Dissociation energies, of benzyl

The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C—H bond, like an allylic one, is weaker than a C—H bond of an alkane, as the bond dissociation energies of toluene, propene, and 2-rnethylpropane attest ... 

Toluene is oxidized to cresols (ortho meta para ratio = 5 1 4) and not to the benzyl derivatives, despite the low dissociation energy of the benzylic C—H bond. p-Xylene undergoes oxidation to 2,5-dimethylphenol. 

Conjugation, too, is effective at stabilizing radicals. We know that radicals next to double bonds are delocalized from their ESR spectra (p. 1025) that they are more stable is evident from the bond dissociation energies of allylic and benzylic C-H bonds. 

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... 

Many compounds [olehns, alcohols, and carboxyhc acids (or other carbonyl chemistry)] will undergo dimerization reactions. Figure A15-7 shows how carboxylic acids can react with an alcohol to form a dimer [6] (note that it should be loss of water and not carbon dioxide). In RP-UPLC under basic conditions, the elution order would be dihunisal in its ionized form < descarboxydiflunisal < the dimer. Indoles have been shown to dimerize under acidic conditions, and phenols have shown to dimerize under free radical initiated oxidative conditions, usually ortho-phenols [1]. Due to the low bond dissociation energy of the benzylic C-H bond and ease of radical formation, dimerization can occur at the benzylic center. Nalidixic acid undergoes dimerization under thermolysis conditions to produce a dimeric structure [26]. 

The chemistry of the alkanes is dominated by the abstraction of a hydrogen atom by various free radicals. The bond dissociation energies of C-H bonds decrease in the order primary > secondary > tertiary > benzylic and allylic, and thus free radical reactions tend to occur at tertiary, benzylic or allylic centres. 

This paper is concerned with certain aspects of the thermodynamics and kinetics of transition metal-alkyl homolytic bond dissociation processes, notably of stable, ligated complexes in solution (L M-R, where L is a ligand and R = alkyl, benzyl, etc.)(l ). The metal-alkyl bond dissociation energy of such a complex (BDE, strictly bond dissociation enthalpy) is defined as the enthalpy of the process represented by Equation 1. 

If, in silane, one hydrogen is replaced by a phenyl group, the silicon-hydrogen bond dissociation enthalpy is only reduced by about 9 7 kJ/mol (19). This is in sharp contrast with the decrease observed for the carbon analogues, where the large stabilization energy of benzyl radical makes D(Me-H)-D(PhCH -H) = 71 4 kJ/mol. The effect of a second phenyl group in the hydrocarbon family is also known, D(Ph CH-H) = 340 8 kJ/mol (27,28) and there is a recent estimate for D(Ph C-H), 338 13 kJ/mol (28). 

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