Phenol disappearance rates

More recently, the reaction advancement of resole syntheses (pH = 8 and 60°C) was monitored using high-performance liquid chromatography (HPLC), 13C NMR, and chemical assays.55,56 The disappearance of phenol and the appearances of various hydroxymethyl-substituted phenolic monomers and dimers have been measured. By assessing the residual monomer as a function of reaction time, this work also demonstrated the unusually high reactivity of 2,6-dihydroxymethyl-phenol. The rate constants for phenolic monomers toward formaldehyde substitution have been measured (Table 7.6). 

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. 

All the surface recombination processes, including back reaction, can be incorporated in a heavy kinetic model [22]. The predicted, and experimentally observed, effect of the back reactions is the presence of a maximum in the donor disappearance rate as a function of its concentration [22], Surface passivation with fluoride also showed a marked effect on back electron transfer processes, suppressing them by the greater distance of reactive species from the surface. The suppression of back reaction has been verified experimentally in the degradation of phenol over an illuminated Ti02/F catalyst [27]. 

The disappearance rate of each monomer on oxidation of an equimolar mixture of phenols is shown for the three monomer pairs in Figure 1. In each pair, the phenol more reactive in homopolymerization is more rapidly consumed. Even for the DMP-DPP and MPP-DPP pairs,... 

Figure 1 shows the conversion of phenol in the presence of various Mn-Ce catalysts as a function of time. A deactivation was observed. At dl compositions, a very high rate of phenol disappearance was observed during the first minutes of the reaction, but a plateau was rapidly reached. The consumption of phenol corresponds to a decrease of total organic carbon in the solution. Very few intermediate molecules (only traces) were identified in the solution. These were benzoquinones, hydroquinone, catechol and some carboxylic acids. Acetic acid was not detected. 

In a kinetic investigation of the catalytic liquid-phase phenol oxidation carried out in a semibatch slurry reactor [6], it has been found that homogeneous stepwise polymerization reactions are enhanced in the bulk liquid-phase due to the high liquid-to-solid volumetric ratio. The rate of phenol disappearance has been expressed on the basis of power-law kinetics as a sum of heterogeneous and homogeneous (polymerization) contributions, thus... 

Phenol o-hydroxylase is an NADPH-dependent flavin mono-oxygenase which catalyses the oxidation of phenols to o-diphenols. Wainwright described a soil enzyme assay based on the colorimetric measurement of substrate (phenol) disappearance. Soils were incubated for Ih with phenol (O.OIM) in a citrate -phosphate buffer pH 4.0 (optimum) at 37°C (temperature optimum 40-45°C). Under these conditions soil activities were linear with time and the rates of reaction were proportional to the amounts of soil. Soil activities were stimulated by Cu (purified phenol o-hydroxylase has a Cu requirement). 

Alkyl-substituted phenols have different reactivities than phenol toward reaction with formaldehyde. Relative reactivities determined by monitoring the disappearance of formaldehyde in phenol-paraformaldehyde reactions (Table 7.3) show that, under basic conditions, meta-cresol reacts with formaldehyde approximately three times faster titan phenol while ortho- and para-cresols react at approximately one-third the rate of phenol.18 Similar trends were observed for the reactivities of acid-catalyzed phenolic monomers with formaldehyde. 

Tab. 4.4. Comparison between HjOj production and the rates of phenol and carbon tetrachloride disappearance at different frequencies (pMmin-1)...

Chemical/Physical. Anticipated products from the reaction of 1,3-dichlorobenzene with atmospheric ozone or OH radicals are chlorinated phenols, ring cleavage products, and nitro compounds (Cupitt, 1980). Based on an assumed base-mediated 1% disappearance after 16 d at 85 C and pH 9.70 (pH 11.26 at 25 C), the hydrolysis half-life was estimated to be >900 yr (Ellington et al., 1988). 1,3-Dichlorobenzene (0.17-0.23 mM) reacted with OH radicals in water (pH 8.7) at a rate of 5.0 x 10 /M-sec (Haag and Yao, 1992). 

Serpone at al. (1994) studied three chorophenols under pulse sonolytic conditions (frequency, 20 kHz power, 50 W) in air-equilibrated aqueous media. These phenols were totally transformed to dechlorinated, hydroxylated intermediate products via first-order kinetics in about 10 hr for 2-CPOH and 3-CPOH and about 15 hr for 4-CPOH rate constants for the disappearance of these phenols were (4.8 0.4) x 10 7 min, (4.8 0.5) x 10 3/min, and (3.3 0.2) x 103/min, respectively, for approximately 80 pM initial concentration. 

Rates of transformation and/or disappearance of phenolic acids in soil solutions have also been determined under a variety of circumstances and in various soils.2 22 23 31 42,44 In general, there is a rapid initial transformation (e.g., loss of the carboxylic acid group) of phenolic acids. For example, 90% of the carboxylic acid carbon of p-hydroxybenzoic acid, syringic acid, and vanillic acid was lost within 1 week.22 Losses of other side chain carbons or ring carbons, however, took... 

The authors suggested that the slow disappearance of pyrene is due to its reaction with radicals (although pyrene is less reactive toward the radicals than the hindered phenol). Another possibility suggested is a low-efficiency reaction of excited pyrene competing with the radiation and the radiationless decay. When pyrene is added together with MBMTP, pyrene somewhat decreases the rate of disappearance of the MBMTP molecules, e.g. at 3 x 105 Gy from 97% to 94%. 

Polcaro et al. (2003, 2005) verified that during the oxidation of organic compounds, such as phenol, diuron, 3,4-dichloroaniline, and triazines, the crucial point to obtain high Faradic yields is the rate of mass transfer of the reactant toward the electrode surface (Fig. 2.11). Thus, they developed an impinging cell that enabled them to obtain high mass-transfer coefficients (e.g., 10-4 m s-1). With this cell, at a current density of 150Am-2, they achieved a Faradic yield of 100%, up to the almost complete disappearance of the organic load. 

The kinetics of the process was considered as oxidative reactions in series with each other, but in parallel with respect to the OH, so that the concentration of OH radicals in the reaction zone was determined by the fastest reaction step. As a consequence, if the value of the imposed current was sufficiently high to complete the first oxidative step of phenol to hydroquinone (i = 2FkmCo which corresponded to y > 0.07) which involves two electrons, the disappearance of phenol was mass-transfer controlled but the total mineralization was not achieved. In these conditions, the concentration of OH was too low to provoke an appreciable reaction rate of the less oxidisable intermediates. Thus, the products of the first oxidative step accumulated in the laminar film, from which they diffused to the bulk solution where they were identified. 

Figure 3 Influence of ferric ions on the rate of disappearance of phenol. 20 ppm C in phenol (Ortiz-Gomez et al., 2008).

For the overall series-parallel reaction scheme, a set of differential equations can be developed to describe the rates of formation and disappearance of phenol and all its aromatic and carboxylic intermediates. It is well known... 

Rates of quenching of excited state triplets have been measured and the influence of substituents on the phenols studied has shown that electron-donating substituents enhance the degradation process (< > 0.5) while phenol itself has a quantum yield for disappearance of only 0.1. 

Resoles are typically generated in aqueous solution under base-catalysed conditions. Early work focused on the rate of reaction, either by the disappearance of phenol and formaldehyde or by the appearance of hydroxymethyl phenols . It was shown that the rate of reaction between phenol and formaldehyde is a function of pH , suggesting that the overall reaction proceeds with the generation of a phenolic anion, followed by the addition of formaldehyde , generating a complex mixture of different hydroxymethyl phenol compounds—the resole resin (Scheme 14). 

Determine the initial rate of disappearance (or loss, or conversion) of phenol, R piienoi, in mol min (note this is a zero-order process). 

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