Isoparametricity paradox in the pyridine-catalyzed reaction of phenyloxirane with 3-nitrobenzoic acid

The combined effect of structure and temperature on rate and activation energy of the X-substituted pyridine-catalyzed reaction between phenyloxirane and 3-nitrobenzoic acid in acetonitrile is studied. It is found that the intense interaction (nonadditivity) of effects of X substituents and temperature manifests itself in cross-reaction series; experimental evidence of isoparametricity in its enthalpy-entropy compensation aspect is obtained. It is determined that at the isoparametric temperature point (the isokinetic temperature), the rate (activation energy) of the catalytic process does not depend on structure of X substituents and a reversal of the order of effect of X on catalytic activity of pyridines is observed upon the transition of this isoparametric point (the isoparametricity paradox). It is shown that at the isoparametric point on the constant of the X substituent, temperature has almost no effect on the rate of the process since the value of the activation enthalpy is close to zero.     

Joint effect of structure and temperature on the rates of the reactions of 3,5-dinitrophenyloxirane with arenesulfonic acids: Compensation effect and isoparametricity

The joint effect of structure and temperature on the rate and free energy of activation of the reactions of 3,5-dinitrophenyloxirane with arenesulfonic acids YC₆H₄SO₃H in a dioxane-diglyme (1: 1 vol/vol) mixture is reported. A correlation analysis of the results of a multifactor kinetic experiment has demonstrated the nonadditivity (interaction) of the effects of the substituent Y and temperature for a cross reaction series. The pronounced kinetic enthalpy-entropy compensation effect provided experimental evidence for the isoparametricity phenomenon: at the isoparametric temperature point (isokinetic temperature), the rate of the reaction is independent of the structure of Y. At the isoparametric point for the constant of the substituent Y, the free energy of activation is temperature-independent and the entropy of activation is ΔS ^≠ = 0. On passing through this point, ΔS ^≠ changes its sign. The isoparametric points for the parameters of the varied factors are used in the interpretation of the mechanism of oxirane ring opening.     

A polylinear regression model taking into account the cross effects of the structure and temperature in the reactions of phenyloxirane with benzoic acids, catalyzed by pyridines: Evidence of the isoparametricity phenomenon

The joint effect of the structure and temperature on the rate of the reactions of phenyloxirane with Y-substituted benzoic acids in acetonitrile, catalyzed by X-substituted pyridines, was studied. A polylinear regression model that adequately describes the effects of cross-varied factors was calculated. Owing to intensive interaction of the effects of the substituents X and temperature, experimental evidence for the isoparametricity phenomenon was obtained.     

Nonadditive effects of structural factors in reactions of aryloxiranes with arenesulfonic acids. Observation of isoparametricity phenomenon

In reactions of aryloxiranes XC₆H₄₍₃₎CH(O)CH₂ with arenesulfonic acids YC₆H₄SO₃H in a mixture of dioxane with diglyme (1: 1) at 265 K the nonadditive effects of substituents X and Y were strongly revealed, therefore it was possible to observe experimentally the isoparametricity phenomenon: In the isoparametric point σ_X ^IP = 1.42 with respect to the X substituent constant the rate of the oxirane ring opening did not depend on the structure of Y (ρ_Y ^X = 0).     

Cross effects of structure and temperature in the reactions of <Emphasis Type="Italic">trans</Emphasis>-2,3-diaryloxiranes with arenesulfonic acids

Cross effects of the reagent structure and the temperature on the rate and activation parameters of the reactions of symmetrically X-substituted trans-2,3-diaryloxiranes with Y-substituted arene-sulfonic acids have been investigated in the temperature range 265–298 K. The isokinetic temperature of 265 K was reached, at which the rate and the free energy of the ring-opening reaction of trans-2,3-bis(3-bromo-5-nitrophenyl)-oxirane practically do not depend on substituent Y. The transition through the isoparametric points on substituent Y was realized with the inversion of the temperature effect on the free energy of activation for the reactions of oxiranes with X = 4-NO₂ and 3-Br-5-NO₂.     

Kinetics of the reaction of 4-nitrophenyl benzoates with 4-chlorophenol in the presence of potassium carbonate in dimethylformamide

The effect of the substituent in the benzoyl group on the relative rate and activation parameters of transesterification of substituted 4-nitrophenyl benzoates with 4-chlorophenol in dimethylformamide in the presence of potassium carbonate was studied by the competing reaction technique. The whole series of benzoates showed the enthalpy-entropy compensation effect. 4-Nitrophenyl benzoates having electronacceptor substituents give rise to isokinetic relationship with an isokinetic temperature β of 382 K. The mechanism of the transesterification process is discussed.     

Enthalpy–entropy compensation effect in reactions of 3,5-dinitrophenyloxiran with arenesulfonic acids: experimental evidence of the phenomenon of isoparametricity

Isoparametricity was observed in reactions of 3,5-dinitrophenyloxiran with arenesulfonic acids YC₆H₄SO₃H in mixtures of dioxane with the dimethyl ether of diethyleneglycol (1:1, v:v) at 265, 287, and 303 K: at the isoparametric point for temperature, the rate of the process of oxiran ring-opening did not depend on the structure of Y, whereas at the isoparametric point for the constant of substituent Y, the free energy of activation did not depend on the temperature.     

Phenomenon of isoparametricity in reactions of <Emphasis Type="Italic">trans</Emphasis>-2,3-diaryloxiranes with arenesulfonic acids. Mechanistic interpretation

Nonadditive effects of substituents X and Y are manifested in reactions of symmetrically X-substituted trans-2,3-diaryloxiranes with Y-substituted arenesulfonic acids. The isoparametric point is reached experimentally: close to the point τ _X ^IP = 4.73 on substituent Х the rate of the oxirane ring opening of trans-2,3-bis(3-bromo-5-nitrophenyl)oxirane (τ_X = 4.38) practically does not depend on substituent Y (ρ _Y ^X = 0.10±0.05). The results of cross correlation analysis of the kinetic data are applied to interpretation of the mechanism of the studied reactions.     

Compensation effect in reactions between <Emphasis Type="Italic">trans</Emphasis>-4,4'-dinitrostilbene oxide and arylsulfonic acids

The effect structure and temperature have on the rate and free activation energy of reactions between trans-4,4'-dinitrostilbene oxide and Y-substituted arylsulfonic acids YC₆H₄SO₃H in a mixture of dioxane with 1,2-dichloroethane (7: 3 vol/vol) at 265, 281, and 298 K is studied. It is found that as a result of the nonadditivity of the joint effect of substituents Y and temperature on the rate of the process of oxirane ring opening, the cross reaction series exhibits isoparametric properties in the aspect of enthalpy–entropy compensation. This allows the experimental determination of an isoparametric point with respect to the constant of substituent Y (σ_Y^IP= 0.52), in which activation entropy ΔS^≠ = 0 and free activation energy ΔG^≠ do not depend on temperature (ΔG^≠ = ΔH^≠), and to conduct the transition through this point with inversion of the order of the effect temperature has on the value of ΔG^≠ as a result of reversing the sign of ΔS^≠: in the series Y (σ_Y) = 4-OCH₃ (–0.27), 4-CH₃ (–0.17), H (0), 4-Cl (0.23), and 3-NO₂ (0.71), the values of ΔS^≠ (J/(mol K)) are–140,–119,–85,–42, and 44, respectively. The possibility of using isoparametric points as quantitative mechanistic criteria is demonstrated.     

Substituent effects on the acidity of weak acids. 2. Calculated gas-phase acidities of substituted benzoic acids

To investigate the origin of substituent effects on the acidity of benzoic acids, the structures of a series of substituted benzoic acids and benzoates have been calculated at the B3LYP/6-311+G* and MP2/6-311+G* theoretical levels. The vibrational frequencies were calculated using B3LYP/6-311+G* and allowed corrections for the change in zero-point energies on ionization, and the change in energy on going from 0 K (corresponding to the calculations) to 298 K. A more satisfactory agreement with the experimental values was obtained by energy calculations at the MP2/ 6-311++G* level using the above structures. The resulting Delta H(acid) values agree very well with the experimental gas-phase acidities. The energies of compounds with pi-electron-accepting or -releasing substituents, rotated to give the transition state geometries, provided rotational barriers that could be compared with those found for the corresponding substituted benzenes. Isodesmic reactions allowed the separate examination of the substituent effects on the energies of the acids and on the anions. Electron-withdrawing groups stabilize the benzoate anions more than they destabilize the benzoic acids. Electron-donating groups stabilize the acids and destabilize the anions by approximately equal amounts. The gas-phase acidities of meta- and para-substituted benzoic acids are linearly related. This is also found for the acidities of substituted phenylacetic acids and benzoic acids. Since direct pi-electron interactions are not possible with the phenylacetic acids, this indicates that the acidities are mainly controlled by a field effect interaction between the charge distribution in the substituted benzene ring and the negative charge of the carboxylate group. The Hammett sigma(M) and sigma(P) values are also linearly related for many small substituents from NO(2) through the halogens and to OH and NH(2). Most of the other substituents fall on a line with a different slope     

Structure-reactivity correlations on the kinetics of substituted 2-phenylethyl m-nitrobenzenesulfonates with substituted pyridines

The rates of the substituted 2-phenylethyl m-nitrobenzenesulfonate(2-PNS) with pyridines were determined in acetonitrile. The reaction was accelerated by an electron - donating substituent on both substrate and nucleophlie. Substitutent effects 2-PNS and pyridine are correlated by Brønsted and Hammett equations, respectively. The sensitivity parameters, β and ϱ, obtained from the free-energy relationships, are inter-related and are themselves sensitive to the reactivity of the system. Thus, β varies from 0.246 for p-MeO 2-PNS to 0.284 for p-NO₂PNS, and are linearly related to the α values for 2-PNS substituents. Likewise the ϱ_z (Z is a substituent of substrate) values are linearly related to pKa of the pyridines and ϱ_y (Y is a substituent of pyridine) values are also correlated to the β values. These data show that electron-withdrawing substituents in 2-PNS increse bond formation between C and N atoms and such subsituents in the pyridines also lead to increased bond formation relatively to bond breaking in the transition state. The More O'Ferral and Swain, Thornton, and Harris approaches were applied for the prediction of substituent effects on above interpretation.     

Cautions regarding the physical interpretation of statistically based separation of the ortho substituent effect into inductive,mesomeric, and steric components—II : Acid catalysed esterification of benzoic acids by methanol and cyclohexanol

In order to attain a better insight into the composition of Taft E_s^o, constants the rate data of hydrion catalysed esterification of both m,p-substituted and o-substiluted benzoic acids by cyclohexanol and methanol were submitted to a statistical analysis using inductive mesomeric and steric substituent constants and various dummy variables differently structured. Furthermore a principal component analysis with subsequent identification of the first principal component via multiple regression analysis was applied. It has been demonstrated that in the reactions of m,p substituted compounds some substituents capable of exerting strong mesomeric effects show peculiar characteristics deviating from the general trend. Since the same result was obtained in the correlations of ortho substituted benzoic acids this effect was taken into account using an appropriate dummy variable which in all cases improved the multiple coefficient of determination. It is concluded that the esterification rates of the ortho substituted compounds depend essentially upon inductive and steric effects (taken away OMe OEt and NO₂) as proposed by Taft. While generally the E_s^o values may be regarded as some measure of a steric effect, this is not true for the methoxy and ethoxy groups.     

Substituent cross-interaction effects on the electronic character of the C=N bridging group in substituted benzylidene anilines--models for molecular cores of mesogenic compounds. A 13C NMR study and comparison with theoretical results

13C NMR chemical shifts delta(C)(C=N) were measured in CDCl3 for a wide set of mesogenic molecule model compounds, viz. the substituted benzylidene anilines p-X-C6H4CH=NC6H4-p-Y (X = NO2, CN, CF3, F, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, F, Cl, H, Me, MeO, or NMe2). The substituent dependence of delta(C)(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on delta(C)(C=N): electron-withdrawing substituents cause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominating over the resonance effects. In contrast, the aniline substituents Y exert normal effects: electron-withdrawing substituents cause deshielding, while electron-donating ones cause shielding of the C=N carbon, the strengths of the inductive and resonance effects being closely similar. Additionally, the presence of a specific cross-interaction between X and Y could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene or aniline ring substituents. Electron-withdrawing substituents on the aniline ring decrease the sensitivity of delta(C)(C=N) to the substitution on the benzylidine ring, while electron-donating substituents have the opposite effect. In contrast, electron-withdrawing substituents on the benzylidene ring increase the sensitivity of delta(C)(C=N) to the substituent on the aniline ring, while electron-donating substituents act in the opposite way. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects). The present NMR characteristics are discussed as regards the computational literature data. Valuable information has been obtained on the effects of the substituents on the molecular core of the mesogenic model compounds.     

Hydrogen migrations in mass spectrometry. VI—the chemical ionization mass spectra of substituted benzoic acids and benzyl alcohols

The H₂ and CH₄ chemical ionization mass spectra of a series of series of substituted benzoic acids and substituted benzyl alcohols have been determined. For the benzoic acids the major fragmentation reactions of the protonated molecule involve elimination of H₂O or elimination of CO₂, the latter reaction involving migration of the carboxylic hydrogen to the aromatic ring. For the benzyl alcohols the major fragmentation reactions of [MH]⁺ involve loss of H₂O or CH₂O, analogous to the CO₂ elimination reaction for the benzoic acids. It is shown that the CO₂ and CH₂O elimination reactions occur only when a conjugated aromatic ring system is present, and that for the carboxylic acid systems, methyl groups and, to a lesser extent, phenyl groups are capable of migrating. The only discernible effect of substituents on the fragmentation of [MH]⁺ is an enhancement of the H₂O loss reaction in the benzoic acid system when an amino, hydroxyl, or halogen substituent is ortho to the carboxyl function. This ‘ortho’ effect, which differs in scope from that observed in electron impact mass spectra, is attributed to an intramolecular catalysis by the ortho substituent of the 1,3 hydrogen migration in the carbonyl protonated acid followed by H₂O elimination. Apparently, this route is favoured over the direct elimination of H₂O from the carbonyl protonated acid, since the latter has a high activation energy barrier because of unfavourable orbital symmetry restrictions.     

Adduct formation of copper(II) chelates of 1-hydroxypyrazole 2-oxides with substituted pyridines

Equilibrium constants have been determined for the adduct formation of 10 copper(II) chelates of the derivatives of 1-hydroxypyrazole 2-oxide with nine substituted pyridines at room temperature in chloroform solution. These adducts were shown to have 1:1 stoichiometry. All the stabilities of the adducts were governed by: (1) σ-donating ability of the nitrogen atom in the substituted pyridines to the copper(II) chelates, (2) electron-attracting forces of substituents at the 3- and 4-positions of the phenyl ring in the chelate ligands, and (3) the magnitude of the polar substituent constant of the substituents in the pyrazole ring of the chelate ligands.     

Is the Hammett's constant free of steric effects?

In this work, we have explored the validity of the hypotheses on which rest the Hammett's approach to quantify the substituent effect on a reaction center, by applying two DFT energy decomposition schemes. This is performed by studying the change in the total electronic energy, ΔΔE, associated with a proton transfer isodesmic equilibrium. For this reaction, two sets of substituted benzoic acids and their corresponding benzoate anions have been considered. One of these sets contains para- and meta-substitutions, whereas the other one includes ortho-substituted benzoic acids. For each case, the gas phase change in the total electronic energy has been calculated, and two DFT energy decomposition schemes have been applied. The experimental σ(X) was found to be nearly proportional to the computed ΔΔE. The results for the para- and meta-substituted benzoic acids lead to the conclusion that it is possible to treat separately and, in an additive manner, the electrostatic and steric contributions; and also that the Hammett constant depends mainly on the electronic contributions to the free energy, while the steric contribution is negligible. However, the results for the ortho-substituted cases lead to the conclusion, as was assumed by Hammett, that there are significant qualitative differences between the effects on a reaction site of substituents in the meta- and para-positions and those in the ortho-position.     

Evidence of substituent-induced electronic interplay. Effect of the remote aromatic ring substituent of phenyl benzoates on the sensitivity of the carbonyl unit to electronic effects of phenyl or benzoyl ring substituents

Carbonyl carbon (13)C NMR chemical shifts delta(C)(C[double bond]O) measured in this work for a wide set of substituted phenyl benzoates p-Y-C(6)H(4)CO(2)C(6)H(4)-p-X (X = NO(2), CN, Cl, Br, H, Me, or MeO; Y = NO(2), Cl, H, Me, MeO, or NMe(2) ) have been used as a tool to study substituent effects on the carbonyl unit. The goal of the work was to study the cross-interaction between X and Y in that respect. Both the phenyl substituents X and the benzoyl substituents Y have a reverse effect on delta(C)(C[double bond]O). Electron-withdrawing substituents cause shielding while electron-donating ones have an opposite influence, with both inductive and resonance effects being significant. The presence of cross-interaction between X and Y could be clearly verified. Electronic effects of the remote aromatic ring substituents systematically modify the sensitivity of the C[double bond]O group to the electronic effects of the phenyl or benzoyl ring substituents. Electron-withdrawing substituents in one ring decrease the sensitivity of delta(C)(C[double bond]O) to the substitution of another ring, while electron-donating substituents inversely affect the sensitivity. It is suggested that the results can be explained by substituent-sensitive balance of the contributions of different resonance structures (electron delocalization, Scheme 1).     

The ortho‐Substituent Effect on the Ag‐Catalysed Decarboxylation of Benzoic Acids

A combined experimental and computational investigation on the Ag‐catalysed decarboxylation of benzoic acids is reported herein. The present study demonstrates that a substituent at the ortho position exerts dual effects in the decarboxylation event. On one hand, ortho‐substituted benzoic acids are inherently destabilised starting materials compared to their meta‐ and para‐substituted counterparts. On the other hand, the presence of an ortho‐electron‐withdrawing group results in an additional stabilisation of the transition state. The combination of both effects results in an overall reduction of the activation energy barrier associated with the decarboxylation event. Furthermore, the Fujita–Nishioka linear free energy relationship model indicates that steric bulk of the substituent can also exert a negative effect by destabilising the transition state of decarboxylation.     

Enthalpy–Entropy Correlations in Reactions of Aryl Benzoates with Potassium Aryloxides in Dimethylformamide

Temperature dependences of the relative reactivity of potassium aryloxides XC₆H₄O^?K⁺ toward 4‐nitrophenyl ( 1 ), 3‐nitrophenyl ( 2 ), 4‐chlorophenyl ( 3 ), and phenyl ( 4 ) benzoates in dimethylformamide (DMF) were studied using the competitive reactions technique. The rate constants k_X for the reactions of 1 with potassium 4‐cyanophenoxide, 2 with potassium 3‐bromophenoxide, 3 with potassium 3‐bromo‐, 4‐bromo‐, and unsubstituted phenoxides, 4 with potassium 4‐methoxy‐ and 3‐methylphenoxides were measured at 25°C. Correlation analysis of the relative rate constants k_X/k_H(3‐Me) and differences in the activation parameters (??Н^≠ and ??S^≠) of competitive reactions revealed the existence of six isokinetic series. We investigated the substituent effect of X on the activation parameters for each isokinetic series and concluded that the reactions of aryl benzoates PhCO₂C₆H₄Y with potassium aryloxides in DMF proceed via a four‐step mechanism. The large ρ⁰(Y) and ρ_XY values at 25°C obtained for the reactions of 1–3 with potassium aryloxides with an electron‐donating substituent refer to the rate‐determining formation of the spiro‐σ‐complex. The Hammett plots for the reactions of 1 and 2 exhibit a downward curvature, causing the motion of the transition state for the rate‐determining step according to a Hammond effect as the substituent in aryloxide changes from electron‐donating to electron‐withdrawing. Analysis of data in the terms of two‐dimensional reaction coordinate diagrams leads to the conclusion that significant anti‐Hammond effects arise in the cases of ortho‐substituted and unsubstituted substrates. It was shown that the isokinetic and compensation effects observed for the reactions of aryl benzoates with potassium aryloxides in DMF can be interpreted in the terms of the electrostatic bonding between the reaction centers.     

Kinetics of the reaction of substituted 4-nitrophenyl benzoates with benzenethiol in the presence of potassium carbonate in dimethylformamide

The effect of the substituent nature on the rate and activation parameters of transesterification of a series of 4-nitrophenyl benzoates with benzenethiol in dimethylformamide in the presence of potassium carbonate was studied by the competing reaction method. In all cases, change of the Gibbs energy of activation is determined mainly by variation of the enthalpy of activation. 4-Nitrophenyl benzoates having electron-withdrawing substituents in the benzoyl fragment were found to fit an isokinetic relation with an isokinetic temperature β of 318 K. Enthalpy-entropy compensation effect was observed in the reactions with all the examined 4-nitrophenyl benzoates. The relation between the reactivity and polarizability of nucleophilic center in S-and O-nucleophiles is discussed.