Rate constants - nitrophenol

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

Table 2. Association constants (K) and second-order rate constants (kL and kc) for p-nitrophenol release from p-nitrophenyl picolinate 1 in the presence of ligand (L) and metal ion (M)a-b...

Table 4. Pseudo-first-order rate constants (kob,d) for p-nitrophenol release from p-nitrophenyl picolinate...

Table 9. Pseudo-first-order rate constants for the release of p-nitrophenol in the reactions of optically active esters in a CTAB micelle...

In the M. trichosporium OB3b system, a third intermediate, T, with kmax at 325 nm (e = 6000 M-1cm 1) was observed in the presence of the substrate nitrobenzene (70). This species was assigned as the product, 4-nitrophenol, bound to the dinuclear iron site, and its absorption was attributed primarily to the 4-nitrophenol moiety. No analogous intermediate was found with the M. capsulatus (Bath) system in the presence of nitrobenzene. For both systems, addition of methane accelerated the rate of disappearance of the optical spectrum of Q (k > 0.065 s-1) without appreciatively affecting its formation rate constant (51, 70). In the absence of substrate, Q decayed slowly (k 0.065 s-1). This decay may be accompanied by oxidation of a protein side chain. 

Cramer and co-workers (1967) have recently measured rate constants as well as equilibrium constants for the association of p-nitrophenol and a series of azo dyes with cydohexaamylose. The general structure of the dyes employed in this study is illustrated in Fig. 4. p-Nitrophenol and p-nitro-phenolate bind to cydohexaamylose with rate constants of about 108 M l sec-1, near the diffusion-controlled limit. Within the series of dyes, however, binding rates decrease by more than seven orders of magnitude as the steric bulk of the dye is increased. Equilibrium constants, on the other hand, are roughly independent of the steric nature of the substrate, indicating that association and dissociation rates are affected by similar... 

More recently, Kaiser and coworkers reported enantiomeric specificity in the reaction of cyclohexaamylose with 3-carboxy-2,2,5,5-tetramethyl-pyrrolidin-l-oxy m-nitrophenyl ester (1), a spin label useful for identifying enzyme-substrate interactions (Flohr et al., 1971). In this case, the catalytic mechanism is identical to the scheme derived for the reactions of the cycloamyloses with phenyl acetates. In fact, the covalent intermediate, an acyl-cyclohexaamylose, was isolated. Maximal rate constants for appearance of m-nitrophenol at pH 8.62 (fc2), rate constants for hydrolysis of the covalent intermediate (fc3), and substrate binding constants (Kd) for the two enantiomers are presented in Table VIII. Significantly, specificity appears in the rates of acylation (fc2) rather than in either the strength of binding or the rate of deacylation. 

Maximal rate constant (kf) for the appearance of p-nitrophenol from the fully complexed ester. 

Fig. 20 A plot of the observed pseudo-first-order rate constants (kobs) for the methanolysis of HPNPP (4 x 10 5moldm ) as a function of [35 2Zn(II)] in the presence of 1 equivalent of added CH30 per complex giving jpH = 9.5, T = 25 + 0.1 °C. Dotted line is presented as a visual aid directed through all actual data collected at 280 nm ( ) or 320 nm (O) which are the wavelengths for disappearance of HPNPP and appearance of /j-nitrophenol solid line is a linear fit of the data corrected for inhibition by triflate counterions at 280 nm ( ) or 320 nm ( ). Reproduced with permission from ref. 95.

Pseudo-first order rate constants at 20°C for release ofp-nitrophenol Relative rates refer to the ratio of [3271/1327 + K+]... 

Photolytic. A photooxidation rate constant of 6 x 10 " cm /molecule-sec at room temperature was reported for the vapor-phase reaction of benzene with OH radicals in air (Atkinson, 1985). The reported rate constant and half-life for the reaction of benzene and OH radicals in the atmosphere are 8.2 x 10 M/sec and 6.8 d, respectively (Mill, 1982). Major photooxidation products in air include nitrobenzene, nitrophenol, phenol, glyoxal, butanedial, formaldehyde, carbon dioxide, and carbon monoxide (Nojima et al., 1975 Finlayson-Pitts and Pitts, 1986). 

When nitrobenzene, with nitrogen as a carrier gas, was passed through a quartz cell and irradiated by two 220-volt arcs, nitrosobenzene and 4-nitrophenol formed as the major products (Hastings and Matsen, 1948). A rate constant of 1.4 x 10cmVmolecule-sec was reported for the gas-phase reaction of nitrobenzene and OH radicals in air (Witte et ah, 1986). 

Chemical/Physical. In an aqueous solution, nitrobenzene (100 pM) reacted with Fenton s reagent (35 pM). After 15 min, 2-, 3-, and 4-nitrophenol were identified as products. After 6 h, about 50% of the nitrobenzene was destroyed. The pH of the solution decreased due to the formation of nitric acid (Lipczynska-Kochany, 1991). August et al. (1998) conducted kinetic studies for the reaction of nitrobenzene (0.2 mM) and other monocyclic aromatics with Fenton s reagent (8 mM hydrogen peroxide [Fe ] = 0.1 mM) at 25 °C. They reported a reaction rate constant of 0.0260/min. 

Photolytic. A second-order reaction rate constant of 9 x lO cmVmolecule-sec was reported for the reaction of 2-nitrophenol and OH radicals in the atmosphere (Atkinson, 1985). 

Anticipated products from the reaction of phenol with ozone or OH radicals in the atmosphere are dihydroxybenzenes, nitrophenols, and ring cleavage products (Cupitt, 1980). Reported rate constants for the reaction of phenol and OH radicals in the atmosphere 2.8 x 10 " cmVmolecule-sec at room temperature (Atkinson, 1985) and with NO3 in the atmosphere 2.1 x lO" cmVmolecule-sec at 296 K (Atkinson et al., 1984). 

Bruice and Sturtevant, (1959) and Bruice, (1959) found extremely facile intramolecular nucleophilic attack by neighbouring imidazole in the hydrolysis of p-nitrophenyl 7-(4-imidazoyl)butyrate [19]. The rate constant for imidazole participation (release of p-nitro-phenolate) in this reaction is nearly identical with the rate constant for a-chymotrypsin catalysed release of p-nitrophenolate ion [190 min in equation (11) at pH 7 and 25°] from p-nitrophenyl acetate. Comparison of the rate constant for intramolecular imidazole participation to that for the analogous bimolecular reaction (imidazole attack on p-nitrophenyl acetate) (s" /m s )... 

If an amine P-NH2 is used in the aqueous solution, one obtains RCONHP instead of RCOOH. Rates of cleavage of three acyl nitrophenyl esters were followed by the appearance of p-nitrophenolate ion as reflected by increased absorbances at 400 nm. The reaction was carried out at pH 9.0, in 0.02 M tris(hydroxymethyl)aminomethane buffer, at 25°C. Rate constants were determined from measurements under pseudo-first-order conditions, with the residue molarity of primary amine present in approximately tenfold excess. First-order rate graphs were linear for at least 80% of the reaction. With nitrophenyl acetate and nitrophenyl caproate, the initial ester concentration was 6.66xlO 5M. With nitrophenyl laur-ate at this concentration, aminolysis by polymer was too fast to follow and, therefore, both substrate and amine were diluted tenfold for rate measurements. 

To compare the catalytic effectiveness of our polymer with that reported for other substances that accentuate nitrophenyl ester cleavage, we26 have carried out a series of experiments (at pH 7.3) in which the residue molar concentration of polymer imidazole groups was substantially in excess of the concentration of substrate, p-nitrophenol acylate. Pseudo-first-order rate constants k[ were determined at each of a number of polymer concentrations. Under these conditions k[ was found to be linear with [P-Im]0, the initial residue concentration of methylene-imidazole groups ... 

For example, Beltran and Alvarez (1996) successfully applied a semi-batch agitated cell for the determination of kL k,a, and the rate constants of synthetic dyes, which react very fast with molecular ozone (direct reaction, kD = 5 105 to 1 108 L mol-1 s l). In conventional stirred tank reactors operated in the semi-batch mode the mass transfer coefficient for ozone kLa(03) was determined from an instantaneous reaction of ozone and 4-nitrophenol (Beltran et al., 1992 a) as well as ozone and resorchinol (l,3-c//hydroxybenzene) or phloroglucinol... 

When the ester is mixed with the enzyme, there is a rapid exponential phase followed by a linear increase in the absorbance due to the nitrophenol. The rate constant for acylation and the dissociation constant of the enzyme-substrate complex may be calculated from the concentration dependence of the rate constant for the exponential phases (Chapter 4, equation 4.46). (The rate constant of the linear portion gives the deacylation rate, but this is a steady state measurement.) Unfortunately, nitrophenyl esters are often so reactive that the acylation rate is too fast for stopped-flow measurement. 

The acylenzyme E—O—COOEt is rapidly formed but hydrolyzes slowly. Note that about 0.63 mol of p-nitrophenol is released per mole of enzyme in the burst. Either the enzyme is only 63% pure (active), or the rate constant formation of the acylenzyme is not sufficiently greater than that for deacylation for the acylenzyme to accumulate fully. [From B. S. Hartley and B. A. Kilby, Biochem. J. 56, 288 (1954).]... 

Solodovnikov (1976) studied the kinetics of the interaction of the 4-nitro-l-chlorobenzene with sodium methylate in dimethylsulfoxide in air via the method of spectrophotometry. Kinetic calculations were made in an assumption that all the anion radicals of 4-nitro-l-chlorobenzene are converted into 4-nitrophenolate. The calculations gave a sum of rate constants for formation of 4-nitroanisole and of the 4-nitro-l-chlorobenzene anion radicals close to the rate constant for the consumption of 4-nitro-l-chlorobenzene. Solodovnokov (1976) concluded that the anion radicals of 4-nitro-l-chlorobenzene are produced by a reaction parallel to substitution. Then it should be assumed that the reaction proceeds either by a nonradical mechanism or by a hidden radical mechanism, which implies that particles of a radical nature are produced and unite in a solvent cage without passing into a solvent pool. This conclusion generated objections (Shein 1983). The discussion deserves our consideration because it reveals features and limitations of the method for discerning the ion radical nature of a reaction. 

Hammett s equation was also established for substituted phenols from the elementary hydroxyl radical rate constants. The Hammett resonance constant was used to derive a QSAR model for substituted phenols. The simple Hammett equation has been shown to fail in the presence of electron-withdrawing or electron-donating substituents, such as an -OH group (Hansch and Leo, 1995). For this reason, the derived resonance constants such as o°, cr, and o+ were tested in different cases. In the case of multiple substituents, the resonance constants were summed. Figure 5.24 demonstrates a Hammett correlation for substituted phenols. The least-substituted compound, phenol, was used as a reference compound. Figure 5.24 shows the effects of different substituents on the degradation rates of phenols. Nitrophenol reacted the fastest, while methoxyphenol and hydroxyphenol reacted at a slower rate. This Hammett correlation can be used to predict degradation rate constants for compounds similar in structure. 

For the reduction of />-nitrophenol, where the irreversible step (35a) is replaced by a reversible two-electron process (36), another equation has been derived (91, 92) for the calculation of the rate constant k of reaction (35 b). The numerical values obtained for the rate constants are claimed (92) to be in agreement with the values obtained by chrono-potentiometric and potentiostatic methods. Reaction (35 b) has also been studied as a consecutive reaction to the reverse reaction of (35c), i.e. oxidation of -aminophenol. 

The oxidation of substituted phenols illustrates the importance of including speciation. Dissociation of the phenolic hydroxyl group results in an equilibrium mixture of the parent compound and its dissociated form, the phenoxide (or phenolate) anion. The undissociated phenol and the phenoxide anion react as independent species with very different rate constants, designated kArOU and kAr0. For the oxidation of 4-nitrophenol (pKa = 7.2) by C102, 1(1,0,11 = 1.4 x 10 1 M 1 s, andkArCr = 4.0 x 103 M 1 s 1 (42). Estimates of the pH-corrected second-order rate constant, kM, can be made using... 

Several kinds of evidence indicate that the reactions are catalytic rather than stoichiometric. When the reaction is followed to completion, linear first order plots are obtained for at least 90% of the reaction 7>. At the ratio of substrate to polymer employed, about 1 1 by weight, nonlinear first order plots would be predicted for a stoichiometric reaction. When a second aliquot of substrate is added after completion of the reaction, the first order rate constant noted with the second aliquot is essentially identical to that of the original7). The liberation of acetate and p-nitrophenol in equimolar proportions is also consistent with an inference of catalysis 7>. 

In an EC2j process, the initial ET step is followed by a second-order irreversible homogeneous reaction. For example, the feedback mode of SECM was employed to study the reductive hydrodimerization of the dimethyl fumarate (DF) radical anion [22]. The experiments were carried out in solutions containing either 5.15 or 11.5 mM DF and 0.1 M tetrabutylammonium tetrafluoroborate in A,A,-dimethyl form amide (DMF). The increase in the feedback current with increasing concentration of DF indicated that the homogeneous step involved in this process is not a first-order reaction. The analysis of the data based on the EC2 theory yielded the fc2 values of 180M-1 s-1 and 160M-1 s-1 for two different concentrations. Another second order reaction studied by the TG/SC mode was oxidative dimerization of 4-nitrophenolate (ArO-) in acetonitrile [23]. In this experiment, the tip was placed at a fixed distance from the substrate. The d value was determined from the positive feedback current of benzoquinone, which did not interfere with the reaction of interest. The dimerization rate constant of (1.2 0.3) x 108 M x s-1 was obtained for different concentrations of ArO-. 

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