occur as a result of hydrolysis of the investigated hydrazones. As a result of this instability, the UV spectra at different pH values (between 1 and 7) were recorded immediately after the preparation of the solutions at room temperature. Simultaneously, the blanks were prepared with the same composition as the working solutions without the investigated hydrazone. For the quantitative characterization of the protonation reaction the measurements were performed with three series of solutions with different pH values (from 1 to 7) at ionic strengths of 0.1, 0.25 and 0.5 M. pKBH+ values were calculated as the average values of these data.
Apparatus and Chemicals The purity of the investigated hydrazones was confirmed by the elemental analysis, as well as by measuring their melting points, while the perchloric acid and sodium perchlorate were of analytical grade (p.a.). The spectrophotometric measurements were made using a Varian Cary 50 spectrophotometer with a 1 cm path length quartz cell. All absorption spectra were digitized at five data points per nanometres in the wavelength region between 190 and 400 nm. The pH values were measured using a digital pH meter with a combined glass electrode (1 < pH < 13), calibrated with two buffer solutions at pH = 4 and 7. All the results were obtained using the computer programs:
503
A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium
Excel, Grams Version 4.10, and Hyper Chem. Version 8.
2
A
a
1.6
RESULTS AND DISCUSSION
1.2
Spectroscopic Behaviour of the Hydrazones In order to investigate the behaviour of hydrazones in the acid medium, the electronic absorption spectra of Nbenzaldehydebenzoilhydrazone (H1), N-benzaldehyde-pmethylbenzoilhydrazone (H2), N-benzaldehyde-p-methoxybenzoilhydrazone (H3), N-benzaldehyde-p-chlorobenzoilhydrazone (H4) and N-benzaldehyde-p-hydroxybenzoilhydrazone (H5) were recorded in the wavelength region from 190 nm to 400 nm. The concentration of the hydrazones in the test solutions was kept constant, while the pH values were varied along the pH range between 1 and 7 at constant ionic strength (0.1, 0.25 and 0.5 M). The obtained UV spectra for the series of solutions of the hydrazones H1 and H5 (at ionic strength of 0.1 M) are presented in Figs. 1a and 2a, respectively. The changes in the spectra of the other hydrazones are similar to those shown in these figures. The reconstruction of the experimental spectra to eliminate the influence of the solvent could be performed via different methods [20]. One most frequently used is the characteristic vector analysis (CVA) which has been described in detail by Simonds [21], and since then has been tested and proved to be applicable to the analysis of spectroscopic data, to investigate problems dealing with protonation [22,23]. CVA is a method of separating independent factors for sets of multivariate response data. The method can be used empirically for estimating the number of independent factors contributing to the total variation observed in a family of UV spectra. If p independent factors are involved in generating the absorbance curve, the sample responses at each wavelength for a given concentration will be given by Eq. (1). A1 = Ā1 + c1v11 + c2v21 +..........+ cpvp1 A2 = Ā2 + c1v12 + c2v22 +..........+ cpvp2 Ar = Ār + c1v1r + c2v2r +..........+ cpvpr
(1)
where the choice of A is arbitrary and the mean values of the absorbance seem to be a convenient choice. v is the characteristic vector, and c is the weighting coefficient. The reconstructed UV spectra using CVA method for
504
0.8 0.4 0 190
240
290
340
390
λ (nm)
2
A
b
1.6 1.2 0.8 0.4 0 190
240
290
340
λ (nm)
390
Fig. 1. The experimental (a) and reconstructed (b) UV spectra of H1 (c = 3.04 × 10-5 M) at pH range from 1 (spectrum 19) to 7 (spectrum 1) and ionic strength of 0.1 M (NaClO4).
compounds H1 and H5 are shown in Figs. 1b and 2b, respectively. In the spectra of the investigated hydrazones obtained in neutral media (pH = 6.6) two absorption bands were noticed with maximum wavelengths around 195 and 300 nm (spectrum 1, Figs. 1a and 2a). The absorption bands at 195 and 300 nm are the results of π→π* and n→π* electron transitions, respectively [24]. The latter band was of interest to our further measurement. When the pH of the investigated solutions decreased, the intensity of the absorption band that appeared around 195 nm increased (spectrum 1, Figs. 1a and 2a), while its position did not change significantly (Table 1). On the other hand, when the investigated solution became acidic, the absorption band around 300 nm shifted hipsochromic and its intensity
Jankulovska et al.
1.6
a
A 1.2
0.8
0.4
0 190
240
290
340
λ (nm)
390
1.6
b
A 1.2
0.8
0.4
0 190
240
290
340
λ (nm)390
Fig. 2. The experimental (a) and reconstructed (b) UV spectra of H5 (c = 3.00 × 10-5 M), at pH range from 1 (spectrum 19) to 7 (spectrum 1) and ionic strength of 0.1 M (NaClO4).
decreased. Below pH 3.1 this band disappeared and a new absorption band appeared at around 250 nm (spectrum 19, Figs. 1a and 2a). Its intensity increased when the pH of the solution decreased and at pH lower than 2.3 its intensity and position did not change anymore. This band is probably a result of the absorption of the protonated form of the hydrazone molecule. That is to say, the hydrazones in acid media behave as weak bases and protonation process is expected to occur. The intensity of the absorption band at around 250 nm increased with decreasing the pH value as a result of increasing the quantity of the protonated form. The position of the absorption bands of the investigated hydrazones was in agreement with literature data for a similar group of compounds [25,26]. The characteristic absorption bands of neutral and protonated forms of the investigated hydrazones and the pH range of protonation are given in Table 1. From Table 1, it can be seen that the hydrazones H4 and H5 were protonated at lower pH values because they are more acidic members of the investigated aromatic hydrazones. When pH of the investigated solutions decreased, the absorption band at 195 nm shifted batochromic by about 5 nm, while the absorption maximum at 300 nm shifted hipsochromic by about 50 nm (Table 1). The appearance of an isosbestic point (Figs. 1b and 2b) in the reconstructed spectra of the investigated compounds indicated the equilibrium between the two forms: i.e., the
Table 1. Characteristic Absorption Bands of the Investigated Hydrazones in Neutral and Acidic Media and pH Range of Protonation
Compound
Neutral form λ1max λ2max pH
Protonated form pH λ1max λ2max
pH Range of protonation
H1
6.59
193
297
2.32
197
243
4.1-2.1
H2
6.50
195
298
2.33
199
248
4.2-2.3
H3
6.50
195
300
2.30
199
252
4.1-2.1
H4
6.53
195
298
2.32
199
246
3.9-1.7
H5
6.60
196
302
2.34
199
252
3.6-1.7
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A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium
molecular and protonated forms. The changes in the UV spectra owing to changes in pH of the solutions could be accounted for by the dependence of the absorbance values on pH. The absorbance vs. pH values at λmax (250 nm) and ionic strength of 0.1 M, for all investigated hydrazones are presented in Fig. 3. The dependence of the absorbance values on pH of the solution has sigmoid form, “S” curve (Fig. 3). From this dependence, it could be noticed that the protonation process occurs in one step (one plateau of the S curve exists), and the pH range of the protonation could be determined. The protonation process of all investigated hydrazones takes place in the pH range between 1.7 and 4.2 (Fig. 3 and Table 1).
1
A 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
pH
7
Fig. 3. The absorbance vs. pH at λmax = 250 nm for hydrazones H1-H5 (c = 3.00 × 10-5 M) at ionic strength of 0.1 M. (3) H1, (▬) H2, (▲) H3, () H4, () H5.
Site of Protonation As it was mentioned before, the investigated hydrazones in perchloric acid media behave as weak bases and could be protonated. The site of protonation of the hydrazones has been the subject of many scientific papers [27,28]. Some results that were obtained from these experimental and theoretical investigations showed that the energies of amino (sp3) and imino (sp2) nitrogen are very similar. Thus, the site of protonation depends on the substituents bonded on hydrazone molecule [29]. In the case of the investigated hydrazones, because of the presence of carbonyl group (>C=O) in the molecule of hydrazones, the sp3 hybridized nitrogen has lower basicity compared to the sp2 hybridized nitrogen which makes proton bonding to imino (sp2) nitrogen easier. To find out where the protonoation of the hydrazones occurs, using the semiempirical methods AM1 [18] and PM3 [19], we calculated the proton affinity (PA) of both nitrogen atoms for hydrazones H1-H5, according to Eq. (1). PA = ∆H f(B) + ∆H f(H ) - ∆H f(BH ) o
o
+
o
+
(1)
where, PA is the proton affinity, ∆Hof(B) is the heat of formation of the molecule, ∆Hof(BH+) is the heat of formation for the cation and ∆Hof(H+) is the heat of formation for the proton (367.15 kcal mol-1) [30]. The obtained results for heats of formation of neutral and protonated (N-sp3 and N-sp2) forms and proton affinities are presented in Table 2. The calculated proton affinities of sp2 hybridized hydrogen
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atom using AM1 and PM3 semiempirical methods are similar, as are those of sp3 hybridized nitrogen atom (See Table 2). Comparing the results shown in Table 2, it could be seen that the proton affinity of the sp2 hybridized nitrogen atom is higher than that of the sp3 hybridized one for all the investigated hydrazones. The information about the protonation site of hydrazones obtained from the literature [27-29], as well as our experimentally calculated values of proton affinities suggests the following protonation reaction for all the investigated hydrazones:
O R
+
C NH
N
CH
+H _ H+
R
O
H
NH
N
C
+
CH
The influence of the substituents on the protonation site of the investigated benzaldehyde hydrazones is not significant, and because of that, the PA values of both nitrogen atoms are similar.
Determination of Ionization Constants The biological activity of hydrazones depends on the ionic form in which they exist in the solution [31]. Therefore, the determination of their ionization constant values in aqueous solutions is necessary for understanding their biological effects, because they are important parameters for determining
Jankulovska et al.
Table 2. Heats of Formation of Molecular and Protonated Forms (N-sp2 and N-sp3) and Proton Affinities of the Hydrazones H1-H5
∆Hof(BH+) ∆Hof(BH+) N-sp2 N-sp3 AM1 H1 61.22 215.48 229.58 H2 53.30 206.81 220.51 H3 22.59 175.53 188.83 54.59 210.35 224.92 H4 H5 16.42 170.20 183.91 PM3 H1 55.67 211.82 222.20 H2 44.61 201.47 211.54 H3 15.60 170.35 181.56 H4 49.29 206.31 216.79 H5 9.984 165.23 175.25 a o o + The units for ∆H f(B), ∆H f(BH ) and PA are kcal mol-1.
Compound
∆Hof(B)a
the extent of ionization of hydrazones in solutions at different pH values. The changes in the absorbance value that occurred when the pH of the solutions was decreased made it possible to determine the pKBH+ values of hydrazones. To this end, eight wavelengths were chosen (Table 3) for each hydrazone around the absorption maximum of the band in neutral (300 nm) and in acid (250 nm) media (for both experimental and reconstructed spectra). It was revealed that the influence of the solvent was of minor importance when the measurements were performed in dilute perchloric acid solution [32] and it did not affect the appearance of the spectra. Thus, the ionization constants have identical values in both cases and they are independent of the selected wavelengths. Using the absorbance values at the selected wavelengths, the molar absorption coefficients of neutral (pH = 6.7) and protonated (pH = 1.6) forms were calculated. The UV spectra were recorded at three different concentrations (2.40 × 10-5, 3.00 × 10-5 and 3.60 × 10-5 M) of the investigated hydrazones in the solution at ionic strengths of 0.1, 0.25 and 0.5 M. The average molar absorption coefficients at ionic strength of 0.1 M are given in Table 3. Then, a system of four equations (for absorbance values)
PA N-sp2
PA N-sp3
212.89 213.64 214.21 211.39 213.37
198.78 199.94 200.91 196.82 199.66
211.00 210.28 212.39 210.13 211.90
200.62 200.21 201.19 199.64 201.88
with two unknown parameters (concentrations of the protonated and unprotonated forms) was used for the determination of the ionization ratio I (I = c(BH+)/c(B)), i.e., the ratio between the concentrations of the protonated (BH+) and unprotonated (B) forms of the compounds. The calculations were made using the molar absorption coefficient values and the spectrophotometric data obtained at the selected wavelengths. The calculations were made in accordance with Beer’s law. Finally, the pKBH+ values were calculated using Eq. (2). pKBH+ = n × pH + logI
(2)
where, pKBH+ is the ionization constant, I is the ionization ratio and n is the number of protons. The calculations were made using the absorbance data from the experimental and the reconstructed spectra. The pKBH+ values obtained from both the experimental and reconstructed spectra at different ionic strengths (0.1, 0.25 and 0.5 M) are presented in Table 4. The statistical data: standard deviations (s), correlation coefficients (R) and variances (V%) of the calculations are also given in Table 4. These calculations were made using standard statistical methods [33]
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A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium
Table 3. Molar Absorption Coefficients of Neutral (pH = 6.7) and Protonated (pH = 1.6) Forms of Hydrazones H1-H5 at Ionic Strength of 0.1 M for Experimental and Reconstructed Spectra
Experimental spectra
λ (nm) a εB
H1
240
245
250
285
295
300
305
103825
92258
84659
83094
259911
294677
292757
274453
177888 240
191490 245
194266 250
183250 255
39069 290
32146 295
27217 300
22136 305
97918
99931
101849
103822
261187
275396
276767
265029
221846
247432
241313
198491
33903
30983
26574
22266
245
250
255
260
290
295
300
305
73730
84069
96228
108313
305520
342412
358714
356592
250181 240
292589 245
296945 250
258404 255
46457 290
37138 295
30795 300
26223 305
107131
107271
105733
104188
246579
260400
262459
250831
λ/nm
241205 245
265587 250
256542 255
209918 260
24907 290
29318 295
33709 300
36492 305
115410
ε
+ BH
λ (nm) εB
H2
ε
+ BH
λ/nm εB
H3
εBH+ λ/nm εB
H4
εBH+
H5
εB
88737
96047
105041
286672
314854
327380
325834
εBH+
237866
275798
275852 232954 39006 Reconstructed spectra
30830
25144
20886
εB
103730
92173
84578
83017
259903
294668
292750
274446
H1
εBH+
177754
191352
194127
183120
38077
32153
27224
22143
εB
97816
99829
101751
103726
261269
275495
276868
265107
221884
247461
241366
198532
33903
30982
26577
22272
72603
83009
95256
107421
305927
343058
359460
357286
ε
250233
292634
296994
258448
46474
37151
30810
26233
εB εBH+
107186
107331
105807
104270
246604
260419
262487
250844
241275
265668
256623
209981
36508
33724
29335
24923
εB
88746
96036
105036
115423
286682
314838
327352
325811
εBH+
237866
275808
275868
232971
39016
30843
25156
20897
H2
ε
+ BH
εB
H3
+ BH
H4 H5 a
235
ε (M ). -1
applying the computer program Excel. The protonation constants could also be determined graphically [34] from the intercept of the dependence of logI on pH (Fig. 4), namely, when c(BH+) = c(B), logI = 0, and the pKBH+ value is equal to the pH value of the solution. As an example, the dependence of logI on pH of the solution for
508
hydrazone H1 from the experimental spectra at ionic strength of 0.1 M is given in Fig. 4. The pKBH+ values obtained graphically are presented in Table 4. Furthermore, the thermodynamic pKBH+ values were determined for the investigated hydrazones. The dependence of the average pKBH+ values at different ionic strengths of the
Jankulovska et al.
Compound
Table 4. pKBH+ Values (Numerically and Graphically) at Ionic Strengths of 0.10, 0.25 and 0.50 M, Thermodynamic pKBH+ Values of Hydrazones H1-H5 (c = 3.00 × 10-5 M), and Statistical Data (s, V%, R) from Experimental and Reconstructed Spectra
I (M)
Experimental spectra
Reconstructed spectra
pKBH+ Numerically (graphically)
pKBH+ numerically (graphically)
1
H1
0.1
0.25
0.5 0a
H2
0.1
0.25
0.5 0
H3
0.1
0.25
0.5 0
3.45 ± 0.054 (3.45) s = 0.091 V = 2.64 R = 0.995 2 3.45 ± 0.044 (3.43) s = 0.075 V = 2.17 R = 0.996 1 3.49 ± 0.026 (3.45) s = 0.037 V = 1.07 R = 0.998 2 3.47 ± 0.024 (3.53) s = 0.034 V = 0.98 R = 0.999 1 3.52 ± 0.028 (3.55) s = 0.041 V = 1.16 R = 0.998 2 3.55 ± 0.023 (3.54) s = 0.033 V = 0.94 R = 0.999 3.44 (3.42) 1 3.13±0.01 (3.14) s = 0.013 V = 0.42 R = 0.999 2 3.15±0.01 (3.11) s = 0.015 V = 0.49 R = 0.999 1 3.16 ± 0.012 (3.17) s = 0.013 V = 0.42 R = 0.999 2 3.18 ± 0.012 (3.19) s = 0.015 V = 0.49 R = 0.999 1 3.20 ± 0.018 (3.22) s = 0.028 V = 0.87 R = 0.999 2 3.22 ± 0.014 (3.27) s = 0.021 V = 0.66 R = 0.999 3.12 (3.10) 1 3.29 ± 0.025 (3.23) s = 0.036 V = 1.11 R = 0.998 2 3.24 ± 0.028 (3.24) s = 0.041 V = 1.26 R = 0.998 1 3.27 ± 0.013 (3.28) s = 0.019 V = 0.58 R = 0.999 2 3.29 ± 0.019 (3.23) s = 0.028 V = 0.87 R = 0.999 1 3.33 ± 0.015 (3.26) s = 0.022 V = 0.65 R = 0.999 2 3.30 ± 0.024 (3.31) s = 0.035 V = 1.07 R = 0.998 3.25 (3.23)
1
3.32 ± 0.026 (3.33) s = 0.044 V = 1.32 R = 0.998 2 3.33 ± 0.025 (3.29) s = 0.043 V = 1.30 R = 0.998 1 3.36 ± 0.017 (3.40) s = 0.024 V = 0.72 R = 0.999 2 3.38 ± 0.039 (3.33) s = 0.057 V = 1.69 R = 0.997 1 3.45 ± 0.01 (3.46) s = 0.013 V = 0.39 R = 0.999 2 3.43 ± 0.036 (3.43) s = 0.052 V = 1.53 R = 0.997 3.29 (3.28) 1 3.13±0.013 (3.12) s = 0.019 V = 0.62 R = 0.999 2 3.14±0.023 (3.13) s = 0.036 V = 1.14 R = 0.998 1 3.13 ± 0.013 (3.12) s = 0.019 V = 0.62 R = 0.999 2 3.14 ± 0.023 (3.13) s = 0.036 V = 1.14 R = 0.998 1 3.17 ± 0.031 (3.19) s = 0.047 V = 1.49 R = 0.997 2 3.19 ± 0.021 (3.24) s = 0.032 V = 0.98 R = 0.999 3.13 (3.11) 1 3.23 ± 0.018 (3.21) s = 0.026 V = 0.82 R = 0.999 2 3.21 ± 0.035 (3.19) s = 0.051 V = 1.59 R = 0.997 1 3.22 ± 0.017 (3.26) s = 0.025 V = 0.78 R = 0.999 2 3.26 ± 0.024 (3.20) s = 0.035 V = 1.07 R = 0.998 1 3.25 ± 0.016 (3.23) s = 0.023 V = 0.72 R = 0.999 2 3.29 ± 0.013 (3.30) s = 0.018 V = 0.57 R = 0.999 3.21 (3.19)
509
A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium
Table 4. Continued 1 2.51 ± 0.018 (2.53) 2.56 ± 0.034 (2.54) s = 0.028 V = 1.12 R = 0.999 s = 0.052 V = 2.04 R = 0.996 0.1 2 2 2.65 ± 0.013 (2.66) 2.61 ± 0.037 (2.60) s = 0.018 V = 0.70 R = 0.999 s = 0.053 V = 2.05 R = 0.995 1 1 2.63 ± 0.019 (2.56) 2.62 ± 0.023 (2.57) s = 0.027 V = 1.05 R = 0.999 s = 0.031 V = 1.17 R = 0.997 0.25 2 2 2.58 ± 0.018 (2.66) 2.58 ± 0.012 (2.60) s = 0.026 V = 1.03 R = 0.999 s = 0.017 V = 0.67 R = 0.999 1 1 2.68 ± 0.018 (2.68) 2.64 ± 0.01 (2.63) s = 0.026 V = 0.97 R = 0.998 s = 0.013 V = 0.51 R = 0.999 0.5 2 2 2.59 ± 0.012 (2.62) 2.61 ± 0.018 (2.58) s = 0.016 V = 0.61 R = 0.998 s = 0.023 V = 0.88 R = 0.998 0 2.56 (2.58) 2.58 (2.56) 1 1 2.74 ± 0.016 (2.75) 2.78 ± 0.01 (2.82) s = 0.024 V = 0.89 R = 0.999 s = 0.043 V = 1.56 R = 0.997 0.1 2 2 2.79 ± 0.013 (2.83) 2.75 ± 0.019 (2.79) s = 0.020 V = 0.76 R = 0.999 s = 0.029 V = 1.08 R = 0.998 1 1 2.81 ± 0.013 (2.80) 2.79 ± 0.018 (2.85) s = 0.021 V = 0.75 R = 0.998 s = 0.029 V = 1.06 R = 0.998 0.25 2 2 2.80 ± 0.018 (2.84) 2.80 ± 0.011 (2.82) s = 0.030 V = 1.06 R = 0.997 s = 0.018 V = 0.64 R = 0.999 1 1 2.88 ± 0.016 (2.90) 2.87 ± 0.023 (2.91) s = 0.024 V = 0.84 R = 0.999 s = 0.033 V = 1.16 R = 0.998 0.5 2 2 2.88 ± 0.026 (2.87) 2.87 ± 0.027 (2.89) s = 0.037 V = 1.30 R = 0.997 s = 0.039 V = 1.36 R = 0.998 0 2.74 (2.77) 2.73 (2.78) a + 1 Thermodynamic pKBH values. λ: H1: 235, 240, 245, 250 H2 and H4: 240, 245, 250, 255 H3 and H5: 245, 250, 255, 260. 2λ: H1: 285, 295, 300, 305 H2-H5: 290, 295, 300, 305 s, standard deviation; V%, variance; R, correlation coefficient.
H5
H4
1
logI
2
1.5 1 0.5 0
-0.5 1.5 -1
2 2.5 3 y = -0.9935x + 3.4353
3.5
4
4.5
5
R2 = 0.9929
-1.5
p Fig. 4. Dependence of logI on pH (1.7-4.5) of H1 (c = 3.04 × 10-5 M) at ionic strength of 0.1 M, experimental spectra.
510
solutions on µ was used [35]. From extrapolation of the lines pKBH+ = f( µ ) to zero ionic strength, the thermodynamic pKBH+ values were obtained as an intercept (Fig. 5). Their values were between 2.56 and 3.44 (Table 4). This method yielded very precise values which were in excellent agreement with those obtained from other methods [35,36]. Table 4 shows that the numerically calculated pKBH+ values are identical to those obtained graphically (3.45 and 3.44 for the hydrazone H1, from the experimental spectra). The pKBH+ values calculated from the absorbance values of the reconstructed spectra are lower than those calculated from the experimental spectra (3.32 and 3.45 for the hydrazone H1, Table 4), the differences are not significant, however. The
Jankulovska et al.
3.6
pKBH+ 3.4 3.2 3 2.8 2.6 2.4 0
0.04
0.08
0.12
0.16
Fig. 5. Dependence of pKBH+ values on
pKBH+
0.2
0.24
µ (thermodynamic
values). () H1, () H2, (▲) H3, (▬) H4, (3)
H5.
pKBH+ values calculated from the selected absorbance values around absorption maximum of 300 nm are similar to those calculated from those around 250 nm. The final pKBH+ values at different ionic strengths are calculated as average values. On the other hand, there were no important differences between the pKBH+ values for hydrazones H1, H2 and H3. This means that the substituents -CH3 and -OCH3 had no significant effect on the protonation process. The hydrazones H4 and H5 had lower pKBH+ values compared to those of the hydrazones H1-H3 because of the influence of the -Cl and -OH groups on their molecules. Halogenated derivative (p-Cl) was the highly acidic member which could be accounted for by the strong electron-attracting affinity of this group. The obtained pKBH+ values were in agreement with those obtained for the similar group of compounds [24,37,38]. The values of standard deviations and variances were lower when the calculations were made from the absorbance values of the reconstructed spectra. The differences, however, were statistically insignificant (See Table 4). The statistical data indicated high reliability of these calculations (R was between 0.995 and 0.999).
curves, Aλmax = f(pH), as well as the isosbestic point observed in the reconstructed spectra, indicated the presence of one acid-base equilibrium. The literature data and the semiempirical calculations (AM1 and PM3) suggested that the protonation process occurred in imino (sp2 hybridized) nitrogen in the hydrazone molecules. The calculations of the pKBH+ were madefrom the absorbance values at four wavelengths around the absorption maximum at 300 nm and 250 nm, and at ionic strengths of 0.1, 0.25 and 0.5 M. The final pKBH+ values were obtained as average values. From these values, the thermodynamic pKBH+ values were evaluated at zero ionic strength. The obtained pKBH+ values for hydrazones H1, H2 and H3 were similar and they were lower compared to those of hydrazones H4 and H5. This implies that the hydrazones H1-H3 are weaker acids than are H4 and H5. There was a good agreement between the obtained pKBH+ values of the investigated hydrazones and those of similar groups of compounds reported in the literature.
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A UV Study of the Behavior of Some Benzaldehyde Hydrazones in Acid Medium
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