DETERMINATION AND INTERRELATION OF BOND HETEROLYSIS AND HOMOLYSIS ENERGIES IN SOLUTION

Heats of homolysis (ΔHhomo) for a variety of carbon–carbon bonds have been determined by combining heats of heterolysis with redox potentials for a series of resonance-stabilized carbenium ions and carbanions by means of a simple thermodynamic cycle. The heats of heterolysis (Δhet) were derived from calorimetrically measured heats of reaction by changing the sign (ΔHhet = –ΔHrxn). Redox potentials were determined by cyclic voltammetry and second harmonic alternating current voltammetry (SHACV), which produced excellent reversible electrochemical data. The difference between ΔHhet and ΔHhomo is the enthalpy of electron transfer, which for the systems studied here is approximated closely by the free energy of electron transfer (ΔGET)—the difference between the free energy of oxidation of the carbanion and the free energy of reduction of the cation. In order to obtain ΔHhomo values from ΔHhet and ΔGET, it is necessary for ΔSET to be negligible. This is shown to be true for the systems reported here. The heats of homolysis and heats of heterolysis determined under this project are the first available thermodynamic data that allow comparison of the two primary modes of bond cleavage in solution for the bonds linking the component fragments of the types of large molecules that are important to the polymer and fossil fuel industries. A variety of correlations are presented between the pKa's of the carbanions, the pKR+'s of the carbenium ions, the redox potentials of both types of ions, the ΔGET's, the ΔHhet's, and the ΔHhomo's. Good linear correlations are obtained between most of the properties that involve the conversion of ions to neutral products or vice versa. Thus, ΔHhet correlates well with ΔGET. However, ΔHhomo, which involves the conversion of a neutral molecular species to a pair of electrostatically neutral free radicals, correlates poorly with these types of data. In general, ΔHhet's and ΔGET's are much more sensitive to structural change in the carbenium ions and carbanions than are the ΔHhomo's to the formed radicals. The carbinol reduction potential (CRP) is defined as the free energy of homolysis for the carbon-oxygen bonds in triarylcarbinols (and related compounds). The CRP is the sum of the pKR+ of the carbenium ion and its reduction potential. It is analogous to the acid oxidation potential (AOP) as defined by Bordwell and Bausch for the free energy of homolysis (relative bond dissociation energies) of carbon-hydrogen bonds in carbon acids. It is shown that to a good approximation ΔHhet = apKa + bpKR+ + constant and ΔHhomo = cAOP + dCRP + constant. Thus, heterolysis and homolysis energies can be determined experimentally from heats of reaction of resonance-stabilized ions and their redox potentials or estimated from their pKa's, pKR+'s, and redox potentials. This work indicates that carbon–carbon bond cleavage may proceed with greater selectivity when resonance-stabilized ionic species rather than free-radical species are the initially formed reactive intermediates. Further research should expand insight as to the generality of these results as well as the factors that control the magnitude of bond cleavage selectivity differences. © 1990, American Chemical Society. All rights reserved.