Phenols reaction mechanism

We examine here a number of reaction pathways for nitric oxide, with the emphasis on assessing their biological relevance. To date, the fastest reaction for nitric oxide with clear toxicological significance is that with superoxide to produce ONOO" (Huie and Padmaja, 1993). Thus, the chemistry and reactivity of ONOO" are discussed at length. In addition, the interaction between ONOO" and nitric oxide is examined with respect to its effects on nitric oxide half-life as well as effects on peroxynitrite reactivity toward phenol. Reaction mechanisms are proposed to account for the nitrated, hydroxylated, and nitrosated phenolic products seen. 

The optimal pH-value for the coupling reaction depends on the reactant. Phenols are predominantly coupled in slightly alkaline solution, in order to first convert an otherwise unreactive phenol into the reactive phenoxide anion. The reaction mechanism can be formulated as electrophilic aromatic substitution taking place at the electron-rich aromatic substrate, with the arenediazonium ion being the electrophile ... 

Pyridinium chloride ([PyHjCl) has also been used in a number ofcyclization reactions of aryl ethers (Scheme 5.1-4) [4, 18]. Presumably the reaction initially proceeds by deallcylation of the methyl ether groups to produce the corresponding phenol. The mechanism of the cyclization is not well understood, but Pagni and Smith have suggested that it proceeds by nucleophilic attack of an Ar-OH or Ar-0 group on the second aromatic ring (in a protonated form) [4]. 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

Melt reaction mechanisms of tertiary aliphatic amine catalyzed phenolic-epoxy reactions were proposed to begin with a trialkylamine abstracting a phenolic hydroxyl proton to form an ion pair (Fig. 7.36). The ion pair was suggested to complex with an epoxy ring, which then dissociated to form a /1-hydroxycther and a regenerated trialkylamine.87... 

Epoxidized novolacs, 411 Epoxy -phenol networks, 411-416 properties of, 413-416 Epoxy-phenolic reaction kinetics of, 413 mechanism of, 411-412 Epoxy structures, 414 e-Caprolactam, 174... 

In this article we describe selective bromination of various phenols under mild conditions and discuss their reaction mechanisms. 

Figure 13, indicates that the first mole of phenol is released in <30 s, the same elapsed time for the chemiluminescence to reach a maximum intensity. In fact, the measured rate constant r, for the rise in the chemiluminescence emission, is identical to the rate of the first phenol s release from the oxalate ester. Furthermore, the slower rate of release of the second phenol ligand has a rate constant that is identical to the chemiluminescence decay rate f. Thus, the model allows a quantitative analysis of the reaction mechanism, heretofore not available to us. We intend to continue this avenue of investigation in order to optimize the chemiluminescence efficiencies under HPLC conditions and to delineate further the mechanism for peroxy-oxalate chemiluminescence. 

The reaction mechanism has not been elucidated the ammonium monovanadate presumably oxidizes the phenols to quinones, that then react with />-anisidine to form quinonimines. 

Figure 3 Possible reaction mechanism of carboxylation of phenol via...

These experiments clearly showed that it is a-oxygen participation that provides FeZSM-5 zeolites with such a remarkable catalytic performance in the reaction of benzene to phenol oxidation. Equations (1-3) written above are the main stages of the reaction mechanism. 

The reaction mechanism is based on protonation of the hydroxyl moiety, rearrangement of the phenyl group and simultaneous cleavage of water, creating a carbocation as intermediate [135]. This cation is hydroxylated by water. Thereby, an unstable hemiacetal is formed that splits into two molecules, phenol and water. 

A Comparison of the Reaction Mechanism for the Gas-Phase Methylation of Phenol with Methanol Catalyzed by Acid and by Basic... 

This chapter compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It is proposed that with the basic catalyst, Mg/Fe/0, the tme alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacts with phenol to yield salicyl alcohol, which rapidly dehydrogenates to salicyladehyde. The latter was isolated in tests made by feeding directly a formalin/phenol aqueous solution. Salicylaldehyde then transforms to o-cresol, the main product of the basic-catalyzed methylation of phenol, likely by means of an intramolecular H-transfer with formaldehyde. With an acid catalyst, H-mordenite, the main products were anisole and cresols moreover, methanol was transformed to alkylaromatics. 

The conclusions derived from the preceding experiments may be summarized with the aid of the reaction mechanism illustrated in Scheme II. The ester undergoes a rapid, reversible association with the cycloamylose, C—OH. An alkoxide ion derived from a secondary hydroxyl group of the cycloamylose may then react with an included ester molecule to liberate a phenolate ion and produce an acylated cycloamylose. This reaction is characterized by a rate constant, jfc2(lim), the maximal rate constant for the appearance of the phenolate ion from the fully complexed ester in the pH range where the cycloamylose is completely ionized. Limiting rates are seldom achieved, however, because of the high pK of cycloamylose. 

The introduction of 0.5 mol L 1 pyridine into 1,1-dimethylethanol containing phenol and hydroperoxide increases the rate constant kn from 10-4 to 1.2 x 10 L mol-1 s 1 (353 K) [121]. At concentrations lower than 0.1 mol L 1, pyridine enhances the rate of the reaction of p-methoxyphenol with hydroperoxide in benzene at 353 K from almost zero to v = 3.7 x 10 2[ArOH][ROOH][C5H5N] [121]. The addition of pyridine to tert-butanol with p-methoxyphenol increases both the reaction rate and the electroconductivity of an ArOH solution. All these results are in agreement with the following reaction mechanism ... 

We emphasize that the above results have been observed only in the oxidation of sulfides and phenols, reactions known to follow radical mechanisms. A thorough investigation of the catalytic potential of the materials in other oxidation reactions (epoxidation, hydroxylations, etc.) is warranted. 

Epoxides can react with alcohols via acidic or basic catalysed reaction mechanisms. However, since both strong acids and bases will degrade the cell wall polymers of wood, the reaction is usually catalysed via the use of amines, which are more strongly nucleophilic than the OH group. For example, whereas the production of epoxy-phenolic resins requires temperatures in the region of 180-205 °C, reaction between epoxides and primary or secondary amines takes place at 15 °C (Turner, 1967). Reaction of epoxides with wood often involves the use of tertiary amines as catalysts (Sherman etal., 1980). The sapwood is more reactive towards epoxides than heartwood (Ahmad and Harun, 1992). 

Figure 3. Reaction mechanism of ortho methylation of phenol using methanol.

In this section, we discuss the high performance of the Rejo cluster/HZSM-5 catalyst, its active structure and dynamic structural transformation during the selechve catalysis, and the reaction mechanism for direct phenol synthesis from benzene and O2 on this novel catalyst [73, 107]. Detailed characterization and determination of active Re species have been conducted by XRD, Al solid-state MAS NMR, conventional XAFS and in situ time-resolved energy dispersive XAFS, which revealed the origin and prospects of high phenol selectivity on the novel Re/HZSM-5 catalyst [73]. 

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