Taft Solvatochromic Parameters Research Articles

Medium Effects on the Initial and Transition States on the Amine-Catalysed Ring-Opening of Azlactone in Water–Acetonitrile Mixtures

The kinetics of ring opening of azlactone in water – acetonitrile mixtures in the presence of morpholine have been studied under pseudo-first order condition in the temperature range 40-60°C. Non-linear plots of log kobs with the reciprocal of relative permittivity of the solvent were obtained while linear plots of log kobs against the Y value were observed. The thermodynamic parameters Δ H≠, Δ S≠ and Δ G≠ have been determined; Δ G≠ increases gradually as the mole fraction of the cosolvent increase, due to a complex quasi-mirror image compensation of Δ H≠ and Δ S≠. The negative values of the entropy of activation and the non-linear relation of log kobs with the reciprocal of relative permittivity suggested selective solvation by the more highly polar water molecules. The isokinetic temperature obtained indicated that the reaction was enthalpy-controlled. The presence of an electron-releasing or electron-withdrawing para-substituent in the benzylidene moiety affected the reaction rate, i.e. it being accelerated by p-Br but retarded by p-OMe substituents. Also the rate was decreased by increasing the content of organic cosolvent. The reactivity was analysed in the light of various simple and multiple regression equations using Kamlet–Taft solvatochromic parameters which were applied successfully in mixed aqueous – acetonitrile mixtures.

Solvent hydrogen bonding and structural effects on nucleophilic substitution reactions: Part 4 — Reaction of 2-chloro-5-nitropyridine with para-substituted anilines in acetonitrile/dimethylformamide mixtures

Substitution reactions of some para-substituted anilines with 2-chloro-5-nitropyridine were carried out conductometrically in acetonitrile/dimethylformamide mixtures. The various thermodynamic parameters for the reactions have been reported and discussed along with the validity of the isokinetic relationship. The correlation of second-order rate constants with Hammett's substituent constants yields a fairly linear straight line with negative slope. The rate data correlate satisfactorily (100 r 2 ~ 93%) with Reichardt's macroscopic solvent parameter, E T(30) . While correlation with Kamlet–Taft's solvatochromic parameters is excellent (100 R 2 ~ 99%) and the computed percentage contribution of these parameters ( P α = 29%, P β = 13%, P π ⁎ = 58%) suggests that both specific and non-specific solute–solvent–solvent interactions influence the reactivity. The solvation model proposed is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. The molar extinction coefficient ( ε max) of aniline varies appreciably up to ~ 25% with the change in mole fraction of the mixture. The multivariate correlation analysis of ɛ max (with α, β, π⁎) suggests that the solvation around NH 2 moiety of aniline through hydrogen bond donor (HBD) property is found to be dominant in the solvation process and consequently in altering the rate. The observation is that the dominance of HBD property in solvation is further confirmed by the cyclic voltammetric oxidation of aniline in the solvent mixture.

Use of Fluorescence Probes for Studying Kamlet−Taft Solvatochromic Parameters of Micellar System Formed by Binary Mixture of Sodium Dodecyl Sulfate and Triton-X 100

Fluorescence probes have been used to estimate Kamlet-Taft solvatochromic parameters alpha and pi* representing hydrogen-bond donation ability and dipolarity/polarizability, respectively, of sodium dodecyl sulfate (SDS)-Triton X 100 (TX100) mixed aggregates with varying compositions. The hydrogen-bond donation ability of the mixed aggregate has been found to increase with SDS composition, whereas the dipolarity/polarizability parameter decreases. The relative contribution of electrostatic and steric effect toward the total free energy of micellization have been calculated for the mixture. The solvatochromic parameters alpha and pi* depend linearly on the total free energy of micellization, indicating a correlation between aggregational and solvatochromic properties.

Solvent hydrogen bonding and structural effects on nucleophilic substitution reactions: Part 3. Reaction of benzenesulfonyl chloride with anilines in benzene/propan‐2‐ol mixtures

AbstractSubstitution reactions of 13 para‐ and meta‐substituted anilines with benzenesulfonyl chloride in varying mole fractions of benzene in propan‐2‐ol have been investigated conductometrically. The second‐order rate constants correlate well with pKa values of anilines and with the Hammett's equation. The negative Hammett reaction constant indicates the formation of an electron‐deficient transition state. The rate data correlate satisfactorily with macroscopic solvent parameters such as relative permittivity, εr, and polarity, ETN. Correlation of rate data with Kamlet–Taft solvatochromic parameters (α, β, π*) suggests that both the specific and nonspecific solute–solvent interactions influence the reactivity. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 657–663, 2007

Solvent Effects on the Kinetics of a Diels−Alder Reaction in Gas-Expanded Liquids

Gas-expanded liquids (GXLs) form a unique class of environmentally benign solvents that offer many of the benefits of both organic liquids and supercritical fluids. A more complete understanding of the interactions between the gas, the organic liquid, and solutes at the molecular level will enable the full exploitation of GXLs. Combining kinetic and solvatochromic studies, we have developed a comprehensive multiparameter approach to add insight into the molecular interactions related to a Diels−Alder reaction in CO2-expanded acetonitrile. We have studied the kinetics of the Diels−Alder reaction of anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione as a function of solvent composition using in situ high-pressure fluorescence spectroscopy. We have also measured the values of the Kamlet−Taft solvatochromic parameters π* (dipolarity/polarizibility), α (hydrogen bonding acidity), and β (hydrogen bonding basicity) in CO2-expanded acetonitrile using in situ high-pressure UV/vis spectroscopy. A linear solvation e...

Homogeneous solvation controlled photoreduction of cobalt(III) complexes in aqueous 2-methyl-2-propanol solutions: Linear solvation energy relationship and cyclic voltammetric analyses

The effect of solvent participation on the ligand-to-metal charge transfer (LMCT, L → Co III) reduction of the of Co III(en) 2Br(RC 6H 4NH 2) 2+ where R = m-OCH 3, p-F, H, m-CH 3, p-CH 3, p-OC 2H 5 and p-OCH 3 were examined in aqueous 2-methyl-2-propanol (Bu t OH) solutions. The change in the reduction behavior of Co III centre was also examined through cyclic voltammetric studies. The observed reduction in quantum yield due to LMCT excitation can mainly be accounted using linear solvation energy relationship (LSER) comprising model correlation equations. These consist of empirical parameters such as Grunwald–Winstein's solvent ionizing power, Y, Dimroth–Richardt's solvent micro-polarity parameter, E T N , Gutmann's donor number, DN N, along with Kamlet–Taft's solvatochromic parameters (hydrogen bond acceptor acidity/basicity α/ β and solvent dipolarity/polarizability, π*). The origin of solvent effect is found to be due to microscopic interaction between the solvent donor and the nitrogen-bound hydrogen of the ligand. Cyclic voltammograms show an irreversible reduction of Co III in DMF using Glassy Carbon Electrode, GCE, the redox peaks for the aniline complexes appear at −0.20 and 0.525 V. Irradiation of the complexes with UV light ( λ = 254 nm) in binary mixtures produce Co II aq and the concentration of this species are highly dependent on x alc ( x alc = mole fraction of alcohol). The observed quantum yield (log Φ Co(II)) is found to be linearly related to mole fraction of organic co-solvent added in the mixture, therefore, log Φ Co(II) = 26.41 × 10 −2 when x 2 = 0.0094 and 43.75 × 10 −2 when x 2 = 0.076 for a typical complex Co III(en) 2Br( p-OCH 3C 6H 4NH 2) 2+ in aqueous 2-methyl-2-propanol at 300 K. Cyclic voltammetry and LSER analyses illustrate the variation of reduction property of Co(III) by the aryl ligand and homogeneous solvation of the excited state of the complex Co III(en) 2Br(RC 6H 4NH 2) 2+ in H 2O/Bu t OH mixtures.

Modelling of retention of pesticides in reversed-phase high-performance liquid chromatography: Quantitative structure-retention relationships based on solute quantum-chemical descriptors and experimental (solvatochromic and spin-probe) mobile phase descriptors

A quantitative structure-retention relationship (QSRR) analysis based on multilinear regression (MLR) and artificial neural networks (ANNs) is carried out to model the combined effect of solute structure and eluent composition on the retention behaviour of pesticides in isocratic reversed-phase high-performance liquid chromatography (RP-HPLC). The octanol–water partition coefficient and four quantum chemical descriptors (the total dipole moment, the mean polarizability, the anisotropy of the polarizability and a descriptor of hydrogen-bonding based on the atomic charges on acidic and basic chemical functionalities) are considered as solute descriptors. In order to identify suitable mobile phase descriptors, encoding composition-dependent properties of both methanol- and acetonitrile-containing mobile phases, the Kamlet–Taft solvatochromic parameters (polarity–dipolarity, hydrogen-bond acidity and hydrogen-bond basicity, π *, α and β, respectively) and the 14N hyperfine-splitting constant ( a N) of a spin-probe dissolved in the eluent are examined. A satisfactory description of mobile phase properties influencing the solute retention is provided by a N and β or alternatively π * and β. The two seven-parameter models resulting from combination of a N and β, or π * and β, with the solute descriptors were tested on a set of 26 pesticides representative of 10 different chemical classes in a wide range of mobile phase composition (30–60% (v/v) water–methanol and 30–70% (v/v) water–acetonitrile). Within the explored experimental range, the acidity of the eluent, as quantified by α, is almost constant, and this parameter is in fact irrelevant. The results reveal that a N and π *, that can be considered as interchangeable mobile phase descriptors, are the most influent variables in the respective models. The predictive ability of the proposed models, as tested on an external data set, is quite good ( Q 2 close to 0.94) when a MLR approach is used, but the modelling capability can be further improved using an artificial neural network.

Solvent and substituent effects on the electrochemical oxidation of substituted benzylamines in 2‐methylpropan‐2‐ol/water medium

AbstractElectrochemical oxidation of various para‐ and meta‐substituted benzylamines in different mole fractions of 2‐methylpropan‐2‐ol in water has been investigated in the presence of 0.1 M sulfuric acid as supporting electrolyte. The oxidation potential data of benzylamines correlates well with Hammett's substituent constants affording negative reaction constants (−1.112 < ρ > −1.529). The correlation of the oxidation potential values with macroscopic solvent parameters is nonlinear, suggesting the operation of both specific and nonspecific solvent–solvent–solute interaction mechanisms. Correlation of the experimental data with Kamlet–Taft solvatochromic parameters is excellent (100r2 > 98%) and the results reveal that the reactivity is influenced by the preferential solvational effects. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 371–377, 2007

Linear free‐energy relationships in chromium(VI) oxidation of substituted benzylamines in nonaqueous media

AbstractThe kinetics of oxidation of 11 para‐ and meta‐substituted benzylamines by imidazolium fluorochromate (IFC) in different organic solvent media has been investigated in the presence of p‐toluenesulfonic acid (TsOH). The reaction was run under pseudo‐first‐order conditions. The rate of the reaction was found to be first order in IFC and found to increase with increase in [TsOH]. Solution IR studies in combination with kinetic measurements were used to get a better insight into the mechanism of the oxidation process. The product analysis was carried out using GC–MS. Various thermodynamic parameters for the oxidation have been reported and discussed along with the validity of the isokinetic relationship. The specific rate of oxidizing species benzylamines reaction (k2) correlates with Hammett's substituent constants affording positive reaction constants. The rate data failed to correlate with macroscopic solvent parameters, such as εr and ENT, while showing satisfactory correlation with Kamlet–Taft's solvatochromic parameters (α, β, and π*) which suggests that the specific solute–solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute–solvent complexation. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 362–369, 2007

Solvent effects on 13C and 1H NMR shielding of cyclic ketones: An experimental and theoretical study

1H and 13C NMR chemical shifts of various cyclic ketones from four to seven-membered rings have been measured and compared with calculated data in 17 different solvents. The solvent effects on NMR spectra were simulated with the polarizable continuum model (PCM). Magnetic shielding tensors were performed using the DFT coupled gauge-included atomic orbital calculations. A multi-linear-regression analysis has been carried out using the Kamlet–Abboud–Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the 1H and 13C chemical shifts of the select compounds. It has been found that there is a good correlation between the solvent induced chemical shifts of 13C and π ∗ scale of solvent polarity. 1H chemical shifts are affected mainly by polarity–polarizability and basicity of the solvent. The agreement between the calculated and the experimental data is good in all instances.

Correlation analysis of reactivity in the oxidation of anilines by nicotinium dichromate in nonaqueous media

AbstractThe kinetics of oxidation of 15 para‐ and meta‐substituted anilines by nicotinium dichromate (NDC) in different organic solvent media in the presence of p‐toluenesulfonic acid (TsOH) has been investigated. The rate of the reaction is zero order with respect to substrate, first order in NDC, and is found to increase with increase in [TsOH]. The various thermodynamic parameters for the oxidation have been reported and discussed along with the validity of the isokinetic relationship. The specific rate of oxidizing species‐anilines reaction (k2) correlates with Hammett's substituent constants affording negative reaction constants. The effect of para‐ and meta‐substituents on the oxidation rates confirms to Swain et al.'s substituent constants F and R, both with negative reaction constants. The rate data failed to correlate with macroscopic solvent parameters such as εr and E while showing satisfactory correlation with Kamlet–Taft's solvatochromic parameters (α, β, and π*). © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 657–665, 2006

Preferential Solvation Effects on the Kinetics and Thermodynamics of Oxidation of Anilines by Chromium(VI)

Summary The imidazolium fluorochromate (IFC) oxidation of fifteen meta - and para-substituted anilines, in varying mole fractions of benzene in 2-methylpropan-2-ol as mixtures, in the presence of p-toluenesulfonic acid (TsOH) is first order in IFC and TsOH and zero order in substrate. The various thermodynamic parameters for the oxidation have been reported and discussed. The validity of the isokinetic relationship and various linear free energy relationships have been tested and discussed. The activation free energy data failed to correlate with macroscopic solvent parameters such as εr and E T N. Correlation of Δ G # with Kamlet–Taft solvatochromic parameters (α, β and π*) suggests that the specific solute–solvent interactions play a major role in governing the reactivity.

Solvent effect on the alkaline hydrolysis of 2‐thiophenyl‐ 3,5‐dinitropyridine

AbstractThe kinetics of the alkaline hydrolysis of 2‐thiophenyl‐3,5‐dinitropyridine were studied spectrophotometrically in different aquo‐organic solvents such as methanol, ethanol, n‐propyl alcohol, iso‐propyl alcohol, t‐butyl alcohol, acetonitrile, dimethyl sulfoxide, dioxane, and acetone at 30°C with various solvent compositions up to 80% (v/v) of organic components. An increase in the organic solvent percentage (v/v) has different effects on the reaction rate constants presumably due to hydrogen bond donor HBD and acceptor HBA of the medium and other solvatochromic parameters. Linear and nonlinear plots of log k against the reciprocal of the dielectric constant of the solvent were obtained. The effects are too complex to be analyzed in terms of a single parameter, but an approach using the Kamlet–Taft solvatochromic parameters is applied successfully to six mixed aquo‐organic solvent systems. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 159–165, 2006

Studies on the kinetics of imidazolium fluorochromate oxidation of some meta‐ and para‐substituted anilines in nonaqueous media

AbstractThe imidazolium fluorochromate (IFC) oxidation of meta‐ and para‐substituted anilines, in seven organic solvents, in the presence of p‐toluenesulfonic acid (TsOH) is first order in IFC and TsOH and is zero order with respect to substrate. The IFC oxidation of 15 meta‐ and para‐substituted anilines at 299–322 K complies with the isokinetic relationship but not to any of the linear free energy relationships; the isokinetic temperature lies within the experimental range. The specific rate of oxidizing species‐anilines reaction (k2) correlates with substituent constants affording negative reaction constants. The rate data failed to correlate with macroscopic solvent parameters such as εr and ENT. A correlation of rate data with Kamlet–Taft solvatochromic parameters (α, β, π*) suggests that the specific solute–solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute–solvent complexation. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 166–175, 2006

The use of free energy relationships to rationalise structural and solvent effects on the photoreduction of cobalt(III) complexes in aqueous organic solvent medium

Quantum yields for the photoreduction of a series of CoIII complex ions, [CoClen2 (RC6H4NH2)]2+ where R=p-OMe, p-OEt, p-Me, m-Me, H and p-F using a low pressure Hg vapour lamp (254 nm) in aqueous ethanol mixtures (15–40% v/v organic cosolvent), increase with increase in mole fraction of the cosolvent in the binary solvent mixture under investigation. The Hammett correlation is linear, affording negative reaction constants, which indicate that the excited state is electron deficient. Correlation of the quantum yield data with Kamlet–Taft's solvatochromic parameters indicates that the specific solvent–solute interactions play a dominant role in governing the reactivity.

Theoretical solvent effects on 13C NMR shielding of some aromatic hydrocarbons. Linear regression analysis

The 13C isotropic shieldings for a series of aromatic hydrocarbons: naphthalene, azulene, acenaphtalene, fluoranthene and phenanthrene are determined. A quantum mechanical calculations of NMR chemical shifts were performed using the DFT coupled gauge-included atomic orbital calculations. The polarisable continuum model approach was used to evaluate the solvent effects on the 13C chemical shifts of the studied molecules. To this end, the solvents: cyclohexane, triethylamine, di- n-butyl ether, diisopropyl ether, diethyl carbonate, tetrahydrofuran, butyronitrile, γ-butyrolactone, propylene carbonate, dimethyl sulfoxide, benzene, toluene, fluorobenzene, anisole, acetophenone, benzonitrile and nitrobenzene are used. A multiple linear regression analysis has been carried out using the Kamlet–Abboud–Taft solvatochromic parameters to quantify the solvent effects on the 13C chemical shifts of the select hydrocarbons. It has been found that there is a good correlation between the solvent induced chemical shifts and π * scale of solvent polarity. The agreement between the calculated and the experimental data is good in all instances.

Use of Fluorescence Probes for Characterization of Solvation Properties of Micelles: A Linear Solvation Energy Relationship Study

Analysis involving the linear solvation energy relationship (LSER) of the energy of maximum fluorescence of seven structurally related probes in a micellar medium has been made to obtain solvation properties of the aqueous micellar phase. Solvation interactions as expressed by the empirical Kamlet−Taft solvatochromic parameters representing dipolarity/polarizability (π*), hydrogen bond donor acidity (α), and hydrogen bond acceptor basicity (β) have been determined for aqueous micelles formed by cetyl trimethylammonium bromide, sodium dodecyl sulfate, and Triton-X 100. The parameters agree well with those reported by other workers using absorption probes. Solvatochromic measurements also allowed the determination of partition coefficient values for the probe molecules between the micellar and the aqueous phase. The observed values have been rationalized in terms of different modes of solute−solvent interaction.

Correlation of gas–liquid partition coefficients using a generalized linear solvation energy relationship

In a number of previous communications, we reported on the utility of the solvatochromic linear solvation energy relationship (LSER) method for the correlation of a number of solute and solvent-dependent properties. In those studies, it was our practice to examine the effect of a variety of solvents on a given solute or a number of solutes in a given solvent. Here we report on a novel generalized LSER in which the solute and solvent were both simultaneously varied so as to assess the validity of the entire LSER concept and define its limits. The Hildebrand solubility parameter, δ H, the Kamlet–Taft solvatochromic parameters, π*, α, β and the solute molar volume, V 2, were used as the explanatory variables. The gas–liquid partition coefficient ( K) was the property of interest. We have found that the correlation using the generalized linear solvation energy relationship is statistically as good as the previous LSER correlations despite the use of a far smaller number of freely adjustable parameters. Furthermore, the new approach is able to give reasonable predictions of K values of systems not included in the data set upon which the regression is based.

Study of Solvent Composition Effects on the Protonation Equilibria of Various Anilines by Multiple Linear Regression and Factor Analysis Applied to the Correlation Between Protonation Constants and Solvatochromic Parameters in Ethanol–Water Mixed Solvents

The stoichiometric protonation constants (logβ) of aniline derivatives were determined potentiometrically over a wide range of solvent composition (0–0.74 mole fraction of ethanol). To explain the variation of the logβ values obtained over the whole composition range studied, the quasi-lattice quasi-chemical theory of preferential solvation was applied. The results were discussed in terms of macroscopic properties of the mixed solvent and different microscopic parameters, such as the Kamlet–Taft solvatochromic parameters to identify the solvent characteristics affecting the logβ values. Kamlet and Taft's general equation was reduced to two terms by using both multiple-linear regression analysis and combined factor analysis and target factor analysis in these mixtures: the independent term and the hydrogen-bond donating ability α (HBD), which is a solvatochromic parameter. Hammett's reaction constant for the protonation of anilines has been obtained for all the solvent mixtures and correlates well with α (HBD) of the solvent.

Solvent and substituent effects on the photoreduction of cobalt(III)-aryl amine complexes in an aquo-organic solvent medium

The photoreduction of a series of CoIII complex ions, [Coen2Cl(RC6H4NH2)]2+ where R = p-OMe, p-OEt, p-Me, m-Me, H, and p-F, has been studied using a low pressure Hg vapour lamp as the light source (254 nm) in an aquo-organic solvent [15–40% (v/v) 2-methylpropan-2-ol] medium. Quantum yields for CoII production by redox decomposition have been determined in all cases. The quantum yield increases considerably with the increase in concentration of the organic co-solvent in the binary solvent mixture under investigation. The Hammett correlation is linear, affording negative reaction constants, which indicate that the excited state is electron deficient. Correlation of the experimental results with Kamlet–Taft's solvatochromic parameters indicates that the solvent hydrogen bond donor acidity plays a dominant role in governing the reactivity.