2009) 3( 22مجلة ابن الهیثم للعلوم الصرفة والتطبیقیة المجلد و بعض بیانات الدینمة الحراریة لبعض قواعد شیف كك حامضيبت تفاحساب ثو أمینو بیرمیدین-2 ثنائى مثیل-6، 4 المشتقة من هاشم جالل عزیز جامعة صالح الدین، كلیة التربیة، قسم الكیمیاء الخالصة بیرمیدین أمینو -ثنائي مثیل-6، 4لعدد من قواعد شیف المشتقة من حساب ثوابت تفكك حامضيفي هذا البحث تم ( X=H,OH;Y=H,OH,OCH3; Z=H,OH حیـث ان 0.003مـاء فـى / حجـم دایوكسـان/حجـم %)50(فـى محـیط KCl موالرى (وكذلك حسبت الطاقـات الثرموداینمیكیـة .وفى ثالث درجات حراریة مختلفة مجهادیا ∆S 0 ، ∆H 0 ، ∆G 0 ( pKa وتم التوصل لعالقة خطیة بین سم ( والعدد الموجى الهتزاز المطي لمجموعة الهیدروكسیل في طیف االشعة ) 1- تحت الحمراء IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Determination of pKa and Thermodynamic Data of Some Schiff Bases Derived From 4,6-Dimethyl 2-Amino Pyrimidine H. J. Aziz Department of Chemistry, College of Education, University of Salahaddin Abstract Acid dissociation constants of some Schiff bases derived from 4, 6-dimethyl 2-amino pyrimidine of the type (1) in 50% V/V dioxane-water mixture in 0.003M KCl, at three different temperatures were determined potentiometrically. The thermodynamic energies were calculated and a good linear correlation was obtained between pKa and IR OH. Stretching frequencies. Introduction Schiff bases have recently received much attention because of their power chelation with traces of metal ions. The formation of hydrogen bonding adducts between 1-phenyl 2- aminopyrimidine and vanillin was done in ethy l alcohol at 1993(1). The pKa values (macroscopic acidity constant K1 and K2) of 1, 2-cyclopropanediammoniumdibromide (0.5M) were determined by potentiometric titration with KOH in carbon dioxide free water, special measures were taken to avoid errors due to cis-trans isomerization or decomposition during the titration([2). The pKa values of pigments were determined by a three-parameter function derived from the Handerson-Hassellbach equation. ∆Amax ∆A = 1+ 10 n(pKa - pH) Where the parameters are ∆A and ∆Amax, the absorbance difference and the maximum absorbance difference, correspondingly, between the protonated and deprotonated states, (n) is the number of protons participating in the above transition, and pKa is the midpoint of transition(3). Under the chromatographic conditions used in the determination of mixtures of carboxylic acids and selected inorganic anions by ion exclusion chromatography; the degree of ionization, of the solutes was determined by the acid dissociation constant (pKa) of the solutes(4). Also acid dissociation constant of some 2-methyl-N-(substituted phenyl)-4,5-dihydrofuran-3-carboxamides in 50% V/V dioxane-water mixture 0.003M KCl at three different temperatures were determined potentiometrically(5). The measurements of pK1 and pK2 for two amino acids (proline and valine) at four different temperatures in the range 293.15-323.15K, were done conductimetrically in water and in glycerol-water mixtures with three different mole fractions of glycerol(6). In the present study the dissociation constant of some Schiff bases derived from 4, 6- dimethyl2-amino pyrimidine was determined potentiometrically in 50% V/V dioxane-water (structure 1) at three different temperatures. ∆G ◦, ∆H◦ and ∆S◦ functions were also calculated. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 S tructure 1 Comp. No X Y Z A H OCH3 OH B OH H OH C H H OH D OH H H E H OH OH F H OH H Experimental Schiff bases of the type (1) were synthesized by the following general procedure(7,8). Equimolar proportion of 4, 6-dimethyl 2-amino pyrimidine and the appropriate aldehyde were heated under reflux in absolute ethanol for 1 hour and the solid yield was recrystalised from absolute ethanol. Schiff bases, obtained, were found to give single spots on TLC and chractreized by their CHN analysis. Potassium hydroxide (0.01M), carbonate free titrant was prepared from BDH (CVS). An electronic pH–meter (model Kent) with a combined glass electrode was used to record the hydrogen ion concentration, in which the pH was maintained to±0.0.1unit. The pH-meter was calibrated with buffers of pH 4.00 and 9.00 supplied from BDH. The IR-spectra was recorded on a Pye-Unicam SP-300S infrared spectrometer, as KBr disk by using polystyrene film as a standard. The following stock solutions were prepared and used: 0.02M solution of Schiff bases was prepared in dioxane-water v/v 50%. 0.1M potassium chloride was prepared by using deionized water. The volumes of the various solutions were added in such amounts that the final concentration in the solution becomes 0.01M ligand and 0.003M KCl. The final volume was completed to 50ml of well-stirred, double wall cell, in which nitrogen was bubbled. Potassium hydroxide was added in small increments through microburette. Constant temperature was maintained with the help of HAAKE E3 thermostat. Results and Discussion pKa determination The calculation of the acid dissociation constants depends on the evaluation of the average number of associated with the reagent(9) ñA, using the equation used by Masoud, which was the protons determined at different pH ُ◌ s ñA = Y – V1 N ◦ /V◦ C ◦ R N N H3C CH3 N CH X Y Z IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Where V1 denotes the volume of alkali required to reach a given pH on the titration curve, V◦ is the initial volume of the ligand, N ◦ is the alkali concentration, C ◦ R is the total concentration of the reagent and Y is the number of displaceable hydrogen atoms in the reagent and the pKa values was obtained by point-wise calculation method(9,10). Table (1) summaries the pKa values obtained at three different temperatures. The data indicates that the compounds under investigation are different in acidic strength, due to the number and position of substitutents (especially compounds B & E) have lower values of pKa, which is attributed to the longer conjugation(11) (structure 2), which leads to stabilize the anion compounds and because all atoms (except H) have an octet state. Also shows that as temperature of the medium increases, the pKa values decreases. S tructure (2) Compound (E) IR spectra The infrared data shown in Table (2), indicates that the band at 3600cm-1 due to free OH is absent and replaced by a band or bands at the range (3140-3320) cm -1 . This may be attributed to the hydrogen bonding (inter and intra). Fig.(1) shows the correlation between the pKa with wave number (cm -1) of OH stretching in IR spectra, and the best indication for the formation of Schiff bases is due to the formation of C=N bonding Table (2) with stretching vibrations (1630-1680)cm-1(12,13). Thermodynamic parameters The change in free energy is related to the other parameters by the following equations: ∆G ◦ = ∆H ◦ – T∆S ◦ ∆G ◦ = -RT ln Ka From the above two equations -RT lnKa = ∆H◦ – T ∆S◦ -2.303 RT log Ka = ∆H ◦ - T∆S ◦ -logKa= ∆H ◦ /2.303 RT - ∆ S◦ / 2.303R N N H3C CH3 N CH OH O N N H3C CH3 N CH OH O IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 A plot of pKa vs.1/T does not deviate much from linearity Fig.(2). The slope of the line gives ∆H ◦ value and the intercept gives – ∆S ◦ value. The thermodynamic parameters, calculated from these relations are reported in Table (3). The positive values of ∆G ◦ indicate that the dissociation process is non spontaneous which support that no weak acid of its own dissociates into ions. LH L - + H+ The higher degree of solvation of the negative ions than that of the undissociated molecule, causes the negative value of ∆S◦ ,and more salvation means a system is more orderly fashion, which implies loss of entropy. References 1.Ahmed, Farage, I.S.; Gad, A.M ; El-shabiny, A.M . and Raybakov, V.B. (1993), Formation of hydrogen – bonding adduct between 1-phenyl-2-amino pyrmidine and vaniline, Cryst. Res. Technol., 28(8): 117-1133. 2.Wolfgang von der saal and Helmut Quast, (1996), Cyclopropanediamines. 6-Autocatalytic conversion of 1-methyl-r-l, t-2-cyclopropanediamine into 4-aminobutan-2-one in aqueous buffer solutions of pH 6.5-10, Azomethine ylide as intermediates in the catalysis by alkylketones, J. Org. Chem., 61: 4920-4928. 3.Rousso, I.; Friedman, N.; Sheves, M. and Ottolenghi, M., (1995), pKa of the protonated Schiff base and Aspartic 85 in the bacteriorhodopsin binding site is controlled by a specific geometry between the two residues, Biochemistry, 34(37): 12060-12065. 4.Magyani, O.F., (1997), Determination of mixtures of carboxylic acids and selected inorganic anions by ion-exclusion chromatography, Philippine journal of Science, 126(1): 19-50. 5.Jameel, R.K., (1994), Determination of pKa, thermodynamic data for some 2-methyl-N- (substituted phenyl)-4,5-dihydrofuran-carboxamides, Mutah Journal for research and studies, 9(2): 105-115. 6.Radha, N.K., (2000), Dissociation constants of praline and valine in solvent mixtures, M.Sc. Thesis, Chem. Dept. College of Education, Salahaddin University. 7.Campillos, E. and Marcos, M., Omenat, A. and Serrano, J.L., (1996), Synthesis and properties of copper(11) and oxovandium (IV) complexes derived from polar Schiff bases, J. Mater. Chem., 6(3): 349-355. 8.Yousif, Y.Z. and Ridha, S.H., (2000), Synthesis and mesogenic behavior of some novel copper (Cull) complexes derived from Schiff bases with polar substitutents, Iraqi. J. Science, 41A(1): 30-54. 9.Masoud, M.S. and El-Zaway, F., (1980), Equilibrium studies of β – diketo aryl azo compounds with cobalt (11), Nickel (11) and Copper (11), Talanta, 27: 766-768. 10.Masoud, M.S. and Adallah, A.A., (1982), J. Chem. & Eng. Data (USA), 27(1): 60. 11.Sykes, P., (1981), A guide book to mechanism in organic chemistry, 5 th Ed., Longmans, New York, p.22. 12.Pavia, D. L.; Lampman, G.M . and Kriz G.S., (1979), Introduction to spectroscopy-guide for students of organic chemistry, W.B. Saunders Company, Toronto, p.26. 13.Nakanishi, K. and Solomon, P.H., (1977), Infrared absorption spectroscopy , second edition, HOLDEN-DAY, INC., San Francisco, p.49. IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Table (1): pKa values for the Schiff bases (A-F) at three different temperatures, ±0.1 ◦ C, in 50% V/V dioxane-water, 0.003M KCl Compound No. pKa ± 0.05 25◦ C pKa ± 0.04 35◦C pKa ± 0.05 45◦ C A 7.02 6.81 6.64 B o.OH(7.9),P-OH(6.7) 7.69 , 6.56 7.46 , 6.43 C 7.05 6.85 6.63 D 6.85 6.70 6.54 E m-OH(7.85),p-OH(6.5) 7.65 , 6.36 7.44 , 6.23 F 7.55 7.37 7.18 Where o=ortho, m=meta, p=para Table (2): IR frequencies in cm-1 for the prepared Schiff bases ◌ِCompound No. OH str. C=N str. A 3180 1635 B 3310 , 3140 1630 C 3190 1635 D 3160 1630 E 3320 , 3110 1645 F 3300 1640 Table (3): Thermodynamic parameter values (∆G ◦, ∆H◦, ∆S◦) for hydroxyl group in the Schiff bases (A-F) in 50% V/V dioxane – water, 0.003M KCl at 25 ◦ C± 0.1 Compound No. ∆G◦ KJ/mole ∆H◦ KJ/mole -∆S J/mole.K◦ A 40.06 34.48 18.72 B 45.08 , 38.23 39.93 , 24.50 17.28 , 46.07 C 40.23 38.11 7.11 D 39.09 28.13 36.78 E 44.79 , 37.09 37.21 , 24.50 25.43 , 42.25 F 43.08 33.55 31.98 IBN AL- HAITHAM J. FO R PURE & APPL. SC I VO L.22 (3) 2009 Fig. (1) Relation between pKa and OH stretching vibration frequency of Schiff bases (A-F). Where Ep,Bp represent the values for para hydroxyl group, while Bo and Em represent the values for ortho and meta hydroxy groups respectivel.y Fig. (2) Plot of pKa at 25 o C against 1/T of Schiff bases (A-F) 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 (1/T )Tem perature p K a B C D E F G H I