O-H bonds in phenols

The influence of an ort/io-imidazole substituent on the bond dissociation energy of the O—H bond in phenol was studied by DFT calculations [00IJQ714]. The imidazole ring is twisted with respect to the phenol ring by 59° and causes a decrease of the bond dissociation energy by about -1 kcal/mol with respect to the unsubstituted molecule only. 

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. 

There are many other examples in the literature where the concept of bond enthalpy contribution (either E or Es) has been applied. Let us return to the case of Cr(CO)3(C6H6) and examine the procedure to estimate E sjCr-CeHe). This is much more complex than the case of the O-H bond in phenol and ethanol, as suggested by figure 5.6. 

R. M. Borges dos Santos, J. A. Martinho Simoes. Energetics of the O-H Bond in Phenol and Substituted Phenols A Critical Evaluation of Literature Data. J. Phys. Chem. Ref. Data 1998,27, 707-739. 

Phenols decrease the intensity of CL 7chi in oxidized hydrocarbons as a result of chain termination by the reaction with peroxyl radicals. Since Icu [R02 ]2 (see Chapter 2), the ratio (/0//)12 was found to be proportional to [ArOH] [7]. The kinetic isotope effect (k0K/k0n 1) proves that the peroxyl radical abstracts a hydrogen atom from the O—H bond of phenol [2,8]. 

Polar solvents block the O—H bond of phenols in the reaction with peroxyl radicals due to the formation of hydrogen bond and decrease the activity of phenols as chain terminating agents [1,9,10]. 

In the reaction of the peroxyl radical with the O—H bond of phenol is cleaved, and hydroperoxide ROOH is formed. The reaction enthalpy AH is... 

The BDE of O—H bonds of phenols were studied intensively during the last 20 years [11 31]. They are collected in handbooks [4,32]. Substituents in the aromatic ring have influence on the BDE of the O—H bond of phenol. A few examples for monosubstituted phenols YC6H4OH are given below (values of BDE are given in kJmol-1, for PhOH D(0—H) = 369 kJ mol-1 [4]. 

These data illustrate that the reaction enthalpy AH varies in an interval from 45 kJ mol 1 due to different BDE of hydroperoxides to 44 kJ mol-1 due to different BDE of chosen phenols. Recently Luo [31] performed a correlation of BDE of O—H bonds of phenols YC6H4OH with <5+(Y) Brown-Okamoto constants and arrived at the following equation ... 

M. Lucarini, P. Pedrielli, G. F. Pedulli, S. Cabiddu, C. Fattuoni. Bond Dissociation Energies of O-H Bonds in Substituted Phenols from Equilibration Studies. J. Org. Chem. 1996, 61, 9259-9263. 

Silylene intermediates have also been proposed for the reaction of alcohols with silicon357. When phenol is reacted with silicon in the presence of CuCl as a catalyst, the major product (PhO)3SiH is obtained in 94% selectivity. This result is explained by insertion of a silylene Si(OPh)2 into the O—H bond of phenol. Addition of ethylene to the reaction produces 5.8% of EtSi(H)(OPh)2, which can be accounted for by the reactions of Scheme 14358. 

Enols are generally more acidic (pA a ca 11-12) than their corresponding carbonyl tautomer (pATa ca 17-25). Exceptions to this arise when the carbonyl derivative is either destabilized relative to the enol component, or when the enol is exceptionally stable, as in the case of phenol [pATa (H20) = 9.95] vs. cyclohexa-2,4-dienone [pATa (H20) = —3 1]. Enol acidity can be controlled by O-H bond strength. In certain cases, the relative proportion of enol content can be determined by the relative strengths of the C=0 and C—H bonds in the carbonyl tautomer versus the C=C and O—H bonds in the enol. ... 

Rg.9.2 Reliilion between the erihalpy of formation of base-phenol adducts and the sirelching frequencies of the O—H bond in the phenol. Bases (a) icelonilnle. (b) ethyl acetate, (c) acetone, (d) letrahydrofuran, (c) dirnethylacetamide. (D pyridine, (g) tnethyl-anune. [From Epiey, T. D., Drago, R. S. J. Am. Chem. Soc. 967 89.5770. Reproduced with permission.]... 

When phenol, the functional group of the Tyr residue, was paired with water, the optimal geometry was found [154] to contain the expected OH- - -O H-bond. The phenol could serve as either proton donor or acceptor, but in either case, one of the C-H groups of the phenol was in position to form a secondary H-bond as illustrated in Fig. 11. No estimate was made of the energetic contribution of this secondary interaction. 

The catalyst Cu/(S S,S)-23a was also efficient for the insertion of O—H bonds of phenols (Scheme 44) [109], Under similar conditions as that for N—H insertion reactions, a wide range of phenol derivatives underwent O—H bond insertion with a-diazopropionates, providing a-aryloxypropionates with excellent enantioselectivities (95-99.6% ee). The Pd/(S, S,5 )-23a-catalyzed asymmetric O—H bond insertion reaction between a-aryl-a-diazoacetates and phenols provided the first enantioselective method for the preparation of chiral a-aryloxy-a-arylacetates, which are ubiquitous in biologically active molecules (Scheme 45) [110]. 

Based on the aforementioned results, it is clear that S-state advancement occurs by PCET mechanisms. Yz oxidation is expected to be proton-coupled, because oxidized Yz is a neutral radical. At least some steps of water oxidation should be proton-coupled, because the O— H bonds in H2O must be broken to make O2. Based on the H/D isotope effects and the activation energies discussed above, the different observations described above can be explained by the models for PCET shown in Figure 23. The oxidation of Yz involves electron transfer to Pa and deprotonation of the phenolic proton to HI90, which subsequently must deprotonate to dissipate... 

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