Bonding 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. 

Hydrogenolysis, without ring reduction, of the carbon-oxygen bond in phenols cannot be depended on, but by conversion of the phenol to a better leaving group, such as is formed by interaction of the phenol with 2-chlorobenzoxazole, l-phenyl-5-chlorotetrazole, phenylisocyanate,... 

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. 

Structures Containing a-Ether Bonds The a-ether bonds in phenolic phenylcoumaran (Fig. 7-27) and pinoresinol structures are readily cleaved by hydroxide ions, usually followed by the release of formaldehyde. Only in the case of open a-aryl ether structures does this reaction result in the fragmentation of lignin. In contrast, the a-ether bonds are stable in all etherified structures. 

Rumynskaya, I. G.. Study of intramolecular hydrogen bonding in phenolic Mannich bases. Deposited doc., 1981, VINITI 4213 81, 2.34 Chem. Ahsir.. 98,. 34145, 1983. 

R. Parthasarathi, V. Subramanian, and N. Sathyamurthy, Hydrogen bonding in phenol, water, and phenol-water clusters, J. Phys. Chem. A 109, 843-850 (2005). 

The more polar OH bond in phenols results in increased acidity compared to alcohols. Phenols are weak acids, much stronger than alcohols, but much weaker than strong acids such as HCl. For example, the pH of 0.1 M solutions of water, ethanol, phenol, and HCl are 7, 7, 5.5, and 1.0, respectively. 

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. 

The positioning of bulky substituents, especially in the oilho-position, leads to an increase in the importance of steric effects [53]. However, in all cases, hydrogen bonding plays an important role in the polymerization [54]. The tendency and strength of hydrogen bonding in phenols can be easily detected by NMR and IR-spectroscopy [54]. 

Thermoset composites are made up of thermoset resin-like polyester epoxy and phenol-formaldehyde resin composites with varying fiber volume fractions and fiber length were prepared by following one of the techniques. Among polyester epoxy and phenol-formaldehyde composites, a phenolic-type resin performed as a better matrix than epoxy and polyester resins with respect to tensile and flexural properties due to the high interfacial bonding in phenolic composites [58,59,100]. 

Dissociation energies of OH and CH bonds in phenol and substrate, respectively... 

Uncoated cBN of all grades is most commonly employed in vitreous or single-layer bonds. In phenolic or polyimide resin bonds, cBN with a thick coating of nickel is the recommended choice, as the coating improves crystal retention and heat dissipation. In metal bonds, Ti-coated monocrystalhne cBN or microcrystalline cBN are typically used. Recommended concentrations are between 50 and 125 for resin bonds, 50 and 200 for vitreous bonds, and 50 and 100 for metal bonds. 

Unlike secondary and tertiary alkyloxonium ions derived from alcohols, phenyloxonium derivatives do not dissociate to form phenyl cations, because such ions have too high an energy content (see Section 22-10). The phenyl-oxygen bond in phenols is very difficult to break. However, after protonation of alkoxybenzenes, the bond between the alkyl group and oxygen is readily cleaved in the presence of nucleophiles such as Br or U (e.g., from HBr or HI) to give phenol and the corresponding haloalkane. 

In view of the minimum energy necessary to photodissociate the weakest bonds in phenol and nitrobenzene and the studies of their photochemistry, Calvert et al. (2002)... 

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