Solvent effects rate constant

Solvent Effects. Rate constants and activation parameters for the reactions... 

As is inversely proportional to solvent viscosity, in sufficiently viscous solvents the rate constant k becomes equal to k y. This concerns, for example, reactions such as isomerizations involving significant rotation around single or double bonds, or dissociations requiring separation of fragments, altiiough it may be difficult to experimentally distinguish between effects due to local solvent structure and solvent friction. 

The transition state of this reaction has a polar structure and therefore this reaction occurs more rapidly in polar solvents (compare rate constants in chlorobenzene and /V,/V-dimethylforma-mide for reactions of styrene and butyl methacrylate in Table 4.4). The effect of multidipole interaction was observed for reactions of polyatomic esters [47 49],... 

A series of steady-state fluorescence experiments were performed in mixtures of propanol and glycerol to investigate the effect of viscosity on the effective second order photosensitization rate constant, k2. Figure 3 illustrates that the effective rate constant decreases as the viscosity of the system is increased. For example, as the reaction solvent is changed from pure propanol to pure glycerol, the viscosity of the system rises by three orders of magnitude, while the effective reaction rate coefficient, k2, decreases by approximately one order of magnitude. 

Effect of Non leaving Ligands on the Solvent Exchange Rate Constants on First-Row Divalent Transition-metal... 

Consider the striking effect of glycerol in reducing the solvent exchange rate constant (Sec. 4.2(a)) of all the first row transition metal ions. 

TABLE 1.2. Effect of Solvent on Rate Constants kg (s 20 °C) for Decay of Cations"... 

In contrast, the need to evaluate the relative rates of competing radical reactions pervades synthetic planning of radical additions and cyclizations. Further, absolute rate constants are now accurately known for many prototypical radical reactions over wide temperature ranges.19,33 3S These absolute rate constants serve to calibrate a much larger body of known relative rates of radical reactions.33 Because rates of radical reactions show small solvent dependence, rate constants that are measured in one solvent can often be applied to reactions in another, especially if the two solvents are similar in polarity. Finally, because the effects of substituents near a radical center are often predictable, and because the effects of substituents at remote centers are often negligible, rate constants measured on simple compounds can often provide useful models for the reactions of complex substrates with similar substitution patterns. 

Experiments aimed at probing solvent dynamical effects in electrochemical kinetics, as in homogeneous electron transfer, are only of very recent origin, fueled in part by a renaissance of theoretical activity in condensed-phase reaction dynamics [47] (Sect. 3.3.1). It has been noted that solvent-dependent rate constants can sometimes be correlated with the medium viscosity, t] [101]. While such behavior may also signal the onset of diffusion-rather than electron-transfer control, if the latter circumstances prevail this finding suggests that the frequency factor is controlled by solvent dynamics since td and hence rL [eqn. (23), Sect. 3.3.1] is often roughly proportional to... 

The solvent effect on the bimolecular rate constant of a Diels-Alder reaction is usually quite small. As a rule, in going from nonpolar to polar solvents, the rate constant increases only by a factor of about 3... 20 [34, 35, 121-130, 531-537]. 

As measured by the criteria of stereospecificity, regioselectivity, kinetic isotope effects, and solvent effects [117-120, 541-543], 1,3-dipolar cycloaddition reactions represent orbital symmetry-allowed [n + n s] cycloadditions, which usually follow concerted pathways Diels-Alder reactions and 1,3-dipolar cycloadditions resemble each other, as demonstrated by the small solvent effects on their bimolecular rate constants. In going from nonpolar to polar solvents, the rate constants of 1,3-dipolar cycloadditions change only by a factor of 2... 10 [120, 131-134]. 

The dissociation rate of the dimer of the triphenylmethyl radical in 28 solvents was studied by Ziegler el al. [167]. The decomposition rate of azobisisobutyronitrile in 36 solvents was measured by different authors [183-185, 562], Despite the great variety of solvents, the rate constants vary only by a factor of 2... 4. This behaviour is typical for reactions involving isopolar transition states and often indicates, but does not prove, a radical-forming reaction. The lack of any marked solvent effects in most free-radical forming reactions will become more apparent after an examination of some further reactions presented in Table 5-8. 

Effect of Solvent Concentration on Reaction Rate. In the kinetic modeling of chemical (such as kraTt) pulping, the effect of cooking liquor on delignificatlon rate Is sometimes considered. For example, the alkali concentration [0H ] can be Included In the rate equation (12,13,32). Since the extraction experiments In this study have been conducted under constant solvent flow (1 g/mln) and the solvent ratios In binary fluid extractions have been maintained at a constant, one can combine the solvent concentration factor Into an effective rate constant (keff). Therefore, Equation 13 can be rewritten as ... 

Because kinetic studies have been carried out employing a range of solvents, it is worthwhile to compare the magnitude of the effect of several solvents on rate constants. In keeping with a reactant-like transition state, the reactivity of pyridine toward Mel only increases by factors of 7 (25°) and 4.5 (35°) on changing from nitrobenzene to dimethyl sulfoxide (DMSO) and to sulfolane, respectively. Similar small changes are found with five-membered ring nucleophiles. ... 

Note that r and the diffusion coefficient D have cancelled from Equation 2.29, because D is inversely proportional to the molecular radii r /2. Hence the rate constant kd depends only on temperature and solvent viscosity in this approximation. A selection of viscosities of common solvents and rate constants of diffusion as calculated by Equation 2.29 is given in Table 8.3. The effect of diffusion on bimolecular reaction rates is often studied by changing either the temperature or the solvent composition at a given temperature. For many solvents,54-56 although not for alcohols,57 the dependence of viscosity on temperature obeys an Arrhenius equation, that is, plots of log rj versus 1 IT are linear over a considerable range of temperatures and so are plots of log(kdr]/T) versus 1/T.56... 

The first systematic investigation on the influence of solvent on reaction rates was reported by Menschutkin(l) as long ago as 1890. Quite soon after this study, chemists began to consider whether or not solvent influences on reaction rates were connected with the effect of solvents on the reactants (i.e. with initial-state effects). However, a careful and extensive investigation by Von Halban(2) in 1913 showed conclusively that for the reaction of trimethylamine with p-nitrobenzyl chloride, solvent effects on the reactants could not account quantitatively for the overall influence of solvent on the reaction rate constant. Little further progress was made on these lines until the advent of transition state theory, when it then became clear that in principle it was possible to dissect the influence of solvent on rate constants into initial-state and transition-state contributions(3-5). 

The rate constants and activation parameters for the electron selfexchange reactions of the [CrCCNdipp) ] and [Cr(CNdipp)g] (CNdipp = 2,6-diisopropylphenyl isocyanide) couples have been determined by H NMR line-broadening as a function of solvent. The rate constants show little variation with the nature of the nonaqueous solvent and agree well with predictions of the Marcus model. The variation in the activation parameters with solvent, displaying an enthalpy/entropy compensation, is not well predicted by the Marcus model. Added BF has little effect on the rate constants and activation parameters (AV = +2 to +11 cm mol ) for the... 

Design an orthogonal array to investigate the effects of the pressure, temperature, cosolvent level (methanol), and extraction time on the yield of raw material. The orthogonal matrix contains four factors, and each factor includes three levels. As a rule, the flow rate of SCFs is 25 kg/h see Notes 8-11) (Table 1). Keep other independent variables (e.g., sample size and solvent flow rate) constant during the above experiments. 

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