Phenolic hydroxyl group, stability

In the original analytical method developed by Freudenberg and coworkers, wood was first subjected to alkaline hydrolysis at high temperature followed by a methylation with dimethyl sulfate The free phenolic hydroxyl groups were converted by this treatment to methoxyl groups and the phenolic units were thus stabilized toward oxidative breakdown The oxidation step was carried out by addition of portions of solid potassium permanganate at a pH at or near 7 until the purple color of the solution remained (Freudenberg et al 1936)... 

Of the various plant sterol conjugates, SFs are known to prevent lipid oxidation in various systems. The activity is based on the capability of ferulic acid to donate hydrogen from the phenolic hydroxyl group to a radical. The resulting SF radical formed is resonance stabilized, and the SF radicals may still effectively interfere with the chain reaction of... 

For the bituminous coal, most of this water is formed at temperatures below 400°C before any other products are observed. For the lignite, neither pyrolytic water nor more than a small amount of hydrocarbons (Figure 3) (7,8) is evolved until temperatures of around 600°-700°C are attained. This behavior supports the view, based on work with coal model substances (27), that water production from phenolic hydroxyl groups deprives the pyrolyzing coal of hydrogen which could otherwise stabilize hydrocarbon species (e.g., tar). 

Levothyroxine is another example of an active pharmaceutical ingredient with relatively poor stability that is available as a USP analytical reference standard in the free acid form, while the drug is formulated as the sodium salt in most commercial preparations.84 Levothyroxine is prone to extensive photochemical decomposition85 that is thought to be exacerbated by the facile ionization of the phenolic hydroxyl group.86 Supply of the USP analytical reference standard as the free acid provides a more stable form through suppression of the ionization of the phenolic hydroxyl. 

Polyesters - Aliphatic crosslinked polyesters undergo chain scission and crosslink breakage during photooxidation while using ESCA phenolic hydroxyl groups have been identified in photooxidised poly(ethylene terephtha-late). ° The presence of an optical brightener has been found to enhance the surface stability of poly(ethylene terephthalate). ... 

The o- and p-nitrosophenols enjoy the possibility of resonance stabilization by jt-electron donation from the phenolic hydroxyl group to the nitroso group, and the o-isomer could also be stabilized by an intramolecular hydrogen bond. These species are also tautomeric with benzoquinone oximes. All of this could confound interpretation of enthalpy of formation values if only they were available—there are seemingly no measured enthalpy of formation values for o-nitrosophenol. The value for p-nitrosophenol will be discussed later in Section VI because of tautomeric ambiguity. The m-species lacks the stabilizing conjugate NO/OH interaction, and so the monomer-dimer equilibrium as found in other nitroso compounds becomes problematic—should the measurement of enthalpy of combustion be available. 

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