Redox and acidity properties of 2,2′- and 4,4′-biphenol and the corresponding phenoxyl radicals

The redox and acidity properties of 2,2'- and 4,4'-biphenol and the corresponding phenoxyl radicals have been determined using UV-vis spectrophotometry, pulse radiolysis, and cyclic voltammetry. The pK(a)'s for 4,4'-HO-Ph-Ph-OH, 4,4'-O--Ph-Ph-OH, 4,4'-O-circle-Ph-Ph-OH, 2,2'-HO-Ph-Ph-OH, 2,2'-O--Ph-Ph-OH, and 2,2'-O-circle-Ph-Ph-OH were determined to be ca. 9.7, ca. 9.7, 6.3, 7.6, 13.7, and 10, respectively. At the same time, the one-electron reduction potentials for 4,4'-O--Ph-Ph-O-circle and 2,2'-HO-Ph-Ph-O-circle were determined to be 0.44 and 1.00 V vs NHE, respectively. By using a thermochemical cycle, the experimentally inaccessible one-electron reduction potentials for 4,4'-HO-Ph-Ph-O-circle and 2,2'-O--Ph-Ph-O-circle were calculated to be 0.64 and 0.78 V vs NHE, respectively. From the redox and acidity data we also estimated the O-H bond dissociation enthalpies for 4,4'-HO-Ph-Ph-OH, 4,4'-O--Ph-Ph-OH, 2,2'-HO-Ph-Ph-OH, and 2,2'-O--Ph-Ph-OH to be 349, 330, 372, and 385 W mol(-1), respectively. The results are discussed in light of previously established substituent effects on the thermochemistry of phenols and phenoxyl radicals.