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-Ph-Ph-OH, 2,2‘-HO-Ph-Ph-OH, 2,2‘--O-Ph-Ph-OH, and 2,2‘-•O-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• and 2,2‘-HO-Ph-Ph-O • 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 • and 2,2‘--O-Ph-Ph-O• 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 kJ mol-1, respectively. The results are discussed in light of previously established substituent effects on the thermochemistry of phenols and phenoxyl radicals.