Acid dissociation constants, pKa, of 2-pyrrolaldehyde phenylsulphonyl hydrazone, 1, and 2-thiophenealdehyde phenylsulphonyl hydrazone, 2, have been determined spectrophotometrically in ethanol-water media of various compositions over the temperature range 25 ℃ - 45 ℃. The obtained results were used in the calculation of the enthalpy, △H °, and the entropy, △S °, of the ionization processes. The slight variations observed in the pKa values of the thiophene compound compared to the pyrrol analogue revealed that neither of the two hetero atoms in the pyrrol or thiophene rings, of the two compounds, is involved in a hydrogen bond chelation. This conclusion was also confirmed through measurements of the dipole moment, IR and NMR spectra.
Aldehyde phenylsulphonyl hydrazones of the type, Ar-CH=N-NH-SO2Ph, were firstly prepared by Grammaticakis [
2-pyrrolaldehye phenylsulphonyl hydrazone, 1, and 2-thiophenealdehyde phenylsulphonyl hydrazone, 2, were prepared by the general procedure described elsewhere [
Measurements of PH were made by Hanna PH meter, model H1 8417 equipped with combined glass electrode. The PH meter readings were converted to hydrogen ion concentration, [H+], by means of the Van Uitert relation [
Electronic absorption spectra were measured on Perkin-Elmer Lambda 3 spectro- photometer.
For the spectrophotometric determination of the acid dissociation constant, PKa, an aliquot of the stock hydrazone solution was diluted with perchloric acid solution, absolute ethanol and water to give a definite overall ionic strength of 0.1 M and final hydrazone strength of 10 - 3 M and contain the required ethanol content of (20%, 40%, 60% and 80% v/v). The absorption spectra of each of these solutions were examined in terms of different PH values and at temperatures of (25˚C, 30˚C, 35˚C, 40˚C and 45˚C). Temperature was controlled by using a double jacket cell connected to an ultra thermostat adjusted to the required temperature (±0.1˚C). This thermostat is connected, in turn, to the spectrophotometric cell in order to verify the same temperature for both measurements.
Static dielectric constant of each solution was measured on a Dipolemeter DMO1 manufactured by WTW, Germany. Dielectric constantɛ, density, d, and refractive index, nD, for hydrazone solutions were measured at 30˚C as described earlier [
where
IR spectra were determined for samples in KBr discs on a Pye Unicam SP3-300.
1H NMR spectra were measured in CDCl3 on a Varian Gimini 200 MHz NMR- Spectrophotometer.
The acid dissociation constants pKa’s of the analogues 1 and 2 were determined spectrophotometrically in 20%, 40%, 60% and 80% (v/v) ethanol-water mixture over the temperature range 25˚C to 45˚C. The pKa values were calculated from the absorbance, A, and PH data, using the Equation (2).
Amax and Amin where are the absorbance values in basic and acidic media (PH = 10 and PH = 1), respectively. The results obtained are given in
The data obtained show that at any given temperature, an increase of ethanol content in the solvent mixture resulted in an increase of the pKa values, as a consequence of decreasing the dielectric constant of the solvent mixture [
Compound 1 | ||||
---|---|---|---|---|
Temp. ˚C | Volume% of ethanol | |||
20 | 40 | 60 | 80 | |
25 | 7.08 | 7.72 | 8.38 | 9.01 |
30 | 7.02 | 7.65 | 8.17 | 8.87 |
35 | 6.95 | 7.50 | 8.11 | 8.72 |
40 | 6.87 | 7.42 | 7.02 | 8.57 |
45 | 6.78 | 7.35 | 7.95 | 8.52 |
Compound 2 | ||||
Temp. ˚C | Volume% of ethanol | |||
20 | 40 | 60 | 80 | |
25 | 7.11 | 7.77 | 8.45 | 9.16 |
30 | 7.03 | 7.68 | 8.34 | 9.01 |
35 | 6.92 | 7.56 | 8.20 | 8.68 |
40 | 6.82 | 7.45 | 8.07 | 8.75 |
45 | 6.71 | 7.32 | 7.96 | 8.61 |
Temp. ˚C | Volume% of ethanol | |||
---|---|---|---|---|
20 | 40 | 60 | 80 | |
25 | 68.93 | 57.72 | 46.45 | 35.40 |
30 | 65.92 | 54.38 | 43.02 | 32.95 |
35 | 63.77 | 52.39 | 41.57 | 30.73 |
40 | 60.51 | 49.56 | 40.05 | 29.94 |
45 | 57.42 | 46.38 | 38.67 | 28.34 |
From the data obtained, both the enthalpy, ∆Ho, and the entropy, ∆So, were calculated using the van’t Hoff Equations (3) and (4).
The results are summarized in
Harned et al. [
where the temperature T, is expressed in degree centigrade, pKm is the minimum value of pKa, at temperature θ, and C is an arbitrary constant of the order, 5 × 10 − 2 degree.
When Equation (5) was applied to our experimental data, the values of pKm and θ were calculated and situated in
Comparison of the pKa values for the compounds 1 and 2 revealed that, replacement of the nitrogen hetero atom in compound 1 by the sulpher atom in compound 2 resulted in a slight increase in the pKa values whatever the working temperature and the solvent composition.
Compund 1 | |||||
---|---|---|---|---|---|
Thermodynamic parameter | Volume% of ethanol | ||||
20 | 40 | 60 | 80 | ||
∆Ho (kJ/mole) (±1) | 22.51 | 31.54 | 39.92 | 47.89 | |
∆So (kJ/mole) (±3) | −53.34 | −36.8 | −20.33 | −6.32 | |
Compund 2 | |||||
Thermodynamic parameter | Volume% of ethanol | ||||
20 | 40 | 60 | 80 | ||
∆Ho (kJ/mole) (±1) | 31.74 | 39.46 | 44.65 | 65.45 | |
∆So (kJ/mole) (±3) | −24.76 | −0.32 | −7.05 | −5.43 |
Compund 1 | ||||
---|---|---|---|---|
Volume% of ethanol | ||||
20 | 40 | 60 | 80 | |
θ | 164.08 | 212.45 | 260.74 | 323.78 |
pKm | 6.04 | 5.86 | 5.43 | 4.52 |
Compund 2 | ||||
Volume% of ethanol | ||||
20 | 40 | 60 | 80 | |
θ | 212.74 | 261.32 | 291.82 | 323.78 |
pKm | 5.23 | 4.90 | 4.83 | 4.52 |
In general, compounds 1 and 2 can be represented by either of the following two structures, (i) and (ii).
According to the structure (i), it might be expected that the acidity is most likely dependent upon the strength of the intramolecular H-bond, X… H-N, in such a manner that, the stronger the bond the less will be the acidity of the compound. The electronegativity of the hetero atom, X, will affect the strength of the H-bond and hence, the acidity of the H-N group through an inductive effect; that is, the higher the electronegativity of the hetero atom, X, the higher is the acidity of the compound. Referring to the structure (i), the acidity is governed by the predominance of one of the above two effects. On the other hand, the structure (ii) suggests that the acidity is mainly governed by the electronegativity of the hetero atom, X. This means that the greater the electronegativity of, X, the greater will be the inductive effect and hence, the higher the acidity.
Based on the above agreements, structure (ii) seems to be more appropriate for compounds, 1 and 2, than structure (i), since the observed pKa values for the compound, 1, are less than those of compound 2. This conclusion was confirmed by dipole moment for the two compounds in benzene at 30˚C and comparing the results obtained with those calculated, using the method of bond moments [
Compd. | DP2 | P2∞ (Cm3) | P2∞ | µ | ||
---|---|---|---|---|---|---|
Cm3 | Graph. | Hedstrand | Palit-Banerjee | |||
1 | 68.9 | 563.76 | 563.81 | 555.47 | 561.01 | 4.92 |
α = 10.717 | A = 0.342 | |||||
β = 0.304 | ɣ = 1.862 | |||||
2 | 70.15 | 536.43 | 536.49 | 527.18 | 533.37 | 4.77 |
α = 9.504 | A = 0.342 | |||||
β = 0.191 | ɣ = 1.640 |
Compd. | Conformations | |||||||
---|---|---|---|---|---|---|---|---|
I | II | III | IV | V | VI | VII | Viii | |
1 | 5.01 | 3.50 | 2.87 | 2.96 | 3.50 | 4.93 | 3.03 | 2.66 |
2 | 4.94 | 3.49 | 3.00 | 2.88 | 3.39 | 4.85 | 2.38 | 1.98 |
compounds, 1 and 2, respectively are close to the calculated value based on conformation VI.
The non-chelated structure (ii) was further confirmed by the observation that no metal complex could be detected spectrophotometrically with both compounds, 1 and 2, where, all trials made to prepare Fe III, Co II, Ni II and Cu II complexes were failed.
The IR and 1H NMR spectra of the two compounds were compared with that of ben- zaldehyde benzenesulphonylhydrazone, 3. The stretching frequencies, νNH, and the proton chemical shift, δNH, of the N-H proton of compounds, 1 - 3, are given in
Compound | δNH (ppm) | νNH (Cm−1 ) |
---|---|---|
1 | 8.25 | 3190 |
2 | 8.35 | 3160 |
3 | 8.44 | 3180 |
The close similarity of the stretching frequencies of NH bond and the chemical shift values of the three compounds (1 - 3), substantiate further our previous conclusion that compounds 1 and 2 exist in a non-chelated form.
Based on determined acid dissociation constant values, pKa, for the two compounds 1 and 2 and the electronegativity of the hetero atom, X, structure (ii), which is the non chelated form, seems to be more appropriate for these compounds. This was also confirmed through comparing the experimentally determined dipole moment values of the two compounds with the calculated values obtained of different conformation structures. Also, these compounds failed to form complexes with Fe III, Co II, Ni II and Cu II, metalions.
Nessim, R.I., Albalaa, I.E. and Assi, M.A. (2016) Effect of Temperature and Solvent Composition on the Acid Dissociation Constants of 2-Pyrrol and 2-Thiophene Aldehyde Phenyl Sulphonyl Hy- drazones. Natural Science, 8, 442-449. http://dx.doi.org/10.4236/ns.2016.810046