EFFECTS OF SOLVATION ON THE ENTHALPIES OF REACTION OF TERT-BUTOXYL RADICALS WITH PHENOL AND ON THE CALCULATED O-H BOND STRENGTH IN PHENOL

The enthalpy for the reaction of di-tert-butyl peroxide with phenol to give tert-butyl alcohol and phenoxyl radical (i.e. t-BuOOBu-t + 2 PhO H --> 2 t-BuOH + 2 PhO(.)) has been determined in a number of solvents using photoacoustic calorimetry. The effect of the solvent on the the rmochemistry of this process is remarkably large with the reaction bei ng ca. 10 kcal mol(-1) more exothermic in acetonitrile or ethyl acetat e than in isooctane. The relationship between the observed enthalpy ch anges and the PhO-H bond energy is discussed in detail. It is shown th at in order to extract the bond energy from the experimental (apparent ) enthalpy change, it is necessary to account for a number of processe s, viz., the volume change for the overall reaction, the solvent effec t associated with the conversion of 1 mol of di-tert-butyl peroxide to 2 mol of tert-butyl alcohol, and the differences in solvation of phen ol and the phenoxyl radical. The contributions from each of these proc esses to the observed reaction enthalpy were derived from a separate s et of experiments with 1,4-cyclohexadiene instead of phenol or from da ta available in the literature. These data allow one to determine solu tion bond energies, i.e., the enthalpy of homolysis for which the stan dard state is the solvated reactant and products, and to quantify the solvent effect on these values. Thus, PhO-H bond energies in isooctane (88 kcal mol(-1)), benzene (89 kcal mol(-1)), carbon tetrachloride (9 0 kcal mol(-1)), ethyl acetate (95 kcal mol(-1)), and acetonitrile (95 kcal mol(-1)) have been obtained. Most of the differences between the se values can be accounted for from the known hydrogen bonding equilib rium between the solvents and the phenol. A number of purported determ inations of the PhO-H ''gas-phase bond energy'' which utilized electro chemical (EC) measurements and, of necessity, highly polar solvents ar e shown to be seriously in error. Similar errors must be present in ma ny other EC ''gas-phase'' bond energies which also were determined in polar solvents.