Factors Affecting the Reaction Rates

S. Signorella, S. Garcia, and L. Sala, An easy experiment to compare factors affecting the reaction rate of structurally related compounds, J. Chem. Ed., 76 (1999) 405 108. 

We will use the chlorination of methane to show how we study a reaction. Before we can propose a detailed mechanism for the chlorination, we must learn everything we can about how the reaction works and what factors affect the reaction rate and the product distribution. 

Equations (2.4), (2.6), and (2.7) are also applicable to the degradation of drug substances in the solid state. However, the factors affecting the reaction rates become more complex because reactions often proceed in heterogeneous physical states. For example, apparent reaction rates depend on solubility and dissolution rates of drug substances when degradation proceeds in water layers adsorbed on the surface of solid drugs. Therefore, these and other additional factors need to be considered. 

Oxidative degradation of a crystalline polyolefin is a complex reaction involving a dissolved gas and a two-phase, impure, inhomogeneous solid. Factors affecting the reaction rate are antioxidant concentrations, crystallinity, UV illumination intensity, UV absorber concentration, and the samples previous oxidation history. Failure of a sample is often mechanical rather than chemical, and cannot be regarded as occurring at a particular degree of oxidation. 

Several therapeutically used penicillin preparations contain carbohydrates. Thus, penicillins are commonly infused in glucose or fructose solutions, sucrose is a common ingredient of penicillin syrups and benzylpenicillin procaine suspensions may contain carboxymethylcellulose as an additive. Because of the antigenic and potentially allergenic effect of penicilloyl-carbohydrate conjugates, their presence in penicillin preparations must be avoided. Information about the kinetics of reaction of penicillins with carbohydrate and factors affecting the reaction rate have recently been reported. 

The factors affecting the catalytic activity are more numerous than in the case of soluble catalysts, and so their combination to obtain optimum results is much more complex. A fundamental role is played by the chemical stability of the catalyst and the mechanical stability of the matrix, with diffusive factors affecting the reaction rate as well. 

Extensive DFT calculations have revealed the mechanism and factors affecting the reaction rate for the intramolecular acid-catalysed hydrolysis of a series of seven iV-methylmaleamic acids (107 R, R = H, H H, Me H, Et Me, Me H, Pr -( 112)3- -(012)4-). The results confirm the three-step pathway formulated from the classical experimental work of Kirby and coworkers in which following a proton transfer from the CO2H group of (107) to the adjacent C=0 group, an intramolecular attack by the carboxylate group of zwitterion (108) yielded a tetrahedral intermediate (109), which after an H-transfer from OH to NHMe to give (110) broke down to a lactone (111) and methylamine (Scheme 35). In the gas phase, the rds was the formation of the tetrahedral intermediate (109), but in solution it was its breakdown. ... 

In Eq. (149) we assume that allosteric regulation affects the reaction rates as a multiplicative factor h(I). Following Ref. [161], a generic functional form for an inhibitory effector is... 

The alkene structure and the solvent polarity markedly affect the reaction rate. However, these effects are not easy to rationalize since, as shown in equation 90, one or more intermediates may be involved and each factor can influence the individual rate constants in a different way. It follows that only when the first step is rate determining can the observed rate constant k0bsd t>e interpreted straightforwardly. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

What other factors might affect the reaction rate of the enzyme catalyzed system shown in Figure 11.10 Which of these can be easily controlled Which would be very difficult to control Which would be expected to exert a large effect on the system Which would be expected to exert only a small effect on the system [See Problem 1.1.]... 

An understanding of reaction rates can be explained by adopting a collision model for chemical reactions. The collision theory assumes chemical reactions are a result of molecules colliding, and the rate of the reaction is dictated by several characteristics of these collisions. An important factor that affects the reaction rate is the frequency of collisions. The reaction rate is directly dependent on the number of collisions that take place, but several other important factors also dictate the speed of a chemical reaction. 

In comparing the general and the simple equations, it is seen that the transfer coefficients play the same role in a multistep, n-electron-transfer reaction as the symmetry factor does in one-step, one-electron transfer reaction, i.e., thea s determine how the input electrical energy (Ft)) affects the reaction rate. Table 15 shows the tabulation of values for y, r, v, y, and n, from which a and a have been evaluated. 

Factor affecting the reaction Types of reaction affected Change made in the condition Effect on rate of reaction... 

High liquid holdup may cause the liquid-phase diffusional resistance to the gaseous reactant to be an important factor affecting the global rate of reaction. 

Since the forward reaction in (29) is exothermic, the equilibrium is displaced to the left by increase in temperature this factor accounts in part for the anomalous temperature coefficient of reaction rate mentioned above. The apparent catalysis by propagating base is also explicable as acid catalysis since the carbamic acid is stoichiometrically derived from the base by reaction (29). That true base catalysis is not operative has been shown by the observation that addition of tertiary bases does not affect the reaction rate [17]. Further, the polymerization is catalysed by other weak acids such as hydrocinnamic [17] and a-picolinic acids [10, 17], which, if present in sufficient concentration under conditions of low CO2 pressure, reduce the order in initiating base to unity. Thus, under such conditions, with hydrocinnamic acid (HX) as catalyst the simple kinetic form (30) is achieved. 

Factors Affecting the Overall Rate of Absorption-Reaction (Lll)... 

The interplay of all these different factors affects the overall rate for any specific substitution reaction. However, given any two compounds, it is usually possible to predict which will react the faster by balancing the various factors against each other. 

To measure enzyme activity reliably, all the factors that affect the reaction rate-other than tlie concentration of active enzyme—must be optimized and rigidly controlled. Furthermore, because the reaction velocity is at or near its maximum under optimal conditions, a larger analytical signal is obtained that can be more accurately and precisely measured than a smaller signal obtained under suboptimal conditions. Much effort has therefore been devoted to determining optimal conditions for measuring the activities of enzymes of clinical importance. 

Although the processes may be described by rather simple equations, there are many factors that can affect the reaction rate and whether or not equilibrium is attained. Each process may occur under a wide variety of conditions of temperature, oxygen partial pressure, and presence of organisms and catalysts. Many of the mechanisms may not be understood in detail and the concentrations of key reactants may be poorly characterized. It is often necessary to parameterize the rates of these processes rather than base them on fundamental thermodynamic and kinetic data. 

The viscosity which yields the maximum reaction rate, shifts to a lower value with increasing temperature in GTA. In KF-54, however, the maximum viscosity stays almost constant. This behavior suggests the existence of a factor which affects the reaction rate but does not contribute to the macroscopic viscosity. If this second contribution to the microscopic friction decreases with increasing temperature, the behavior shown in Fig. 3.28 is to be expected. This second factor may be related to local segmental rotation of the siloxane chain. However, further experiments are necessary for detailed analysis. 

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