CALORIMETRIC STUDY OF 2,4,6-TRI-T-BUTYLPHENOXY RADICAL IN SOLUTION

Direct experimental determinations of the differences in the heats of formation in solution of a free radical and its hydrogenated precursor have been carried out for the first time. The method consists of calorimetric measurements of the heat of the quantitative reaction of hydrazobenzene with 2,4,6-tri-t-butylphenoxy radical (I) to produce trans-azobenzene and 2,4,6-tri-t-butylphenol (II). Values of (∆Ht)II– (∆H,f)i calculated from these studies vary from 28.13 ± 0.13 kcal/mol in pure carbon tetrachloride to 29.73 ± 0.07 kcal/mol in the mixed solvent carbon tetrachloride-dioxane (mole fraction 0.53). Calorimetric studies of the reaction of 4-t-butylphenol with I to produce II and 4-(4-t-butylphenoxy)-2,4,6-tri-t-butyl-2,5-cyclohexadiene-1-one (IV) were also carried out in a number of solvents and the results of those studies are internally consistent with the results of the hydrazobenzene reaction. A comparison of the solution properties of I with those of 1,3,5-tri-t-butylbenzene (V) reveals that specific solvent interactions with the radical due to its free electron are small (±0.5 kcal/mol) and thus V is a good model for I in solution. The difference in the heats of formation of a number of other stable free radicals, B •, and the molecules from which they are formed, BH, is then calculated by combining the results of the present study and the results of earlier studies of the reactions of I with BH to produce II and B•. The strain energy in II associated with steric interaction of 2,6-di-t-butylphenol group is calculated in the gas phase as equal to 8.0 ± 2.0 kcal/mol. The manifestations of this strain energy in the phenomena of the synergistic behavior of mixtures of hindered and nonhindered phenols as antioxidants and the ease of disassociation of dimers derived from 2,5-di-t-butylphenoxy radicals are then discussed. © 1969, American Chemical Society. All rights reserved.