Phenols, dissociation with resonance

The spectra of niclosamide in methanol (Fig. 3a) and methanolic base (Fig. 3c), show four bands with the same 2max values but max values increase in base. In methanolic acid (Fig. 3b) only two bands appeared [19]. This could be explained in terms of resonance effects as well as the dissociation of the phenolic-OH group to phenolate in base [20,21]. The possible resonance structures of niclosamide are shown below ... 

This preparative scheme leads to only 30% yield due to the side reactions between the meto-astatoaniline diazonium salt and astato-phenol, which cannot be eliminated even by continuous extraction of the product with n-heptane (167). All the astatophenols synthesized to date have been identified by either HPLC (99,104) or TLC (160,166,167). Their dissociation constants (KJ have been established from extraction experiments by measuring the relative distribution of compounds between aqueous borax buffer solutions and n-heptane as a function of acidity. On the basis of these derived values, the Hammett a-constants and hence the field (F) and resonance (R) effects have been estimated for these compounds (167) (see Table VI). The field effect for astatine was found to be considerably weaker than that for other halogens the resonance effect was similar to that for iodine (162). 

A simple case where the general a constants in Table 8.5 do not succeed in correlating acidity constants is when the acid or base function is in direct resonance with the substituent. This may occur in cases such as substituted phenols, anilines, and pyridines. For example, owing to resonance (see Fig. 8.4), a para nitro group decreases the pKa of phenol much more than would be predicted from the o para constant obtained from the dissociation of p-nitrobenzoic acid. In such resonance cases (another example would be the anilines), a special set of o values (denoted as oJpara) has been derived (Table 8.5) to try to account for both inductive and resonance... 

Why this difference May et al. s°) have examined the rate of displacement of 4-nitrocatechol from protocatechuate 3,4-dioxygenase by substrate and 3-fluoro-4-hydroxybenzoate. Biphasic kinetics are observed and the dissociation of 4-nitrocate-chol is very slow (ca. 10-2 s-1). It has been suggested that, because of its rather acidic p-hydroxyl proton, 4-nitrocatechol may bind upside down with respect to the substrate, i.e. the phenolate binds at the carboxylate site. The resonance Raman data suggests that the 4-nitrocatechol does not bind to the active site iron in protocatechuate 3,4-dioxygenase and lends credence to May s suggestion. 

Dissociation of phenols or anilinium ions, however, can involve substantial resonance stabilisation of product base but not of the reactant acid (Schemes 4 and 5) and the relevant values do not correlate well with Hammett a values as illustrated in Figure 4."... 

In the case of the alkaline hydrolysis of aryl carbamates the p" is 2.64 0,44 which is, within experimental error, the same as the p for the dissociation of substituted phenols (2.23) this is consistent with complete expression of the resonance interaction between developing oxyanion and the substituent. 

The predictive ability of the MLC QRAR model is compared in Fig. 9.12 with a three-variable QSAR model. The predicted value of log 1/C was calculated from the fitted log 1/C vs. k and log 1/C vs. (log Pow, acid dissociation constant pK and resonance parameter R) plots, by using the leave-one-out technique (the compound predicted was left out in the derivation of the model). As observed, the QSAR model had difficulties in predicting the toxicity of highly lipophilic phenols, as indicated by the curvature in this region. The results show that a single MLC retention... 

The experiments on GFP illustrated in Fig. 12.2 were aimed partly at the question of how excitation of the chromophore leads to dissociation of a proton from the phenolic -OH group (Fig. 5.9). Comparisons of resonance Raman spectra of GFP with spectra of the chromophore in ordinary and deuterated ethanol, together with normal-mode assignments of the Raman bands, indicated that stretching of the O-H bond is not strongly coupled to the initial excitation, and must develop later in the evolution of the excited state [59, 60]. 

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. 

Phenols are known to quench DBO with higher rate constants (ca. 10 M s" ), which can be traced back to their low OH bond dissociation energies [183], resulting from the resonance stabilization of phenoxyl radicals [174]. The observation of substantial deuterium isotope effects for the fluorescence quenching of DBO by phenols corroborates, however, that hydrogen transfer operates for phenols as well (cf. Table 3.5). 

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