Sequential reactions, kinetics

Saddle point. 170 Salt effects. 206-214 Scavenging (see Reactions, trapping) Second-order kinetics. 18-22, 24 in one component, 18-19 in two components (mixed), 19-22 Selectivity. 112 Sensitivity analysis. 118 Sensitivity factor, 239-240 Sequential reactions (see Consecutive reactions)... 

This approach uses a kinetic sequential principle to carry out multicomponent CL-based determinations. In fact, when the half-lives of the CL reactions involved in the determination of the analytes in mixture are appreciably different, the CL intensity-versus-time curve exhibits two peaks that are separate in time (in the case of a binary mixture) this allows both analytes to be directly determined from their corresponding calibration plots. In general, commercially available chemiluminometers have been used in these determinations, so the CL reaction was initially started by addition of one or two reaction ingredients. Thus, in the analysis of binary mixtures of cysteine and gluthatione, appropriate time-resolved response curves were obtained provided that equal volumes of peroxidase and luminol were mixed and saturated with oxygen and that copper(H) and aminothiol solutions were simultaneously injected [62, 63],... 

The design of the mass spectrometer may influence its use in a particular kind of measurement. The study of electronic state-specific ions and their reactions has mainly been carried out using the GIB method. Metastable ions (ions produced by the ionization process but decomposing on the way to detection) can be observed in many of Type (1) mass spectrometers and metastable ions aid our understanding of the ionization process and stability of ions. Sequential reactions and kinetic studies of ion-molecule reactions are difficult with the simpler mass spectrometers of Type 1 and so more complex hybrid mass spectrometers have to be used. The ions observed in micro- or milliseconds after the ionization process may or may not be the same as ion observed seconds after the ionization process, which is a limitation in the use of Type 1 mass spectrometers. 

At about the same time, hydroxamic adds and oximes were found to react directly with organophosphorus compounds.85 2-PAM I was found to react In vitro with sarin with marked deviation from first-order kinetics that suggested that the reaction actually consists of (at least) two sequential reactions. Green showed that quatemlzed pyridine aldoximes react with an organophosphorus (OP) compound In three steps ... 

Although the simple rate expressions, Eqs. (2-6) and (2-9), may serve as first approximations they are inadequate for the complete description of the kinetics of many epoxy resin curing reactions. Complex parallel or sequential reactions requiring more than one rate constant may be involved. For example these reactions are often auto-catalytic in nature and the rate may become diffusion-controlled as the viscosity of the system increases. If processes of differing heat of reaction are involved, then the deconvolution of the DSC data is difficult and may require information from other analytical techniques. Some approaches to the interpretation of data using more complex kinetic models are discussed in Chapter 4. 

Kinetic models which consider demetallation as a complex reaction network of consecutive and parallel reactions taught by model compound studies have been recognized with real feedstocks. Tamm et al. (1981) suggest a sequential mechanism where the metal compounds are activated by H2S. Model compound reaction pathway studies in the absence of H2S, discussed in Section IV,A,1, and experiments in which H2S was present in excess (Pazos et al., 1983) indicate that sequential reactions are inherent to the chemistry of the metal compounds irrespective of the presence of H2S. However, it is possible that both mechanisms contribute to metal removal. 

Shein (1983) pointed out that Solodovnikov s (1976) kinetic result may also be caused by a sequential reaction that led to the formation of the anion radicals of 4-nitro-1 -chlorobenzene and then of 4-nitroanosole. This is really possible when, say, the formation of the anion radical of the initial substrate as a result of its interaction with the methylate anion is the... 

Valuable insights into how DNA polymerases process their substrates were obtained as a result of detailed kinetic studies of the enzymes. Benkovic and coworkers employed rapid quenching techniques to study the kinetics of transient intermediates in the reaction pathway of DNA polymerases [5]. Intensive studies revealed that E. coli DNA polymerase I follows an ordered sequential reaction pathway when promoting DNA synthesis. Important aspects of these results for DNA polymerase fidelity are conformational changes before and after the chemical step and the occurrence of different rate-limiting steps for insertion of canonical and non-canonical nucleotides. E. coli DNA polymerase I discriminates between canonical and non-canonical nucleotide insertion by formation of the chemical bond. Bond formation proceeds at a rate more than several thousand times slower when an incorrect dNTP is processed compared with canonical nucleotide insertion. 

Type II deiodinase activity is low in unsupplemented tissue homogenates but is stimulated by DTT [71-74,82,83] and to a lesser extent also by GSH [72]. The DTT concentrations required for maximal enzyme stimulation in the CNS and pituitary seem higher than in BAT and also than those necessary for the type I deiodinase in liver and kidney. Kinetic analysis of the deiodination of varying substrate (T4, rT3) concentrations at different cofactor (DTT) levels have indicated a sequential reaction mechanism for the type II deiodinase [73,82,83]. This is very suggestive of the formation of a ternary enzyme-substrate-cofactor complex in the catalytic process [82], The physiological cofactor of the type II deiodinase has not been identified but it has been observed that GSH depletion with diamide or diethylmaleate impairs T4 to T3 conversion in GH3 pituitary tumor cells [93]. 

Prerequisite for hydrodemetallisation is the diffusion of the large porphyrins into the catalyst porous texture prior to the sequential reaction mechanism. Diffusion of these large molecules can be limited by geometric exclusion and hydrodynamic drag. When the solute molecular size is significant as compared to the pore size, a restrictive factor can be introduced to account for the reduction in difftisivity. As a consequence, clarification of detailed HDM reaction kinetics may be obscured by diffusion limitations. 

Simulation, Modeling, and Design Feasibility Because reaction and separation phenomena are closely coupled in a reactive distillation process, simulation and design are significantly more complex than those of sequential reaction and separation processes. In spite of the complexity, however, most commercial computer process modeling packages offer reliable and flexible routines for simulating steady-state reactive distillation columns, with either equilibrium or kinetically controlled reaction models... 

The main deficiencies with the preceding approaches are that only arylene-amines give good yields (and there are other routes to 5-arylimidazolcs), and since unsubstituted vinylamines (R = H Scheme 4.3.3) are not available this means that 4- and 5-unsubstituted imidazoles are not accessible. An alternative route involving silyl enol ethers (6) has been reported to overcome these deficiencies (Scheme 4.3.4). Silyl enol ethers can be made either by treating a ketone with chlorotrimethylsilane and triethylamine in DMF solution, or by sequential reactions of the ketone with LDA and chlorotrimethylsilane in 1,2-dimethoxyethane. This normally gives a mixture in which the less highly substituted enol ether is the major product (enolate formation is kinetically controlled) [25], When (6) is heated with an At-chloroamidine for 12-24h in chloroform solution in the presence of an equimolar amount of dry pyridine 1,2-disubstituted (5) (R = H) or 1,2,5-trisubstituted (5) imidazoles are... 

The kinetic equations for this simple sequential reaction can be solved analytically and the rate depends on two rate constants, and (see Scheme 1.2). The value of k is measured independently by similarly monitoring the time dependence of the intensity of the ethylidyne infrared signal of an initially ethylidyne-covered surface in the presence of hydrogen using infrared spectroscopy [69]. 

Further support for the concept of a sequential reaction leading to metals deposition comes from an analysis of the deposition profiles of vanadium [17]. A kinetic analysis of a two step process for V removal was developed, in which the first step was suggested to involve hydrogenation of the V containing porphyrin, while the second produces deposits of vanadium sulphide on the surface. Increased levels of HzS in the gas stream inhibited HDS of the material but increased HDV. For vanadium, the second reaction leading to metal deposits, with levels of H2S expected in an industrial reactor, was found to be some 85 x faster than the first hydrogenation reaction [17]. Under these conditions, it is not surprising that the bulk of the deposition of vanadium occurs near the pellet surface. 

The deposited solid is the final result of all these subprocesses. The rate of the subprocess varies in a wide range. For these sequential reactions or subprocesses the overall deposition rate is controlled by the slowest subprocess. In this section the CH3SiCl3-H2 deposition system has been chosen as the system for discussion in both homogeneous chemical reactions and heterogeneous chemical reactions in order to give a more in-depth understanding of the kinetics of a chosen CVD process. 

This problem prompted a closer examination and ultimately a rederivation of the theory describing the link between mechanistic features in generalized sequential reaction schemes and the values of experimentally accessible transfer coefficients upon which the conclusions on mechanism were based. We endeavor here to develop this hnk, which is built upon the quasi-equilibrium approximation for dealing with the kinetics of multistep reactions, clearly and concisely, giving attention to the limits of its application. We hence justify its significance in relation to determination of the reaction mechanism. 

One interesting aspect of asymmetric catalysis is that sequential reactions with a chiral catalyst can often lead to an enhancement in the enantioselectivity over a single transformation with the same catalyst in a process called kinetic amplification. Doyle was able to exploit this phenomenon in the synthesis of novel tricyclic products from the bis-diazoacetate of irans-1,4-cyclohexanediol (56, Scheme 12) [71]. Although formation of C2-symmetric product 58 was expected, resulting from the typically preferred five-membered insertion event, it was found that 57 could be produced preferentially with appropriate choice of catalyst, and with very high ee (95-99%). Bis- )-lactone 59 was never the major product, but could be formed as up to 34% of the product mixture. Notably, similar catalyst-controlled mixtures of [1- and y-lactone products were also obtained with diazoacetates derived from cholesterol derivatives [72],... 

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