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 bondsfor α-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 α-tocopherol, estimated by combining their pKHA values with the oxidation potentials of the conjugate anions, Eox(A−), according to the equation BDE = 1·37pKHA + 23·1Eox(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.