Solvent effect on the rate of the Mannich reaction of butyraldehyde with formaldehyde

The Mannich reaction of formaldehyde with butyraldehyde and diethylamine in hydrophilic solvents ensuring homogeneity of the medium follows the kinetic relations typical of an irreversible second-order reaction. The rate constants are determined by the ability of solvents to undergo self-association and their electrophilic solvation power; additional inclusion of the solvent polarity via multiparameter Koppel’-Pal’m equation is necessary to obtain a satisfactory quantitative correlation.     

Effect of the medium on the reaction of <Emphasis Type="Italic">tert</Emphasis>-butyl hypochlorite with hydrocarbons

The selectivity in the reaction of tert-butyl hypochlorite with 2,3-dimethylbutane in various solvents may be described by a three-parameter correlation equation taking into account the ability of solvents to nonspecific solvation and their cohesion energy density.     

Solvent interactions in the allylation of piperidine

The reaction between allylbromide and piperidine has been studied in different protic and aprotic solvents. The reaction is first order with respect to [allylbromide] and [piperidine]. A correlation analysis of the rate data with solvent properties shows that polarity (Y), polarizability (P), and electrophilicity (E) of the solvent simultaneously influence the rate of reaction. From the regression analysis, information regarding the relative solvation of the reactants and the activated complex is obtained and a solvation model is proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 421–425, 2009     

Effect of solvation on the complexation of 18-crown-6 with amino acids in aqueous-organic media

Our own and published data on the effect of mixed solvents on the thermodynamic parameters of molecular complexation of 18-crown-6 with glycine, D,L-alanine, and L-phenylalanine in aqueous ethanol, dimethyl sulfoxide, and acetone have been generalized. In all cases, decrease of the water fraction in mixed solvents increases the exothermic effect of complex formation. The change in the reaction enthalpy is determined mainly by variation of the enthalpies of solvation of the molecular complex and 18-crown-6, whereas the contribution of solvation of the amino acid is insignificant.     

Solvation model in the allylation of 2-mercaptobenzothiazole

The kinetics of the reaction of allyl bromide with 2-mercaptobenzothiazole has been studied in different protic and aprotic solvents conductometrically. The rate data were correlated with solvation parameters using linear multiple regression analysis. From the regression coefficients, which describe the susceptibility towards rate of different solvent parameters, information regarding solvent interactions is obtained and solvation models are proposed. The reaction has also been studied at different temperatures and thermodynamic parameters ∆H^≠, ∆S^≠, ∆G^≠ are computed.     

Effect of Solvation on the Decomposition Rate of Azodiisobutyronitrile

The log rate constants of monomolecular decomposition of azodiisobutyronitrile can be quantitatively correlated with solvent properties by means of linear multiparameter equations including various solvation effects; the key factors are the ability of solvents for electrophilic solvation, which favors decomposition, and cohesive energy density, which unfavors the reaction.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 196–200.Original Russian Text Copyright © 2005 by Makitra, Polyuzhin, Golovata.     

Experimental and theoretical studies on the isomerization of allyl thiocyanate to allyl isothiocyanate

The mechanism of isomerization of allyl thiocyanate to allyl isothiocyanate has been investigated both experimentally and theoretically. The kinetic study indicates that the reaction is unimolecular and is not ionic. The entropy of activation suggests strongly that the mechanism involves a cyclic transition state. The rate of reaction was retarded to a small extent in polar solvents relative to that in nonpolar solvents. Ab initio MO calculations indicate, in agreement with the experimental results, that the reaction proceeds through a cyclic transition state, one in which the SCN moiety is almost linear. Thus, this is a [3,3] sigmatropic rearrangement. The charge separation in the transition state was substantial. The retardation of the reaction in polar solvents was attributed to the difference in solvation in the original state and in the transition state. © 1997 John Wiley & Sons, Inc.     

Influence of inter- and intramolecular hydrogen bonding on kemp decarboxylations from QM/MM simulations

The Kemp decarboxylation reaction for benzisoxazole-3-carboxylic acid derivatives has been investigated using QM/MM calculations in protic and dipolar aprotic solvents. Aprotic solvents have been shown to accelerate the rates of reaction by 7-8 orders of magnitude over water; however, the inclusion of an internal hydrogen bond effectively inhibits the reaction with near solvent independence. The effects of solvation and intramolecular hydrogen bonding on the reactants, transition structures, and the rate of reaction are elucidated using two-dimensional potentials of mean force (PMF) derived from free energy perturbation calculations in Monte Carlo simulations (MC/FEP). Free energies of activation in six solvents have been computed to be in close agreement with experiment. Solute-solvent interaction energies show that poorer solvation of the reactant anion in the dipolar aprotic solvents is primarily responsible for the observed rate enhancements over protic media. In addition, a discrepancy for the experimental rate in chloroform has been studied in detail with the conclusion that ion-pairing between the reactant anion and tetramethylguanidinium counterion is responsible for the anomalously slow reaction rate. The overall quantitative success of the computations supports the present QM/MM/MC approach, which features PDDG/PM3 as the QM method.     

Calculating the solvation contributions from reagents to the change in enthalpy of silver(I) complexation with 18-crown-6 ether in binary methanol-acetonitrile solvents

The enthalpies of the transfer of silver(I) ions from methanol to mixed methanol-acetonitrile solvents are calculated using the literature data. An estimative calculation of the enthalpies of transfer of the reaction of formation of [Ag18C6]⁺ upon a change in the solvent composition MeOH → AN is performed on the basis of previously found laws of change in the thermodynamic characteristics of complexation and solvation of reagents. It is shown that the governing factor in the change in the energy of the reaction is the enhancement of the solvation of the central ion.     

二氯二氰基苯醌及其阴离子自交换电子转移反应的理论研究

基于非平衡溶剂化能的约束平衡方法和溶剂重组能的新表达式, 实现了电子转移反应溶剂重组能的数值解, 研究了二氯二氰基苯醌(DDQ)及其阴离子体系DDQ-之间的自交换电子转移反应. 考虑了DDQ与DDQ-分子以平行方式形成受体-给体络合物时的两种构型. 引入线性反应坐标, 计算了该反应在不同溶剂中的溶剂重组能. 基于两态变分模型得到了反应的电子耦合矩阵元. 根据电子转移动力学模型, 计算了该自交换电子转移反应的速率常数.     

Correlations of the rate constants of phosphorylation reactions with the empirical parameters of the solvent polarity

Regression analysis of the solvent effects on the rate constants of nucleophilic substitution at the phosphoryl group was performed with the use of the empirical parameters of solvent polarity which characterize the ability of the solvents to electrophilic and nucleophilic solvation. The nucleophilic solvation of reagents by solvents, as a rule, favors the phosphorylation reactions. In the phosphorylation reactions of anionic nucleophiles, the electrophilic solvation of anions influences negatively the reactions rates. The phosphorylation of amines by chlorides of phosphorus acids is facilitated by the electrophilic solvation of a separated anion. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 271–274, February, 1998.     

Correlating the Solvating Power of Solvents with the Strength of Ion-Dipole Interaction in Electrolytes of Lithium-ion Batteries

The solvation structure of Li⁺ plays a significant role in determining the physicochemical properties of electrolytes. However, to date, there is still no clear definition of the solvating power of different electrolyte solvents, and even the solvents that preferentially participate in the solvation structure remain controversial. In this study, we comprehensively discuss the solvating power and solvation process of Li⁺ ions using both experimental characterizations and theoretical calculations. Our findings reveal that the solvating power is dependent on the strength of the Li⁺-solvent (ion-dipole) interaction. Additionally, we uncover that the anions tend to enter the solvation sheath in most electrolyte systems through Li⁺-anion (ion-ion) interaction, which is weakened by the shielding effect of solvents. The competition between the Li⁺-solvent and Li⁺-anion interactions ultimately determines the final solvation structures. This insight into the fundamental understanding of the solvation structure of Li⁺ provides inspiration for the design of multifunctional mixed-solvent electrolytes for advanced batteries.     

A solvolysis model for 2-chloro-2-methyladamantane based on the linear solvation energy approach

Solvolysis/dehydrohalogenation rates of 2-chloro-2-methyladamantane (CMA) in 15 hydrogen-bond acidic and/or basic solvents are studied. The rates of reaction in these solvents have been correlated with the solvation equation developed by Kamlet, Abraham, and Taft. The linear solvation energy relationship (LSER) derived from this study is given by the following equation: log k = -5.409 + 2.219 + 2.505alpha(1) - 1.823beta(1) where , alpha(1), and beta(1) are the solvation parameters that measure the solvent dipolarity/polarizability, hydrogen-bond acidity (electrophilicity), and hydrogen-bond basicity (nucleophilicity). A high correlation coefficient (r = 0.996, SD = 0.191) was achieved. The cavity term, which includes the Hildebrand parameter for solvent cohesive energy density, delta(H), was not found to be statistically significant for this reaction substrate. The resulting equation allows calculated rates of reaction in other solvents and provides insight into the reaction pathway. In a previously reported correlation for another tertiary chloride, tert-butyl chloride (TBC), the coefficients for alpha(1) and are significantly larger and the coefficient for is statistically significant. In addition, the coefficient for beta(1) in the TBC correlation is positive, rather than negative, indicating that the transition states for TBC and CMA are significantly different. These results demonstrate why the uses of simple solvolytic correlation methods may be invalid even for comparisons of similar type substrates, e.g., tertiary chlorides. Also, these results provide confidence in the use of multiple linear regression analysis for predicting solvolytic rates in additional solvents.     

A kinetic investigation of the addition of diazomethane to styrenes. Role of dipolar aprotic solvents in predicting direction of dipole orientation

The addition of diazomethane to a series of substituted styrenes has been kinetically investigated with emphasis on polar, solvent, and solvation effects. Although the reaction is generally insensitive to solvent polarity, a sizeable substituent effect (ρ = +0.90) has been observed. Also, a sizeable increase in rate was generally observed in protic solvents such as water, and a considerable rate enhancement occurred in the case of nitrostyrene in the dipolar aprotic solvent, dimethylformamide. The results are discussed in terms of a concerted mechanism involving a partially-charged transition state II, with negative charge formation on the α-carbon of the styrene. The methoxystyrene offers an exception to the Hammett equation and the unexpectedly high rate constant for this compound could be discussed in terms of a possible participation of the highly nucleophilic resonance form of p-methoxystyrene and an α-addition via the transition state III. However, solvation effects in dimethylformamide (failure to obtain the anticipated rate increase) and the formation of a 3-substituted pyrazoline adduct from the cycloaddition reaction indicate steric control to predominate over electronic factors and β-addition to prevail unequivocally. It is proposed that solvation effects in protic solvents can be utilized to advantage in cycloaddition reactions leading to 1-pyrazolines, for which no other routes are available at the present time.     

Solvent effect on the kinetics of Beckmann rearrangement

The rate constants for the Beckmann rearrangement of cyclohexanone oxime p-toluenesulfonate in 11 solvents are satisfactorily described by a three-parameter linear correlation including polarizability, electrophilicity, and molar volume of the solvent. The first two factors favor the reaction, whereas increase of the solvent molar volume makes the reaction slower, presumably due to steric hindrances to solvation.     

Computational Analysis of Solute–Solvent Coupling Magnitude in the Z/E Isomerization Reaction of Nitroazobenzene and Benzylideneanilines

The dynamic solvent effect often arises in solution reactions, where coupling between chemical reaction and solvent fluctuation plays a decisive role in the reaction kinetics. In this study, the Z/E isomerization reaction of nitoroazobenzene and benzylideneanilines in the ground state was computationally studied by molecular dynamics simulations. The non-equilibrium solvation effect was analyzed using two approaches: (1) metadynamics Gibbs energy surface exploration and (2) solvation Gibbs energy evaluation using a frozen solvation droplet model. The solute–solvent coupling parameter (C_coupled) was estimated by the ratio of the solvent fluctuation Gibbs energy over the corresponding isomerization activation Gibbs energy. The results were discussed in comparison with the ones estimated by means of the analytical models based on a reaction–diffusion equation with a sink term. The second approach using a frozen solvation droplet reached qualitative agreement with the analytical models, while the first metadynamics approach failed. This is because the second approach explicitly considers the non-equilibrium solvation in the droplet, which consists of a solute at the reactant geometry immersed in the pre-organized solvents fitted with the solute at the transition state geometry.     

Deep eutectic solvent as an operative media on structure-reactivity relationships

Deep eutectic solvents seem to be environmentally friendly solvents, particularly because they are prepared easily and have very low-vapor pressures under ambient conditions. They are suitable candidates as green solvents for reaction media with special properties. To present this behavior, substitution reactions of some para- and meta-substituted anilines with 1-fluoro-2,4-dinitrobenzene have been spectrophotometrically investigated in varying mole fractions of ethaline as a deep eutectic solvent in dimethyl sulfoxide (DMSO). The measured rate coefficients of the reaction demonstrated a noticeable variation with the increasing mole fraction of ethaline in ethaline-DMSO mixtures. The linear free energy relationship (LFER) of second-order rate coefficients based on Hammett's substituent constants demonstrates a reasonably linear straight line with a negative slope in different mole fractions of ethaline-DMSO mixtures. Another LFER investigation based on the polarity parameters of the media showed a good agreement with hydrogen bond donor and acceptor abilities of the solvent. Non-LFER assay according to the preferential solvation model confirmed differences between the microsphere solvation of the solute molecules and the bulk composition of the solvents.     

A phenomenological model of dynamical arrest of electron transfer in solvents in the glass-transition region

A phenomenological model of electron transfer reactions in solvents undergoing glass transition is discussed. The reaction constant cuts off slow polarization modes from the spectrum of nuclear thermal motions active on the observation time scale. The arrest of nuclear solvation in turn affects the reaction activation barrier making it dependent on the rate. The resultant rate constant is sought from a self-consistent equation. The model describes well the sharp change in the solvent Stokes shift of optical lines in the glass-transition region. It is also applied to describe the temperature dependence of primary charge separation and reduction of primary pair in photosynthetic reaction centers. The model shows that a weak dependence of the primary charge separation rate on temperature can be explained by dynamical arrest of nuclear solvation on the picosecond time scale of electron transfer. For reduction of primary pair by cytochrome, the model yields a sharp turnover of the reaction kinetics at the transition temperature when nuclear solvation freezes in.     

Pendant unit effect on electron tunneling in U-shaped molecules

The electron transfer reactions of three U-shaped donor-bridge-acceptor molecules with different pendant groups have been studied in different solvents as a function of temperature. Analysis of the electron transfer kinetics in nonpolar and weakly polar solvents provides experimental reaction Gibbs energies that are used to parameterize a molecular solvation model. This solvation model is then used to predict energetic parameters in the electron transfer rate constant expression and allow the electronic coupling between the electron donor and electron acceptor groups to be determined from the rate data. The U-shaped molecules differ by alkylation of the aromatic pendant group, which lies in the ‘line-of-sight’ between the donor and acceptor groups. The findings show that the electronic coupling through the pendant group is similar for these molecules.     

Decomposition in aprotic solvents of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one,a hydroxamic acid from cereals

The decomposition of the title compound (DIMBOA, 1) in aprotic solvents was analysed in terms of linear solvation energy relationships using donor numbers. The results indicate rate-limiting cyclic hemiacetal opening in low donor number solvents and rate-limiting isocyanate formation in high donor number solvents. The addition of H₂O to DIMBOA decomposing in high donor number solvents had no effect upon the reaction rate, allowing one of the two proposed mechanisms to be rejected.