Phenol dissociation enthalpy

Table 5.2 ArO-H bond dissociation enthalpies ofpora-monosubstituted phenols in benzene (I = 298.15 K) relative to DH° n(PhO-H) [73], Data in kJ mol-1.

Though some more traditional thermodynamicists will be dismayed by the concept of solution phase bond dissociation enthalpy, the fact is that the database involving these quantities is growing fast. When used judiciously, they may provide important chemical insights—as is indeed the case for the stability of the O-H bond in phenolic compounds. Although solution phase bond dissociation enthalpies are not true bond dissociation enthalpies, because they include some contribution from intermolecular forces, a series of solution values like those in table 5.2 may be (and often is) taken as a good approximation of the trend in the gas-phase. 

Figure 5.5 Thermochemical cycles relating O-H bond enthalpy contributions ( s) with bond dissociation enthalpies (DH°) in phenol and ethanol. ER are reorganization energies (see text).

K. U. Ingold. Critical Re-evaluation of the O-H Bond Dissociation Enthalpy in Phenol. J. Phys. Chem. A 2005,109, 2647-2655. 

M. M. Bizarro, B. J. Costa Cabral, R. M. Borges dos Santos, J. A. Martinho Simoes. Substituent Effects on the O-H Bond Dissociation Enthalpies in Phenolic Compounds Agreements and Controversies. Pure Appl. Chem. 1999, 71, 1249-1256. 

The different solvation energetics of R and R- will also lead to errors in the bond dissociation enthalpies calculated with equation 16.33. For instance, in the case of phenol, whose interactions with proton-acceptor solvents (like DMSO) are obviously stronger than those for the phenoxy radical, a negative correction should be applied to the value of Z)//°(PhO-H) calculated from equation 16.33 (see also equation 16.32). It is probably unwise to ascribe the 7 kJ mol-1 difference between the electrochemical and the recommended DH° (PhO—R) value to the differential solvation effects. Although this discrepancy is in the correct direction, it lies within the suggested uncertainty of the method. 

In conclusion, therefore, a judicious use of CV methodology may lead to absolute thermodynamic data that are accurate to ca. 15 kJ mol-1. Relative values (i.e., differences between bond dissociation enthalpies in similar compounds) can be more reliable, but the approximations described suggest that some caution be exercised when using the results to draw conclusions that rely on small differences between bond dissociation enthalpies. This is the case, for example, for ring substituent effects on the O-H bond dissociation enthalpies in substituted phenols [346,349],... 

M. Lucarini, G. F. Pedulli, M. Cipollone. Bond Dissociation Enthalpy of a-Tocopherol and Other Phenolic Antioxidants. J. Org. Chem. 1994, 59, 5063-5070. 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

Ab initio molecular mechanics calculations have been used to examine the electronic states of selenoxanthene 9 and selenoxanthone 10. These results were used to explain differences in the formation of bromine adducts of these compounds (see Section 7.11.6.1.2) <1998JOC8373>. Bond dissociation enthalpies and adiabatic ionization potentials of phenolic antioxidants containing selenium and tellurium have been carried out using DFT models in an attempt to design novel vitamin E analogues such as 11 <20060BC846>. 

It is well known that substituents have a profound effect on the hydrogen atom donating ability of phenols. Indeed, only those phenols bearing electron donating substituents, particularly at the ortho and/or para positions, are active as antioxidants. In general, this is as expected since such groups are expected to lower the phenolic O—H bond dissociation enthalpy and increase the reaction rates with peroxyl radicals. 

Lithoxoidou, A.T., and Bakalbassis, E.G., PCM study of the solvent and substituent effects on the conformers, intramolecular hydrogen bonds and bond dissociation enthalpies of 2-substituted phenols, J. Phys. Chem. A, 109, 366-377 (2005). 

In principle, the pK of the toluene radical cation can be estimated from the one-electron reduction potential of the radical cation and the C-H bond dissociation enthalpy for toluene (368 kj mor )[62] using equation (5). The resulting pR is ca -10, i.e., considerably more acidic than the phenol radical cation. Nicholas and Arnold have estimated the pfC of the toluene radical cation to between -9 and -13 in acetonitrile which is weU in line with the estimate given here. [32] Since the C-H bond dissociation enthalpies of substituted toluenes seem to be almost invariant with substituent,[63-66] the substituent effect on the pK of... 

EG. Bordwell, X.-M. Zhang, A.V. Satish, and J.-P. Cheng, Assessment of the importance of changes in ground-state energies on the bond dissociation enthalpies of the 0-H bonds in phenols and the S-H bonds in thiophe-nols, J. Am. Chem. Soc. 116 (1994), pp. 6605-6610. 

ESR can equally be used for detection of radicals in masticated rubber their identification in relation to the chemical structure might be approached with specific techniques such as electron nuclear double resonance (ENDOR). ESR studies also contribute to the understanding of the char forming process of various polymers [815], to the study of mechanical fracture, which produces free radicals, grafting reactions, etc. Pedulli et al. [816,817] have determined the bond dissociation enthalpies of a-tocopherol and other phenolic AOs by means of ESR. The determination of the O—H bond dissociation enthalpies of phenolic molecules is of considerable practical interest since this class of chemical compounds includes most of the synthetic and naturally occurring antioxidants which exert their action via an initial hydrogen transfer reaction whose rate constant depends on the strength of the O—H bond. 

---