Spectrophotometric determination of the dissociation constants of crown ethers with grafted acridone unit in methanol based on Benesi-Hildebrand evaluation

Mihály Kádár, András Biró, Klára Tóth, Borbála Vermes, Péter Huszthy

Research output: Contribution to journalArticle

33 Citations (Scopus)

Abstract

As potential chromo- and fluorophores for optical sensors a number of acridone grafted crown ethers containing NO2 group(s) and/or Br or Cl atom(s) in the aromatic rings (compounds 1-8) were studied by spectrophotometric method. In the first step of this work the acid-base and complexing properties of these compounds as well as those of the acridone (9), thioacridone (10) and 4,5-dinitroacridone (11) were investigated. Compounds 1-11 proved to be very week acids and therefore the conventional spectrophotometric method based on the measurement of the ratio of the protonated/deprotonated forms of the compounds was not applicable for the determination of the dissociation constants (pKa values). Thus, a new spectrophotometric approach was elaborated for the pKa determination of these compounds, which is based on spectrophotometric titration in methanol with strong base and the titration results were evaluated by the Benesi-Hildebrand method. (In the studies with TEAOH, TMAOH and LiOMe, the complex formation between the ionophores and the cations of the bases could be excluded.) As it was expected, the experimentally determined pKa values depended on the nature of the substituents of the acridone moiety and pKa values ranged between 12.6 and 14.9. The lower pKa value of thioacridone compared to the acridone can be explained by the larger size of the sulfur atom. The outstandingly larger pKa value for 4,5-dinitroacridone can be attributed to the formation of intramolecular hydrogen-bonds between the ortho-position nitro group and the NH proton. As a trend, the strong electron withdrawing nitro substituents decreased considerably the pKa values of compounds 3, 7 and 8 compared to that of the halogen (Cl and Br) atoms (for example compounds 5 and 6) of weaker withdrawing effect.

Original languageEnglish
Pages (from-to)1032-1038
Number of pages7
JournalSpectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
Volume62
Issue number4-5
DOIs
Publication statusPublished - Dec 1 2005

Keywords

  • Acridones
  • Benesi-Hildebrand evaluation
  • Chromoionophores
  • Fluoroionophores
  • Proton-ionizable crown ethers
  • Spectrophotometric titration
  • pK determination

ASJC Scopus subject areas

  • Analytical Chemistry
  • Atomic and Molecular Physics, and Optics
  • Instrumentation
  • Spectroscopy

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