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UV Spectral Properties of Benzophenone. Influence of Solvents and Substituents
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Determination of the pKa of Benzophenones in Ethanol-Water

G. T. Castro
S. E. Blanco
1,* and
O. S. Giordano
Química-Física, Facultad de Química, Bioquímica y Farmacia. Universidad Nacional de San Luis. Chacabuco y Pedernera, (5700) San Luis, Argentina
Química Orgánica. Facultad de Química, Bioquímica y Farmacia. Universidad Nacional de San Luis. Chacabuco y Pedernera, (5700) San Luis, Argentina
Author to whom correspondence should be addressed.
Molecules 2000, 5(3), 426-427;
Published: 22 March 2000


The pKa of monohydroxylated benzophenones was determined by UV spectros-copy. The values obtained are coherent with the resonant forms and hydrogen bond in-tramolecular of the analyzed compounds.


The pKa of a drug is one of the most important parameters to explain its physicochemical behavior and acid-base properties [1], to program assays of pharmaceutical preformulation [2], etc. In this work, the pKa values of monohydroxylated benzophenones (BP) were determined by UV-visible spectros-copy. These substances exhibit interesting adsorptive properties, they act as ligands in the complexa- tion of metallic ions and, also, they have biochemical applications as antimicrobial agents and in in-dustry as commercial sun blocks [3].


Molecules 05 00426 i001
The figure shows the basic structure of the analyzed aromatic ketones: 1: 4(OH)-BP (X2=H, X4=OH); 2: 2(OH)-BP (X2=OH, X4=H); 3: 2(OH),4(CH3O)-BP (X2=OH, X4=CH3O). For pKa deter-mination, a UV-vis spectroscopic procedure based on the Henderson-Hasselbalch [4] was used. The buffer solutions used were: a) HCl-KCl, pH 1.5; b) NaOH-KCl, pH 12.5; c) KH2PO4-Na2HPO4 0.01M for a 7.2-8.0 pH interval; d) Na2CO3-NaHCO3 0.01M, pH 9.2-10.0. All buffer solutions were prepared with a 20% w/w ethanol-water mixture, keeping the ionic strength constant (0.05) with KCl.

Results and Discussion

The maximum absorption wavelengths record for the acid (λa) and ionized (λb) forms of the BP were: λa (1) 294.5nm, λb (1) 348nm; λa (2) 334nm, λb (2) 382nm; λa (3) 320nm, λb (3) 372nm. The pKa determined were the following: pKa (1)=7.83; pKa (2)=9.54 and pKa (3)=9.60. The carbonyl group of the benzophenones interacts with the adjacent aromatic rings through the σ and π bonds, fa-voring the π-electronic delocalization of the molecules. The participation of the C=O group in the conjugate molecular system is reflected in its bonding characteristics and in the influence it exerts on the acid-base properties of the hydroxyl groups. The increase of the pKa values in the order pKa (1) < pKa (2) < pKa (3) is coherent with the resonant forms and intramolecular hydrogen bonds exhibited by BPs 2 and 3. The pKa of 1 is markedly lower than that of 4(OH)-chalcone, pKa=8.17 [5]. This indi-cates that the C=O α,β-unsaturated group of the chalcone, increase the π-electronic delocalization of the molecule, decreasing the acidity of the H atom the OH group at position 4.


This work was supported by Secretaría de Ciencia y Técnica de la Universidad Nacional de San Luis and by CONICET.

References and Notes

  1. Isaacs, N.I. Physical Organic Chemistry, 2nd ed.; Addison Wesley Longman: England, 1996; pp. 235–286. [Google Scholar]
  2. Wells, J.I. Pharmaceutical Preformulation, 1st ed.; Ellis Horwood: NY, 1988; pp. 25–28, 70–80, 106–108. [Google Scholar]
  3. Collins, P.; Ferguson, J. British J. Dermat. 1994, 131, 124–129.
  4. Albert, A.; Serjeant, E. P. The Determination of Ionization Constants; Chapman and Hall: USA, 1984; pp. 70–101. [Google Scholar]
  5. Gasull, E.I.; Blanco, S.E.; Ferretti, F.H. An. Asoc. Quím. Argent. 1999, 87, 73–82.

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MDPI and ACS Style

Castro, G.T.; Blanco, S.E.; Giordano, O.S. Determination of the pKa of Benzophenones in Ethanol-Water. Molecules 2000, 5, 426-427.

AMA Style

Castro GT, Blanco SE, Giordano OS. Determination of the pKa of Benzophenones in Ethanol-Water. Molecules. 2000; 5(3):426-427.

Chicago/Turabian Style

Castro, G. T., S. E. Blanco, and O. S. Giordano. 2000. "Determination of the pKa of Benzophenones in Ethanol-Water" Molecules 5, no. 3: 426-427.

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