Solvents effects on rates

Table 8-1. Predicted Solvent Effects on Rates of Sn Reactions...

Ultimately physical theories should be expressed in quantitative terms for testing and use, but because of the eomplexity of liquid systems this can only be accomplished by making severe approximations. For example, it is often neeessary to treat the solvent as a continuous homogeneous medium eharaeterized by bulk properties such as dielectric constant and density, whereas we know that the solvent is a molecular assemblage with short-range structure. This is the basis of the current inability of physical theories to account satisfactorily for the full scope of solvent effects on rates, although they certainly can provide valuable insights and they undoubtedly capture some of the essential features and even cause-effect relationships in solution kinetics. Section 8.3 discusses physical theories in more detail. 

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

We now consider a type of analysis in which the data (which may consist of solvent properties or of solvent effects on rates, equilibria, and spectra) again are expressed as a linear combination of products as in Eq. (8-81), but now the statistical treatment yields estimates of both a, and jc,. This method is called principal component analysis or factor analysis. A key difference between multiple linear regression analysis and principal component analysis (in the chemical setting) is that regression analysis adopts chemical models a priori, whereas in factor analysis the chemical significance of the factors emerges (if desired) as a result of the analysis. We will not explore the statistical procedure, but will cite some results. We have already encountered examples in Section 8.2 on the classification of solvents and in the present section in the form of the Swain et al. treatment leading to Eq. (8-74). 

PEDRIELLI p, PEDULLi G F and SKIBSTED L H (2001a) Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-braining reaction of quercetin and epicatechin in autoxidation of methyl linoleate, JAgric Food Chem, 49, 3034-40. 

Full -polarization in diradicals can give rise to zwitterionic products. First examples were studied in detail by Carpenter and coworker who investigated solvent effects on rates and product distribution in Myers-Saito cyclizations.64 Polar solvents and substitution patterns that stabilize either positive or negative charges (or both) favor the zwitterionic products. For example, the presence of a dimethylamino group leads to stabilization of cations and isolation of pyrrolo-quinolines, rather than pyrido-indoles from eneyne-carbodiimides, as reported by Wang and coworkers (Scheme 14).65... 

Reports on solvent effects on rate constants for aquation of diimine complexes include those on [Fe(5Brphen)3] + and [Fe(4,7-Me2phen)3] " " in methanol- and ethanol-water, [Fe(bipy)3] +, [Fe(phen)3] +, and [Fe(5N02phen)3] + in aqueous methyl D-glycopyranosides, and... 

Figure 2 Diagrammatic summary of selected structural, substituent, and solvent effects on rate constants (kj, at 298 K) for base hydrolysis of low spin iron(II)-diimine complexes. Ligand abbreviations not appearing in the list at the end of this chapter are apmi = (73) with = Me BOH cage = (78) with X = OH ...

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

Reactivity Structure of R Determining factor Nature of X Solvent effect on rate 3°>2°>r>CH, Stability of R RI > RBr > RCI > RF Rate increase in polar solvents CH,>1 >2 >3° Steric hindrance in R group RI > RBr > RCI > RF With Nu there is a large rate increase in polar aprotic solvents... 

The use of the Gutmann41 donor and acceptor numbers for describing solvent effects on rates, equilibria and other physicochemical properties has met with some success in organic chemistry. 62 63 However, because the donor and acceptor numbers of mixtures of solvents can not be inferred from the values of the pure solvents but must be determined experimentally, and also because the relationships describing the effects of solvent on chemical reactions were found to apply to non-associated solvents of medium to high dielectric constant, there has been very little attempt to introduce this approach into inorganic systems where the commonly used solvents are protic, i.e. associated. However, one such reaction that has been studied was63 equation (34) ... 

Equation (35) is an example of a multiparametric equation17 18 64 for describing solvent effects on equilibria and is similar to the multiparametric equations used to describe solvent effects on rates. The use of these equations in describing solvent effects on equilibria involving coordination compounds is, like their application to reaction rates of coordination compounds, not common. Equilibrium (36), where L, U and X are the same as in reaction (12),... 

Reactants Activated complex Reaction type Solvent effect on rate... 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

Solvent effect on rate constants. In this section, the rate constant will be predicted qualitatively in CO2 for the Diels-Alder cycloaddition of isoprene and maleic anhydride, a reaction which has been well-characterized in the liquid state (23,24). In a previous paper, we used E data for phenol blue in ethylene to predict the rate constant of the Menschutkin reaction of tripropylamine and methyliodide (19). The reaction mechanisms are quite different, yet the solvent effect on the rate constant of both reactions can be correlated with E of phenol blue in liquid solvents. The dipole moment increases in the Menschutkin reaction going from the reactant state to the transition state and in phenol blue during electronic excitation, so that the two phenomena are correlated. In the above Diels-Alder reaction, the reaction coordinate is isopolar with a negative activation volume (8,23),... 

Qualitative Theory of Solvent Effects on Reaction Rates 165 Table 5-4. Predicted solvent effects on rates of nucleophilic substitution reactions [16, 44-46],... 

Concerted + n s] cycloadditions are, in principle, forbidden by orbital symmetry [90]. This restriction is bypassed when these reactions occur via zwitterions or biradicals, or by the symmetry-allowed [A + 2 ] process. Since cycloadditions proceeding through zwitterionic intermediates or dipolar activated complexes should be affected by solvent polarity, the investigation of the solvent effects on rates can be of considerable value when considering potential models for the activated complex and the reaction mechanism [91-93]. The possible solvent effects on one-step and two-step cycloaddition reactions are shown schematically in Fig. 5-6 [92] ... 

Likewise, 1,3-dipolar (and other) cycloreversions have been studied in a variety of solvents [546-549]. As expected, only rather small or negligible solvent effects on rate have been observed. 

The solvent-dependence of cheletropic reactions has also been investigated [158aj. The thermolysis of 3-methyl-2,5-dihydrothiophene-l,1-dioxide appears to involve a concerted fission of the two (T-bonds. This is in accordance with the very small solvent effect on rate observed in six solvents of different polarity [158a]. 

Cyclic cu-azoalkanes e.g. pyrazolines, pyridazines, and others) exhibit a similar solvent effect on rate [193, 549], but angle expansion is of course impossible in these compounds. 

The rate of addition decreases moderately with increasing solvent polarity there is a 35-fold rate deceleration in going from cyclohexane to dimethyl sulfoxide. In polar solvents, the dipolar reactant thiyl radical is more stabilized than the less dipolar activated complex. The stabilization of the thiyl radical by solvation has been proven by its strong positive solvatochromism [i. e. bathochromic shift of Imax with increasing solvent polarity) [576]. Similar solvent effects on rate have been observed in the addition of the 4-aminobenzenethiyl radical to styrene [577]. 

The solvent effects on rates shown by these two reactions were determined employing the solvents chloroform, dichloromethane, acetonitrile, ethyl acetate, benzene, tetrahydrofuran and dioxane. Solvents which react with TCNE, such as nitromethane, dimethylformamide and protic solvents, as well as cyclohexane, carbon tetrachloride and tetrachloroethylene, in which the reactants have very low solubility, were deliberately excluded from the study. The observed solvent effects were virtually identical for both Diels-Alder and [2 + 2] cycloaddition processes. Statistical correlations of rate data using a multiparameter equation with dependencies based on acceptor properties, polarizability and inherent polarity of the solvents gave nearly identical coefficients through the regression analyses for each term for both reactions, and excellent linear fits to the rate data. 

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