ACIDITIES AND HOMOLYTIC BOND-DISSOCIATION ENTHALPIES OF 4-SUBSTITUTED-2,6-DI-TERT-BUTYLPHENOLS

Equilibrium acidities and estimates of homolytic bond dissociation enthalpies (BDEs) in DMSO of the O-H bonds for alpha-tocopherol, 2,6-di-tert-butylphenol, six 4-substituted-2,6-di-tert-butylphenols, and ten related phenols are reported. The presence of a 2,6-di-tert-butyl group in a phenol increases its acidity and makes substituent effects on the acidity caused by a para electron-withdrawing group larger. The BDEs of the O-H bonds in 2,4,6-tri-tert-butylphenol, 4-methoxy-2,6-di-tert-butylphenol, 4-methyl-2,6-di-tert-butylphenol and alpha-tocopherol, estimated by combining their pK(HA) values with the oxidation potentials of the conjugate anions, E(ox)(A(-)), according to the equation BDE = 1.37pK(HA) + 23.1E(ox)(A(-)) + C have been found to agree to within 2 kcal mol(-1) with literature values (1 kcal = 4.184 kJ). Introduction of 2,6-di-tert-butyl groups into phenol and six 4-substituted phenols weakens the O-H bonds by amounts ranging from 3.6 to 10.3 kcal mol(-1). These effects are attributed to increases in ground-state energies which introduce strains that are relieved when homolytic cleavage of the O-H bond forms an oxygen-centered radical where the odd electron can be delocalized into the benzene ring.