Pseudo-first-order reaction rates with compounds

Reactions were studied under the pseudo first-order condition of [substrate] much greater than [initial dihydroflavin]. Under these conditions, the reactions are characterized by a burst in the production of Flox followed by a much slower rate of Flox formation until completion of reaction. The initial burst is provided by the competition between parallel pseudo first-order Reactions a and b of Scheme 3. These convert dihydroflavin and carbonyl compound to an equilibrium mixture of carbinolamine and imine (Reaction a), and to Flox and alcohol (Reaction b), respectively. The slower production of Flox, following the initial burst, occurs by the conversion of carbinolamine back to reduced flavin and substrate and, more importantly, by the disproportionation of product Flox with carbinolamine (Reaction c followed by d). Reactions c and d constitute an autocatalysis by oxidized flavin of the conversion of carbinolamine back to starting dihydroflavin and substrate. In the course of these studies, the contribution of acid-base catalysis to the reactions of Scheme 3 were determined. The significant feature to be pointed out here is that carbinolamine does not undergo an elimination reaction to yield Flox and lactic acid (Equation 25). The carbinolamine (N(5)-covalent adduct) is formed in a... 

Many hydrolysis reactions technically follow second-order kinetics, but since water is present in such an excess, its concentration negligibly changes with time. Thus, the rate is dependent only on the rate of decomposition of the drug in solution this type of reaction is termed a pseudo-first-order reaction. Most compounds that degrade in solution follow this order. Second-order kinetics are observed when a reactant reacts with itself or when the reaction rate depends on the concentration of more than one reactant. 

This first example is relevant for environments where reduced sulfur species (e.g., HS") are present (e.g., produced by microbial sulfate reduction). As already mentioned above, in the presence of hydrogen sulfide, dissolved natural organic matter (NOM) constituents may act as electron transfer mediators for NAC reduction (Figure 4). Such NOM constituents most probably include hydroquinone structures (11,60,64), and sulfur derivatives that result from addition reactions of quinone type structures with hydrogen sulfide (54). Dunnivant et al. (11) found that pseudo-first-order reduction rate constants, kobs (Figure 4), determined for a series of substituted nitrobenzenes and nitrophenols were proportional to NOM concentrations. For a given compound and at given conditions (T, pH, [H2S]tot), they calculated a carbon-normalized second-order rate constant, k oM... 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

Such a reaction is described as first order and the proportionality constant k is known as the rate constant. Such first-order kinetics is observed for unimolecular processes in which a molecule of A is converted into product P in a given time interval with a probability that does not depend on interaction with another molecule. An example is radioactive decay. Enzyme-substrate complexes often react by unimolecular processes. In other cases, a reaction is pseudo-first order compound A actually reacts with a second molecule such as water, which is present in such excess that its concentration does not change during the experiment. Consequently, the velocity is apparently proportional only to [A]. 

Table 10.7 illustrates the results of Gloyna and Li (1993) for treatment of hazardous wastes by SCWO. Pressure is given in psi, where 1 bar = 14.50 psi. The following equations describe global reaction rates for the SCWO of six different substituted phenols. The rates of phenolic compound expressions were obtained at 460°C and 250 atm with [organic] = 100 pmol/L and [02] = 7 mmol/L. This rate was calculated as pseudo first order for SCWO of each phenolic compound. 

Similarly, if the emission rate of the compound due to chemistry, Es, is a significant fraction of primary emission rates or the rate of entry by ventilation, then it is worth considering. Increases in reaction products may be relevant even if rate constants are small. The pseudo first-order rate constant for the reaction of OH with gas-phase organics is predicted to be small (Nazaroff and Weschler, 2004), but the reaction has been shown to be responsible for much aerosol generation (Fan et al., 2003). 

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