رفة والتطبیقیة المجلد 2009) 3( 22مجلة ابن الهیثم للعلوم الص 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 ( p وتم التوصل لعالقة خطیة بین Ka سم ( والعدد الموجى -1 الهتزاز المطي لمجموعة الهیدروكسیل في طیف االشعة ) 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 Departme nt of Chemistry, College of Education, Unive rsity of Salahaddin Abstract Acid dissociation constants of some Schiff b ases derived from 4, 6-d imethy l 2-amino py rimidine of the type (1) in 50% V/V dioxane-water mixture in 0.003M KCl, at three different temp eratures were determined p otentiometrically. The thermody namic ener gies were calculated and a good lin ear correlation was obt ained between pKa and IR OH. Stretching frequencies. Introduction Schiff bases have recently received much att ention because of their p ower chelation with traces of metal ions. The formation of hy drogen bonding adducts between 1-p heny l 2- aminopy rimidine and vanillin was done in ethy l alcohol at 1993(1). T he pKa values (macroscopic acidity constant K1 and K2) of 1, 2-cyclop rop anediammon iumdibromide (0.5M ) were determined by p otentiometric titration with KOH in carbon dioxide free water, sp ecial measures were taken to avoid errors due to cis-trans isomerization or decomp osition during the titration([2). The p Ka values of p igments were determined by a three-p arameter function derived from the Handerson-Hassellbach equ ation. ∆Am ax ∆A = 1+ 10 n(pKa - pH) Where the p arameters are ∆A and ∆Amax, the absorbance difference and the maximum absorbance differ ence, corresp ondingly , between the p rotonated and dep rotonated states, (n) is the number of p rotons p articip ating in the above transition, and p Ka is the midp oint of transition(3). Under the chromatogr aphic conditions used in the d etermination of mixtures of carbo xy lic acids and selected inorganic anions by ion exclusion chromato graphy ; the degree of ionization, of the solutes was determined by the acid d issociation constant (p Ka) of the solutes(4). Also acid dissociation constant of some 2-methy l-N-(substituted phenyl)-4,5-dihy drofuran-3-carboxamid es in 50% V/V d ioxane-water mixture 0.003M KCl at t hree d ifferent t emp eratures were determined p otentiometrically(5). The measurements of p K1 and p K2 for two amino acids (p roline and valine) at four different t emp eratures in the range 293.15-323.15K, were done conductimetrically in water and in glycerol-water mixtures with three different mole fr actions of glycerol(6). In the p resent study the dissociation constant of some Schiff bases derived from 4, 6- dimethy l2-amino py rimidine was determined p otentiometrically in 50% V/V dioxane-water (st ructure 1) at t hree different t emp eratures. ∆G ◦ , ∆H ◦ and ∆S ◦ functions were also c alculated. 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 Experime ntal Schiff bases of the typ e (1) were sy nthesized by the following general p rocedure(7,8). Equimolar p rop ortion of 4, 6-dimethy l 2-amino py rimidine and the approp riate aldehyde were heated under reflu x in absolute ethanol for 1 hour and the solid y ield was recry st alised from absolute ethanol. Schiff bases, obtain ed, were found to give single sp ots on T LC and chractreized by their CHN analysis. Pot assium hy droxide (0.01M ), carbonate free titrant was p repared from BDH (CVS). An electronic p H–meter (model Kent) with a comb ined glass electrode was used to record the hy drogen ion concentration, in which the pH was maintained to±0.0.1unit. The pH-meter was calibrated with buffers of p H 4.00 and 9.00 supp lied from BDH. The IR-sp ectra was recorded on a Py e-Unicam SP-300 S infrared sp ectrometer, as KBr disk by using p olystyrene film as a st andard. The following st ock solutions were p repared and used: 0.02M solution of Schiff bases was p repared in dioxane-water v/v 50%. 0.1M p otassium chloride was p repared 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 comp leted to 50ml of we ll-st irred, double wall cell, in which nitrogen was bubbled. Pot assium hy droxide was added in small increments through microburette. Constant temp erature was maintained with the help of HAAKE E3 thermostat. Results and Discussion pKa determination The calcu lation of the acid d issociation constants depends on the evaluation of the average number of associated with the reagent(9) ñA, using the equation used by M asoud, which was t he p rotons determined at different p H ُ◌ 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 t he volume of alkali requir ed to reach a given p H on t he titration curve, V◦ is the initial volume of the ligand, N ◦ is the alkali concentration, C ◦ R is the tot al concentration of the reagent and Y is the number of disp laceable hy drogen atoms in the reagent and the p Ka values was obt ained by p oint-wise calculation method(9,10). Table (1) summaries the pKa values obtained at three d ifferent temp eratures. The data indicates that the comp ounds under invest igation are d ifferent in acidic st rength, due to the number and p osition of substitutents (esp ecially comp ounds B & E) have lower values of p Ka, which is attributed to the longer conju gation(11) (st ructure 2), which leads t o st abilize the anion comp ounds and because all atoms (except H) have an octet st ate. Also shows that as temp erature of the medium increases, t he pKa values decreases. S tructure (2) Compound (E) IR spectra The infrared data shown in Table (2), indicates that t he band at 3600cm -1 due to free OH is absent and replaced by a band or bands at t he 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 sp ectra, and the best indication for the formation of Schiff bases is due to the formation of C=N bonding Table (2) with st retching vibrations (1630-1680)cm -1 (12,13). Thermodynamic parameters The change in free ener gy is related to the other p arameters 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 p lot of p Ka vs.1/T does not deviate much from linearity Fig.(2). The slope of the line gives ∆H ◦ value and the intercep t gives – ∆ S ◦ value. The thermodynamic p arameters, calculated from these relations are reported in Table (3). The positive values of ∆G ◦ indicate that t he dissociation p rocess is non sp ontaneous which supp ort 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 molecu le, causes the negative valu e of ∆S ◦ ,and more salvation means a sy stem is more orderly fashion, which imp lies loss of entropy . Re ferences 1.Ahmed, Farage, I.S. ; Gad, A.M ; El-shabiny, A.M . and Raybakov, V.B. (1993), Formation of hy drogen – bonding adduct between 1-p henyl-2-amino p y rmidine and vaniline, Cry st . Res. Technol., 28(8): 117-1133. 2.Wolf gang von der sa al and Helmut Quast , (1996), Cy clop rop anediamines. 6-Aut ocataly tic conversion of 1- methy l-r-l, t-2-cyclop rop anediamin e into 4-aminobutan-2-one in aqueous buffer solutions of p H 6.5-10, Azomethine y lide as intermediates in the cataly sis by alky lketones, J. Org. Chem., 61: 4920-4928. 3.Rousso, I.; Friedman, N.; Sheves, M . and Ottolenghi, M ., (1995), p Ka of the protonated Schiff base and Aspartic 85 in the bacteriorhodop sin binding site is controlled by a sp ecific geometry between the two residues, Biochemist ry , 34(37): 12060-12065. 4.M agy ani, O.F., (1997), Determination of mixtures of carboxy lic acids and selected inorganic anions by ion-exclusion chromatography , Philipp ine journal of Science, 126(1): 19-50. 5.Jameel, R.K., (1994), Determination of p Ka, thermody namic data for some 2-methy l-N- (substituted p heny l)-4,5-dihy drofuran-carboxamides, M utah Journal for research and st udies, 9(2): 105-115. 6.Radha, N.K., (2000), Dissociation constants of p raline and v aline in solvent mixtures, M .Sc. Thesis, Chem. Dept. College of Education, Salahaddin University . 7.Camp illos, E. and M arcos, M., Omenat, A. and Serrano, J.L., (1996), Sy nthesis and prop erties of copper(11) and oxovandiu m (IV) complexes derived from p olar Schiff bases, J. M ater. Chem., 6(3): 349-355. 8.Yousif, Y.Z. and Ridh a, S.H., (2000), Sy nthesis and meso genic behavior of some novel cop p er (Cull) comp lexes derived fro m Schiff bases with p olar substitutents, Iraqi. J. Science, 41A(1): 30-54. 9.M asoud, M .S. and El-Zaway , F., (1980), Equilibrium studies of β – diketo aryl azo compounds with cobalt (11), Nickel (11) and Copp er (11), Talanta, 27: 766-768. 10.M asoud, M.S. and Adallah, A.A., (1982), J. Chem. & Eng. Data (USA), 27(1): 60. 11.Sy kes, 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 sp ectroscopy -guide for st udents of organic chemistry , W.B. Saunders Comp any, Toronto, p .26. 13.Nakanishi, K. and So lomon, P.H., (1977), Infrared absorp tion sp ectroscopy , 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 value s for the S chi ff bases (A-F) at three different te mperatures, ±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=p ara Table (2): IR frequencies in cm -1 for the prepared S chiff 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): Thermodynami c paramete r value s (∆G ◦ , ∆H ◦ , ∆S ◦ ) for hydroxyl group in the S chiff 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) Rel ation between pKa and OH stretching vibration frequency of S chiff base s (A-F). Whe re Ep,Bp represe nt the values for para hydroxyl group, whi le Bo and Em represe nt the values for ortho and meta hydroxy groups respectivel.y Fig. (2) Plot of pKa at 25 o C agai nst 1/T of S chiff 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 pe ra ture p K a B C D E F G H I