Radical cations fragmentation, thermodynamics

Burton, R.D., Bartberger, M.D., Zhang, Y, Eyler, J.R., and Schanze, K.S., Carbon-carbon bond fragmentation in aminoalcohol radical cations. Kinetics, Thermodynamic correlations and mechanism,/.Am. Chem. Soc., 118, 5655, 1996. 

These studies taken together show that, from a thermodynamic perspective, three factors can be utilized to predict the bond dissociation energy of a homobenzylic ether radical cation (BDE(RC)) and thereby its propensity to fragment (1) the bond dissociation energy of the benzylic carbon-carbon bond in the neutral substrate (BDE(S)), (2) the oxidation potential of the substrate (Ep (S)), and (3) the oxidation... 

The free energy for radical cation bond cleavage, AGf, can be estimated by means of simple thermodynamic cycles [126, 128], where, regardless of the cleavage mode, AGf depends on the free energy of homolysis of the radical cation precursor (AGh) and the dilference between the reduction potentials hE) of the radical cation and the ionic fragment formed—AGf = AGh - AE. 

Thermodynamically cleavage of a C-H bond in an alkylaromatic radical cation is strongly favored over C-C fragmentation due to the much higher solvation free energy of the proton as compared to a carbocation. However, the former process is characterized by significantly higher intrinsic barriers (0.5-0.6 eV for C-H... 

These radical cations are usually very reactive and the two main modes of reaction are side-chain fragmentation and attack by nucleophiles. The reactivity patterns of radical cations in general have recently been reviewed by Sdunittel and Burghart.[l] For many reactions, there is a correlation between the thermodynamic stability of the radical cation and the kinetics of the reaction. 

Thus, Hine (1966a) used PLNM successfully to rationalise the sites of attack on conjugated reactive intermediates (cations, radicals and anions). The data is puzzling since the thermodynamically less stable non-conjugated isomers predominate protonation of the cyclohexadienyl anion, for example, yields predominantly cyclohexa-1,4-diene. The PLNM rationalisation of this result is set out in Scheme 14 in terms of the resonance structures of the pentadienyl anion fragment. 

In various cation radicals in which benzylic C-H and C-C fragmentations are possible, the latter process usually predominates provided that it is not overly endothermic, despite the fact that deprotonation would in all cases be the thermodynamically preferred reaction. Thus, in the photochemical reaction of... 

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