PKfl values

Potential 2-hydroxy- and 2-mercapto-l,3,4-thiadiazoles have been examined both by infrared and by ultraviolet spectra in the solid state and in solution by Sheinker et They concluded that these compounds exist in the 2-oxo and 2-thione forms, respectively. To 2,5-dimercapto-l,3,4-thiadiazole the 2-mercapto-5-thione structure (155) was given. The structure of this compound has been the subject of some controversy. Stanovnik and TiSler have added some valuable arguments to the discussion. They measured the pKfl values of 155, its iV-methyl, iV -phenyl, and iV -phenyl-/S-methyl derivatives (156), and of the conjugate acids of these and the S-methyl derivative (pKn ) (Table III). In all compounds 156 with R = H, the infrared spectrum showed an absorption band near 2300 cm characteristic of the SH group. They also had pK i values near —1.5, Avhich in connection with the infrared evidence was taken as characteristic of an SH group in this situation. Since the 2,5-dithiol structure is excluded by ultraviolet spectral evidence, the 2-mercapto-5-thione structure (155) seems rather well established. It has previously been shown bj Thorn to predominate in chloroform solution, whereas he concluded that the dithione form (157) should predominate in ethanol solution. However, the pK i value for 155, — 1.36, makes it rather probable that Thorn used the monoanion of 155 instead of the acid itself for spectral comparison, and in that case his conclusions have a very weak foundation. 

For two nucleophiles with similar pKfl values, for example, an oxime and phenol, Kh is constant, is common, and hence K is determined by K2. In other words, the affinity of the oxime toward a carbonium center is greater than the affinity of a normal nucleophile of comparable pKa. 

The values of pA),) ) and pKfl) for chromic acid (H2Cr04) are 0.74 and 6.49 respectively, (a) Determine values of A), for each dissociation step, (b) Write equations to represent the dissociation steps of chromic acid in aqueous solution. 

Substitution of the 3 carbon of a fttran with a nitrogen produces oxazole (Fig. 4.52). The rritrogen is a very weak base (pKfl = 0.8) because of the high value of the oxygen... 

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