Using thermodynamic cycles to study reactive intermediates
The observation of the near identity of the acidities in DMSO of benzoic acid, cyclohexane-l,3-dione enol a, and tropolone (2J has provided experimental support for the importance of resonance in carboxylate ions. And the observation of the near identity of the homolytic bond dissociation enthalpies (BDEs) of the 0-H bond in 1 and the C-H bond in CH,COCH2-H, estimated by eq 3, has proviged experimental support for the importance of allylic resonance in radicals of the type C-C=O c) C=C-0'. The BDEs of the 0-H bonds in r-Bu2C=NOH and r-Bu(1-Ad)C=NOH ketoximes, estimated by eq 3, of 83 and 81 kcallrnol, respectively, are in excellent agreement with the values of 81 and 80 kcallmol, respectively, determined by a direct calorimetric method. The BDE of the 0-H bond in anti-benzaldoxirne is 17 kcal/mol lower than that of the 0-H bond in acetaldoxime, indicating that the PhCH=NO' iminoxy radical is a x, rather than a 6, radi- cal. The 15 kcallmol lower BDE of the 0-H bond in r-Bu,C=NOH ketoxime than that of the Me2C=NOH ketoxime is attributed to relief of steric strain on homolytic cleavage. Relief of steric strain is believed to account also for the 8 kcal/mol lower BDE of the 0-H bond in 2,4,6-tri-r-butylphenol than that in phenol itself.