Model phenolics

Comparison of the oxidation products of model phenol and benzoquinone compounds obtained with Pt and Sn02 anodes show two main differences ... 

While the examples outlined in the previous section all pertain to attack by a basic N-atom, another possibility is intramolecular attack by an acidic N-atom, i.e., a deprotonated amide. For example, in AT-(2-carbamoylphe-nyljcarbamalcs of model phenols (8.135, X = H, Cl, or MeO, Fig. 8.11), the deprotonated carboxamido group attacks the carbamate carbonyl C-atom to form a quinazoline-2,4-dione with release of the phenol [173]. In acidic media, formation of the quinazoline-2,4-dione is decreased by competitive breakdown of the intermediate to an anthranilate and C02 in addition to the phenol (not shown). 

This last possibility appeared to offer the most fruitful line of investigation. Therefore, we decided to determine what kinds of structural features would confer unusual free radical stability on simple model phenols which had been oxidized. Several phenols were oxidized in benzene solution by one-electron oxidants such as Pb02, alkaline ferricyanide,... 

Reaction of Procyanidins with Formaldehyde. Our knowledge of the kinetics and stoichiometry of the reaction of proanthocyanidin polymers with formaldehyde to produce crosslinked resins is based mainly on three studies of the reaction of model phenols or catechin with formaldehyde 34-36), These studies showed that, at lower temperatures and pH values between 2 and 9, the stoichiometry of the reaction was near equimolar in catechin and formaldehyde. 

In addition to attack by a basic nitrogen, there exists also the possibility of an intramolecular attack by an anionic nitrogen, i.e., a deprotonated amido nitrogen. This is exemplified by A-(2-carbamoylphenyl)carba-mates of model phenols (Fig. 5 X = H, Cl,... 

Wang YT. 1992. Effect of chemical oxidation on anaerobic biodegradation of model phenolic compounds. Water Environ Res 64(3) 268-273. 

Dihydroxybenzoquinones BenzothiazoUnone hydrazones 980, 987 Benzoxazine, reactions with model phenols 1473, 1474 Benzoxazoles,... 

Cl mass spectra of 292, 293 coupling constants for 349 p-Hydroxybenzylamine, reactions with model phenols 1474, 1475 Hydroxybenzyl cations 265, 295... 

Thus Flaig (1966a) has examined hypothesis 1 by analyzing the humic products of rotting straw. Other workers have oxidized mixtures of model phenols and quinones to support hypothesis 2. Martin and Haider (1980) have treated mixtures of phenol and amino acids with fungal and bacterial enzymes to simulate hypothesis 4. Even artificial landfills have produced humates (Fuller and Artiola-Fortuny, 1982). 

Filip, Z., Haider, K., Beutelspacher, H., and Martin, J. P. (1974). Comparison of IR-spectra from melanins of microscopic soil fungi, humic acids, and model phenol polymers. Geoderma 11, 37-52. 

In a series of papers, Aulin-Erdtman et al. [15-17] compiled extensive data on UV-visible spectra of lignin model phenols and the corresponding phenolate ions. A selection of these is presented in Table 3.4. 

UV-Visible Data for Selected Lignin Model Phenols and Phenolate Anions ... 

The first example of singlet oxygen reactions with lignin model phenols was reported by Matsnnra et al. [59,60], Singlet oxygen produced by photosensitization oxidizes... 

Relatively little basic information has been published regarding the kinetics of phenol-formaldehyde intermediates, especially of phenols, methylol phenols, benzyl alcohol and benzylic ethers with isocyanates. Due to the fact that a typical resole contains both phenolic and benzylic hydroxyl groups, it was of interest to determine their reactivity toward isocyanates in the presence of various catalysts, as well as the effect of substitution on their reactivity. This investigation describes the kinetics of model phenols and model benzyl alcohols with phenyl isocyanate catalyzed with either a tertiary amine (dimethylcyclo-hexylamine, DMCHA) or an organotin catalyst, dibutyltin dilaurate (DBTDL) in either dioxane or dimethylformamide solution. 

The reactivity of the model phenols and benzyl alcohols with phenyl isocyanate was determined in the presence of a tertiary amine (DMCHA) and a tin catalyst (DBTDL) by measurement of the reaction kinetics. The experimental results based on initial equal concentrations of phenyl isocyanate and protic reactants showed that the catalyzed reactions followed second order reaction with respect to the disappearance of isocyanate groups (see Figure 1). It was also found that a linear relationship exists between the experimental rate constant kexp, and the initial concentration of the amine catalyst (see Figure 2). In the case of the tin catalyst, the reaction with respect to catalyst concentration was found to be one-half order (see Figures 3-4). A similar relationship for the tin catalyzed urethane reaction was found by Borkent... 

In Table I are summarized the kinetic results for the model phenols with phenyl isocyanate in the presence of DMCHA catalyst in dioxane solution. It is apparent that substitution in the 2 and 6 positions decrease significantly the reaction constant kcat. In the case of model benzyl alcohols using DMCHA catalyst, the... 

EFFECT OF SUBSTITUTION ON REACTIVITY OF MODEL PHENOLS WITH PHENYL ISOCYANATE IN DIOXANE AT 25°C... 

Figure 5a Three-dimensional section of modelled phenol plume in groundwater, showing the distribution of contaminant concentration (moll, ) away from the source area.

The mechanism of formation has been postulated to involve rapid one-electron oxidation of the aromatic ring by the heme radical cation to give a Tyr radical cation [Eq. (8) (101, 112)]. Using model phenols such species undergo rapid deprotonation to give a phenoxyl radical with rate constants ca. 1010 dm3 mol 1 s 1 (125, 126). Thus, these model studies are consistent with the formation of a radical within the globin polypeptide ... 

The lack of reaction between methylolated phenol and cellulose reported by Allan and Neogi seems to contradict the findings of Chow and coworkers. One possible explanation for this disparity could be the difference In available free formaldehyde In their systems. Allan s model phenolic adhesive would have the equivalent of only one mole of formaldehyde per mole of phenol and would not be expected to have significant quantities of free formaldehyde. The resins used by Chow and coworkers had about 2 moles of combined formaldehyde per mole of phenol. Such resins are able to release formaldehyde during cure idien condensation occurs between two methylol groups. This formaldehyde ml t then add at the aliphatic hydroxyls on cellulose or lignin resulting In condensation, as proposed by Chow, between the methylolated wood components and the phenolic resins. 

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