Correlations solvent parameter

There are numerous attempts to correlate solvent parameters with the reaction rate of Diels-Alder reactions122. Examples are the Brownstein Polarity Parameter S123, the Solvophobicity Parameter Sp124,125 the D-it parameter (based on the solvent effect on the reaction of tetracyanoethylene and diazodiphenylmethane with benzene as the reference solvent)126 or the Acceptor Number / /V127, l2X (a parameter which describes the ability of a solvent to act as an electron pair acceptor)129. These examples included either reactions that were next to insensitive to solvent effects (like that in Table 9) or reactions in which the reactants mainly interact with the electron pair on the donor atom of the solvent130. 

There are numerous attempts to correlate solvent parameters with the reaction rate of Diels-Alder reactions. Examples are the Brownstein Polarity Parameter 5, the Solvophobicity Parameter the D-n parameter (based on the solvent effect on... 

Predicting the solvent or density dependence of rate constants by equation (A3.6.29) or equation (A3.6.31) requires the same ingredients as the calculation of TST rate constants plus an estimate of and a suitable model for the friction coefficient y and its density dependence. While in the framework of molecular dynamics simulations it may be worthwhile to numerically calculate friction coefficients from the average of the relevant time correlation fiinctions, for practical purposes in the analysis of kinetic data it is much more convenient and instructive to use experimentally detemiined macroscopic solvent parameters. 

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

Univariate LSERs may possess the conventional LEER form, as exemplified by Eq. (8-67), the Grunwald-Winstein equation, or they may simply be plots of log k against a solvent parameter such as Z, (30), or ir. Brownstein developed an LEER form for the latter type of correlation, writing... 

D. Miscellaneous.—A study of the racemization of ( + )-methylphenyl-n-propylphosphine has shown that the rate of racemization has no dependence on solvent polarity and could not be correlated with any known solvent parameters. ... 

The best-known solvent parameters are the donor number [21] and acceptor number [22] proposed by Gutmann and coworkers. The donor number (DN) for a donor solvent D is defined as the positive value of the enthalpy difference AH (kcalmol ) for the reaction of D with an acceptor-halide SbCls (D + SbCls D SbCls) in an inert medium such as 1,2-dichloroethane. DN is a fair measure for the donor properties of a solvent. The correlations of DN with the solvation energies are known to be good particularly for solvation of cations. A typical example [19] is shown in Fig. 3. 

AN is known to show good correlations with the solvation energies of anions. Also, AN has good correlations with other solvent parameters defined in different reaction systems, e.g., Grunwald and Winstein s T-value [24], Kosower s Z-value [25], Dimroth and Reichardt s T Value [26,27], etc. 

Many approaches have been used to correlate solvent effects. The approach used most often is based on the electrostatic theory, the theoretical development of which has been described in detail by Amis [114]. The reaction rate is correlated with some bulk parameter of the solvent, such as the dielectric constant or its various algebraic functions. The search for empirical parameters of solvent polarity and their applications in multiparameter equations has recently been intensified, and this approach is described in the book by Reich-ardt [115] and more recently in the chapter on medium effects in Connor s text on chemical kinetics [110]. 

Ab initio SCRF/MO methods have been applied to the hydrolysis and methanol-ysis of methanesulfonyl chloride (334). ° The aminolysis by aromatic amines of sulfonyl and acyl chlorides has been examined in terms of solvent parameters, the former being the more solvent-dependent process.Solvent effects on the reactions of dansyl chloride (335) with substituted pyridines in MeOH-MeCN were studied using two parameters of Taft s solvatochromatic correlation and four parameters of the Kirkwood-Onsager, Parker, Marcus and Hildebrand equations. MeCN solvent molecules accelerate charge separation of the reactants and stabilize the transition... 

The mobile phase and the nature of H-donor and non-donor solvents all have a profound influence on primary conversions. In contrast, distillate or oil yields often correlate with parameters reflecting the aliphaticity of coals (H/C ratio -decreasing vitrinite reflectance - CHj content - Z), better correlations being achieved for low-rank coals if yields are expressed on a free" basis... 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

Solvent effects on enzymatic reactions have been most thoroughly studied for esterification reactions. It has been observed that those reactions are favorably carried out in relatively hydrophobic solvents, while the equilibrium position is less favorable for esterification in more hydrophilic solvents. Correlations between equilibrium constants and solvent parameters have been evaluated. It was shown that the solubility of water in the solvent (Sw/0) gave better correlation with esterification equilibrium constants than log P and other simple solvent descriptors [61]. 

One other consideration should always be borne in mind when attempting to correlate quantitatively the rate of reaction with solvent parameters, which is that the mechanism of the reaction must remain constant. Indeed, the breakdown of the linear plot of Ink vs. 1/sr for ionic reactions... 

Solvent effects on the kinetics and mechanism of unimolecular heterolysis of commercial organohalogen compounds have been investigated.9-11 The reaction rate is satisfactorily correlated by parameters for polarity, electrophilicity, and cohesion of the solvent, whereas the solvent nucleophilicity and polarizability exert no effect. 

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