Paruthi, H. K.

Prasad, C. S. G.

Taneja, A. D.

Tyagi, J. S.

English

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l~DIAN J. (HEM., VOL. 14A, FEBRUARY 1976Ionization Constants ofSome Hydroxycoumarins in Dioxane-Water &Ethanol- Water MediaH. K. PARUTHI & J. s. TVAGIDepartment of Chemistry, Meerut College, Meeruta"clA. D. TANEJA & C. S. G. PRASADDepartment of Chemistry, BITS, PilaniReceived 14 Nouember 1974; accepted 29 March 1975The ionization constants (pKa) of8-amino-7 -hydroxy-4-methylcoumarin (AHMCH) and 7-hydroxy-4-me-thyl-coumarin-6-carboxylic acid (HMCCH,) have beendetermined potentiometrically in 50% dioxane (vjv)and 50% ethanol (vjv) at various ionic strengths.The ionization constant of 7-hydroxy-4-methyl-6-nitrocoumarin (NHMCH) has been determined in50% dioxane only due to its insolubility in ethanol.Thermodynamic dissociation constants (PK~) havebeen obtained by extrapolation to zero ionic strength.The values of AG, AH and AS have also been calcu-lated for the ionization process.THE acid-base equilibria of hydroxycoumarinderivatives have not been studied much.Except for the work of Foffani et al.", Mattoov+,Adachi and Ejima" and Manku", no other work ofany consequence is available. It was therefore ofinterest to determine the ionization constants ofsome hydroxy coumarin derivatives, viz., 8-amino-7-hydroxy-4-methylcoumarin (AHMCH), 7-hydroxy-4-methylcoumarin-6-carboxylic acid (HMCCH2),7-hydroxy-4-methyl-6-nitrocoumarin (NHMCH).AHMCH, HMCCHz and NHMCH were prepared asdescribed elsewhere". The PKa values were deter-mened/-" in 50% dioxane (v(v) and 50% ethanol(v(v) by the direct pH titration technique asdescribed by Albert and Sarjeant".Most of the chemicals used were of analar (BDH)quality. NaCl04.2HzO (Reidel) was used for adjust-ing the ionic strength. Dioxane (Baker, USA) andethanol were purified as described in literature",Carbonate-free potassium hydroxide (BDH) wasprepared according to Albert and Sarjeant".The pH measurements were made on a BeckmanPH meter (model H2) using a glass-calomel electrodeassembly.For finding the thermodynamic ionization con-stants of these compounds, studies were carried outat 25° and at different ionic strengths viz. 0·05,0·10, 0·15 and 0·20 and the results were extra-polated to zero ionic strength to obtain PK!. Forthe determination of thermodynamic parameters,the studies were carried out at 15°, 25°, 35° and 45°and fixed ionic strength of 0·10.Free energies of these reactions have been calcu-lated using the equation [).G =-RT In K =2·303 TpK. Plots of log K vs l(T were linear andAH and AS values were calculated by using theusual expressions.The pKa values at different temperatures andthe AG, AH and AS values are recorded in Table 1.126An examination of the data in Table 1 showsthat HMCCHz ionizes in two steps, while AHMCHand NHMCH have only one ionization step. Thetwo dissociation steps of HMCCH2 are obviouslydue to the carboxylic acid group and the phenolicgroup. pKl of HMCCH2 is due to the ionization ofthe - COOH group and pKz is due to the phenolic-OH. In the other two compounds the pKl aredue to the phenolic -OH.A comparison of pK's in 50% dioxane, due to thephenolic hydroxyl shows that NHMCH is thestrongest acid (PK~ = 6·12) and HMCCH- is theweakest (PK~ = 10·42). AHMCH has an inter-mediate value of pK~ (8·59).NHl\ICH has a strong deactivating nitro groupin position-S, which facilitates the release of theproton from -OH. Reverse is true for AHMCH,the -·NH2 at position-8 being an activatinggroup, the O-H bond gets strengthened, and therelease of the proton becomes more difficult. ThepKI of HMCCH2 is due to a combination of factors.After the ionization of the carboxylic acid group,the molecule is negatively charged, and a secondionization destabilizes the molecule, due to therepulsive interaction between two adjacent negativecharges. A more important factor is presumablythe stabilization of the singly ionized acid by intra-molecular hydrogen bonding.The pKa values of HMCCH2 and AHMCH werealso determined in 50% ethanol. NHMCH couldnot be studied in this medium as it was practicallyinsoluble. The trend in 50% ethanol is the sameas in 50% dioxane (d. Table 1). An interestingpoint is that HMCCH2 and AHMCH are slightlystronger acids in ethanol than in dioxane. Themajor reason seems to be the greater solvation ofthe acid anions in a polar solvent like 50% ethanol,than in a less polar solvent like 50% dioxane. Itmay also be noted that the dielectric constant of50'10 ethanol is greater than that of 50% dioxaneand this equally explains the lowering of pKa from50(% dioxane to 50% ethanol, because charge sepa-ration is favoured by increased dielectric constantof the medium. It has been observed by Dewar-?in the case of keto-enol tautomerism that the morepolar tautomer is always more stable in a morepolar solvent, due to its greater solvation in thatsolvent. This mechanism seems to be applicablein the present case also, and this greater solvationis energetically more favourable to the acid anionthan to the undissociated acid. In the case of pKzof HMCCH2, apart from this solvation, there is apossibility of greater solvent -COO- hydrogen bond-ing, which competes with the intramolecular H-bond-ing, significantly reducing the singly ionized acidsstability in 50% ethanol in comparison to itsstability in 50% dioxane. This destabilizationprobably makes the second ionization easier in 50%ethanol than in 50% dioxane.The variation in pKa with temperature (in dioxane)of all these compounds shows that an increase intemperature decreases the value of pKa (d. Table 1).This is normally observed+ for most acids in thetemperature range studied. The AG and AH valuesare positive and the AS values are negative.KOTESTABLE 1 - IONIZATION CONSTANTS AND THERMODYNAMIC FUNCTIONS OF SOME HypROXYCOUMARIN DERIVATiVES(Ionic strength = 0'1)Compound Temp. Ionization PK AG AH* ASOK constant kcaljrno " aljrnol e.u.50% DIOXANEHMCCH. 288'16 4'60 6'07,288·16 pI<.1 4·47 6·10 I 5'040±0'01O -3'548±0'015308·16 4·35 6·13 }318·16 4·23 6·16HMCCH, 288·16 10·35 13-651298·16 PI(. 10·15 13·85 8·400±0·010 -18·250±0·012308·16 9'95 14·03318·16 9·75 14'19 JAHMCH 288·16 8'49 tHO}298·16 PI<.l 8·30 11·32 8-400±0·015 -9·766±0·1O308·16 8·10 11'42318·16 7·90 IHONHMCH 288·16 5·97 7-871298·16 PK1 5'82 7·94 6·300±0·015 -5-480±0'02308·16 5·67 8·00 I318·16 5·52 8'03J50% ETHANOLHMCCH2 288·16 4'25 5-61")298·16 pKl 4·12 5-62 } 5·050±0·20 -1'947 ±0'015308·16 4·00 5·64318·16 3·88 5·65HMCCH2 288·16 10·10 13.321298·16 PJ(2 9·95 13'57 ; 6·088±0·10 - 25·095 ±O'015308·16 9'81 13'83 r318'16 9·66 14·06 JAH1ICH 288·16 8·15 10'75")298·16 1'1(1 8·02 10·91 t 6'505±0'01S -14·760±0·020308·16 7'85 11·07318·16 7'70 11·20NHMCH is not soluble in ethanol.*Average values.AH was assumed constant over the range oftemperatures studied and was evaluated from theslope of the plot of pKa vs liT. From this averagevalue of AH, AS was calculated using the standardrelationshipAG =AH-TASThe authors are thankful to the authorities of theMeerut College, Meerut, for providing necessaryfacilities.References1. FOFFANI, A., FORNASARI, E. & FOFFAN, M. R., Ricarc asct., 27 Suppl. A Polarographic, 3 (1957), 113.2. MATTOO, B. N., Trans Faraday s»., 53 (1957),760.3. MATTOO B. N., Trans. Faraday ,'oe., 52 (1956),1184.4. MATTOO, B. N., Trans. Faraday soc., 54 (1958), 19.5. ADACHI, T. & EJIMA, A., Eisei Sbikensho Hokoku, 81(1963), 20.6. MANKU, G. S., Aust. J. Chem., 24 (1971), 925.7. RASTOGI, D. K., Infrared spectral studies of metalchelates of some hydroxy coumarins, Ph.D. thesis, AgraUniversity, Agra, 1971.8. ALBERT, D. & SARJltANT, E. P., Ionization constant ofacid! and bases (Methuen, London), 1962.9. STROUT, C. R. N., 'VILSON, H. N. & PARRY-JONES, R. T.,Chemical analysis, Vol. II (Clarendon Press, Oxford),1962, 218.10. DEWAR, M. J. S. Electronic theory of organic chemistry(Clarendon Press, Oxford), 1949, 92.11. HARNED, H. S. & KAZANJIAN, L., J. Am. chem. s».,58 (1936), 1912.Ion-Solvent Interaction in PropyleneCarbonate from Conductivity DataSUDHA SINGH, J. S. JHA & RAM GOPALDepartment of Chemistry, Lucknow University, Luckr..owReceived 26 February 1975; accepted 10 April 1975Ion-solvent interaction of some tetraalkylammoniumand common ions in propylene carbonate has beenstudied ustng electrolytic conductivity data and thederived Walden product. The R.N+ and halide ions donot appear to interact appreciably with the solvent butLl+, Na+, K+, Rb" and Cs+ ions are solvated.THE ion-solvent interaction of tetraalkylammo-nium and Li+, Na", K+, Rb" and Cs" ions has127

Ionization Constants of Some Hydroxycoumarins in Dioxane-Water & Ethanol- Water Media

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Ionization Constants of Some Hydroxycoumarins in Dioxane-Water & Ethanol- Water Media

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