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Report | Regular issue | Vol 20, No. 11, 1983, pp.2211-2224
Published online, 1st January, 1970
DOI: 10.3987/R-1983-11-2211
The Acidities and the Tautomeric Structure of 5-Aryl-2-mercapto-1,3,4-oxadiazoles

Ahmad S. Shawali, Mahmoud S. Rizk, Abdou O. Abdelhamid, Magda A. Abdalla, Cyril Párkányi, and Magdalena E. Wojciechowska

*Department of Chemistry, The University of Texas at El Paso, El Paso, Texaz 79968-0513, U.S.A.


The acidity constants of a series of substituted 5-phenyl-2-mercapto-1,3,4-oxadiazoles were determined by potentiometric and spectrophotometric methods in 80% (vol.) ethanol-water at 25 °C. The data obtained by the two methods are in good agreement. The pKa values correlate with the σ*XC6H4 constants of the substituted phenyl group (ρ = -0.985, r = 0.936), however, a better correlation of the pKa data with the Hammett substituent constants σX (ρ = -0.983, r = 0.959) was obtained. These linear correlations exclude the possibility of the presence of the thiol tautomer 1 in equilibrium with the thioamide tautomer 2 and indicate that the ionization of the compounds takes place in the form of the thioamide tautomer 2. According to the results of the HMO calculations, the thioamide form 2 is more stable than the thiol tautomer 1. A satisfactory correlation between the observed pKa values and the difference between the π-electronic energies (ΔEπ) of the thioamide tautomer and of the common resonance-stabilized anion was obtained, thus supporting the assigned tautomeric structure 2 for the series under study.