Solvent reaction rate constants

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 rate law draws attention to the role of component concentrations. AH other influences are lumped into coefficients called reaction rate constants. The are not supposed to change as concentrations change during the course of the reaction. Although are referred to as rate constants, they change with temperature, solvent, and other reaction conditions, even if the form of the rate law remains the same. 

Other considerations aside, the use of dilute reagents minimizes effects of nonideality. This allows the use of concentrations in place of activities. Of course, the time scale, the sensitivity of the analytical method at different concentrations, and the use of other reaction components introduce additional considerations. Tied closely to this decision is the choice of solvent. Reaction rates may (or may not) be affected by such variables as polarity, dielectric constant, hydrogen-bonding ability, donor capacity, and viscosity. A change in solvent may change not only the rate but also the mechanism and possibly even the products. One cannot even assume that the net reaction is the... 

Effect of solvent on reaction rate constant of reaction between carbon dioxide and glycidyl methacrylate using Aliquat 336 as a catalyst... 

Absorption rates of carbon dioxide were measured in organic solutions of glycidyl methacrylate at 101.3 kPa to obtain the reaction kinetics between carbon dioxide and glycidyl methacrylate using tricaprylylmethylammonium chloride(Aliquat 336) as catalysts. The reaction rate constants were estimated by the mass transfer mechanism accompanied by the pseudo-first-order fast reaction. An empirical correlation between the reaction rate constants and the solubility parameters of solvents, such as toluene, A-methyl-2-pirrolidinone, and dimethyl sulfoxide was presented. 

In this study, the absorption rates of carbon dioxide into the solution of GMA and Aliquat 336 in such organic solvents as toluene, N-methyl-2-pirrolidinone(NMP), and dimethyl sulfoxide(DMSO) was measured to determine the pseudo-first-order reaction constant, which was used to obtain the elementary reaction rate constants. 

The rate constants in organic reaction in a solvent generally reflect the solvent effect. Various empirical measures of the solvent effect have been proposed and correlated with the reaction rate constant [5]. Of these, some measures have a linear relation to the solubility parameter of the solvent. The logarithms of kj and k2/ki were plotted against the solubility parameter of toluene, NMP and DMSO[6] in Fig. 2. As shown in Fig.2, the plots satisfied the linear relationship. The solvent polarity is increased by the increase of solubility parameter of the solvent. It may be assumed that increase of unstability and solvation of Ci due to the increase of solvent polarity make the dissociation reaction of Ci and the reaction between Ci and COisuch as SNi by solvation[7] easier, respectively, and then, k2/ki and ks increases as increasing the solubility parameter as shown in Fig. 2. 

The overall reaction between CO2 and GMA was assumed to consist of two elementary reactions such as a reversible reaction of GMA and catalyst to form an intermediate and an irreversible reaction of this intermediate and carbon dioxide to form five-membered cyclic carbonate. Absorption data for CO2 in the solution at 101.3 N/m were interpreted to obtain pseudo-first-order reaction rate constant, which was used to obtain the elementary reaction rate constants. The effects of the solubility parameter of solvent on lc2/k and IC3 were explained using the solvent polarity. 

Fig.2. Relationship between reaction rate constant and solubility parameter of solvent in the reaction of CO2 with GMA using Aliquat 336 at 85 C.

The theoretical approach involved the derivation of a kinetic model based upon the chiral reaction mechanism proposed by Halpem (3), Brown (4) and Landis (3, 5). Major and minor manifolds were included in this reaction model. The minor manifold produces the desired enantiomer while the major manifold produces the undesired enantiomer. Since the EP in our synthesis was over 99%, the major manifold was neglected to reduce the complexity of the kinetic model. In addition, we made three modifications to the original Halpem-Brown-Landis mechanism. First, precatalyst is used instead of active catalyst in om synthesis. The conversion of precatalyst to the active catalyst is assumed to be irreversible, and a complete conversion of precatalyst to active catalyst is assumed in the kinetic model. Second, the coordination step is considered to be irreversible because the ratio of the forward to the reverse reaction rate constant is high (3). Third, the product release step is assumed to be significantly faster than the solvent insertion step hence, the product release step is not considered in our model. With these modifications the product formation rate was predicted by using the Bodenstein approximation. Three possible cases for reaction rate control were derived and experimental data were used for verification of the model. 

The term reaction rate constant is actually a misnomer, since k may vary with temperature, the solvent for the reaction, and the concentrations of any catalysts that may be present in the reaction system. The term is in universal use, however, because it implies that the parameter k is independent of the concentrations of reactant and product species. 

An exothermic reaction with the stoichiometry A 2B takes place in organic solution. It is to be carried out in a cascade of two CSTR s in series. In order to equalize the heat load on each of the reactors it will be necessary to operate them at different temperatures. The reaction rates in each reactor will be the same, however. In order to minimize solvent losses by evaporation it will be necessary to operate the second reactor at 120 °C where the reaction rate constant is equal to 1.5 m3/kmole-ksec. If the effluent from the second reactor corresponds to 90% conversion and if the molal feed rate to the cascade is equal to 28 moles/ksec when the feed concentration is equal to 1.0 kmole/m3, how large must the reactors be If the activation energy for the reaction is 84 kJ/mole, at what temperature should the first reactor be operated ... 

Since the second reaction rate constant is orders of magnitude greater than the first at temperatures near room temperature, the first reaction may be regarded as the rate controlling step. Since ethanol is used as the solvent, the reaction will follow pseudo first-order kinetics. The rate of this liquid phase reaction can be expressed as... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

The determination of large values of the rate constant ratio ks/kpfrom the low yields of alkene product that forms by partitioning of carbocations in nucleophilic solvents. These rate constant ratios may then be combined with absolute rate constants for the overall decay of the carbocation to give absolute values of kp (s ).14 16 For example, the reaction of the l-(4-methylphenyl)ethyl carbocation in 50/50 (v/v) trifluoroethanol/water gives mainly the solvent adducts and a 0.07% yield of 4-methylstyrene from proton transfer to solvent, which corresponds to kjkp = 1400. This can be combined with ks = 6 x 109 s V4 to give kp = 4.2 x 106 s l (Table 1). 

It is found that the rate of substitution reaction between Mn(CO)sBr and As(C6H5)3 varies somewhat with the solvent. The rate constant at 40 °C when the solvent is cyclohexane is 7.44 X 10-8 sec-1, and when the solvent is nitrobenzene it is 1.08 X 10-8 sec-1. In light of the principles described in Chapter 6, what does this observation indicate about the mechanism of the reaction What would you expect a reasonable value for the rate constant to be if the solvent is chloroform See Table 6.7. 

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],... 

The addition of dioxygen to sulfonyl radicals occurs very rapidly and is also limited by the diffusion of reactants in the solvent. The rate constant of the reaction... 

The steric factor P is high enough for the bimolecular reaction to occur when two radicals met in the cage. This reaction is limited only by translational diffusion of the reacting radicals and depends on the viscosity of the solvent. The rate constant of such reaction is close to the frequency of encounters of radicals, namely, k = a, kn = 0.25 x 47rrABZ)AB = RT 100007] where spin-statistical factor, rAB is the sum of radii of reactants A and B, and DAB is the sum of their diffusion coefficients. 

Reaction rate constants, 21 340 pressure variation and, 13 406 407 of solvents, 10 107 Reaction rates, relative, 10 425 Reactions. See also Chemical reactions Inorganic chemistry reactions Organic chemistry reactions hydrogen peroxide, 14 38—39 methods of initiating, 13 422 microfluidic control of, 26 967—968 Reaction schemes/mechanisms, in kinetic studies, 14 623-625 Reaction solvents, in large-scale... 

It is well known that the surrounding solvent environment plays a crucial role in a chemical reaction. For example, the formation of tetraethylammonium iodide has been studied in many nonpolar and polar solvents. It is found that the rate of the reaction is quite sensitive to the solvent. From the least polar (hexane) to the most polar (nitrobenzene) solvent, the rate constant increases by 2700 times [1]. The polar transition state of this reaction is stabilized in a high dielectric constant medium. Since the... 

The role of solvent dynamics in the electron transfer reaction at a Pt electrode was discussed utilizing the theory of Zusman. In this work, ° the solvent-dependent rate constant for the electron transfer reaction at an electrode was given as... 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

Comparison of polystyrene-supported phosphonium ion catalysts 1 in the reaction of 1-bromooctane with iodide ion showed a 4.5 % CL catalyst to be only half as active as a 2% CL catalyst74 . Use of decane, toluene and o-dichlorobenzene as solvents gave rate constants that increased as the swelling ability of the solvent increased. Swelling ratios were measured at 90 °C, the reaction temperature. 

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

Sherwood and Pigford (S9) have discussed the problem of the absorption of a solute A by a solvent S upon solution, A may be converted into B according to the reaction A = B (k/ and krf being the forward and reverse reaction-rate constants, and K = k//k/). The concentration of A is maintained at cAo at the surface of the liquid S, and it is assumed that S is semiinfinite in extent. It is further assumed that B is nonvolatile that is, it cannot escape from solvent S. Equation (51) is then used to explain the diffusion of A and B, with DAg and DBs taken as concentration independent, and the term containing the molar average velocity w is neglected. Hence the mathematical statement of the problem is (for very dilute solutions of A and B)... 

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