Negative Reaction Constant Research Articles

A paradigm shift in rate determining step from single electron transfer between phenylsulfinylacetic acids and iron(III) polypyridyl complexes to nucleophilic attack of water to the produced sulfoxide radical cation: a non‐linear Hammett

Mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAAs) by iron(III) polypyridyl complexes in aqueous acetonitrile medium has been investigated spectrophotometrically. An initial intermediate formation between PSAA and [Fe(NN)3]3+ is confirmed from the observed Michaelis–Menten kinetics and fractional order dependence on PSAA. Significant rate retardation with concentration of [Fe(NN)3]3+ is rationalized on the basis of coordination of a water molecule at the carbon atom adjacent to the ring nitrogen of the metal polypyridyl complexes by nucleophilic attack at higher concentrations. Electron‐withdrawing and electron‐releasing substituents in PSAA facilitate the reaction and Hammett correlation gives an upward ‘V’ shaped curve. The apparent upward curvature is rationalized based on the change in the rate determining step from electron transfer to nucleophilic attack, by changing the substituents from electron‐releasing to electron‐withdrawing groups. Electron‐releasing substituents in PSAA accelerate the electron transfer from PSAA to the complex and also stabilize the intermediate through resonance interaction leading to negative reaction constants (ρ). Conversely, electron‐withdrawing groups, while retarding the electron transfer exert an accelerating effect on the nucleophilic attack of H2O which leading to low magnitude of ρ+ compared to high ρ− values of electron‐releasing groups. Marcus theory is applied, and a fair agreement is seen with the experimental values. Copyright © 2016 John Wiley & Sons, Ltd.

Reactivities of mixed organozinc and mixed organocopper reagents. Part 13 Kinetic study for phosphine‐catalyzed acylation of alkylarylzincs and effect of residual group on the transfer rate of alkyl group

AbstractKinetics of reactions of di‐n‐butylzinc, n‐Bu2Zn, and mixed n‐butyl(substituted phenyl)zinc reagents and n‐Bu(functional group (FG)C6H4)Zn with benzoyl chloride in the presence of tri‐n‐butylphosphine have been investigated. Reaction rates of transferable n‐butyl group have been determined in tetrahydrofuran at 0 °C to compare the transfer rate of n‐butyl group in homo and mixed diorganozincs. Rate law is consistent with a third‐order reaction, which is first order in diorganozinc, benzoyl chloride, and n‐Bu3P, and a mechanism was proposed. The lower reaction rate of n‐BuPhZn than that of n‐Bu2Zn and negative reaction constant in Hammett plot are in accordance with the carbanionic charge of transferable n‐butyl group in the acylation reaction. These findings support the hypothesis that the reaction rate of transferable group, RT, changes depending upon the residual group, RR, in RRRTZn reagents. Copyright © 2016 John Wiley & Sons, Ltd.

English

The kinetics of oxidation of some substituted N,a-diphenylnitrones by N-chlorosuccinimide (NCS) to yield benzaldehyde and nitrosobenzene was studied. The reaction follows first order kinetics with respect to (NCS) and fractional order with respect to (nitrone). The order with respect to alkali was inverse fractional order. The reaction exhibited both negative and positive Hammett reaction constants depending upon the nature of substituents. A plot of rate constant (log k) and s gave a concave downward curve. The electron releasing substituents fall on one side of curve and electron withdrawing substituents fall on another side. The activation parameters were calculated. A probable mechanism consistent with experimental observations has been proposed.

Kinetics and Correlation Analysis of Reactivity in the Oxidation of Some Alkyl Phenyl Sulfides by Benzimidazolium Dichromate

Abstract The oxidation of some alkyl phenyl sulfides by benzimidazolium dichromate (BIDC) in dimethyl sulfoxide, to the corresponding sulfoxides, is first order with respect to BIDC. Michaelis–Menten type kinetics was observed with respect to sulfide. Thermodynamic parameters for the complex formation and activation parameters for the decomposition of the complex were calculated. The reaction is proton-catalyzed and the order is less than one. It appears that the BIDCH+ is the active electrophile. An intermediate of composition sulfide–BIDC–H+ has been envisaged. The high value of negative entropy of activation suggests a cyclic transition state in the rate-determining step. The reaction exhibited negative polar reaction constants and a small degree of steric hindrance. The rate of oxidation of sulfide was determined in 19 organic solvents. An analysis of the solvent effect by Swain’s equation indicated that though both the anion and cation solvating powers of the solvent contribute to the observed solvent effect, the role of cation solvation is major. A mechanism involving the rate-determining oxygen atom transfer in complex, formed by a nucleophilic attack of sulfide-sulfur on BIDCH+ in a rapid pre-equilibrium, is suggested to give corresponding sulfoxide via. a cyclic transition state.

Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO4) for pyridines binding: Ligand binding assay

Abstract Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO 4 ) toward L = pyridine and its derivatives, based on the equilibrium constant for the reaction OsO 4 ·L + MTO = MTO·L + OsO 4 , has been measured. A successful correlation of log K eq with the Hammett σ constants of the substituents on the ligands was realized. A negative reaction constant, obtained for the reactions, shows that a more positive charge expands on the pyridine nitrogen in the complex MTO·L as compared with the complex OsO 4 ·L. So, the rhenium center acts as a better electron acceptor than osmium center. The thermodynamic parameters have been obtained and an excellent linear relationship was observed between the enthalpy and entropy of the reactions.

Kinetics and mechanism of acid catalyzed oxidation of phenol and substituted phenols by isoquinolinium bromochromate

Oxidation of phenols by isoquinolinium bromochromate (IQBC) in aqueous acetic acid leads to the formation of corresponding quinones. The reaction is first order with respect to both phenol and IQBC and catalysed by hydrogen ion. The rate of oxidation decreases with increase in dielectric constant of solvent indicating ion-dipole interaction. The rate of oxidation decreases with increase in concentration of KCl, this may be due to the formation of less reactive species by interaction of Cl− and protonated IQBC. The specific rates of oxidizing phenol reaction correlate with substituents constants affording negative reaction constants. Hammett plot is found to be valid and the correlation between enthalpies and free energies of activation is reasonably linear with an isokinetic temperature of 320 K.

Symmetry analysis for the nonlinear model of diffusion and reaction in porous catalysts

In this paper the nonlinear problem arising due to diffusion and reaction in catalyst is investigated. This problem was studied before using the Adomian decomposition, finite differences and homotopy analysis methods. Mathematical analysis for the case of the n th order reactions is performed and exact analytical solutions are investigated by the Lie symmetry method. Moreover, we look for exact solutions for negative reaction constants and show that the solutions exist for various values of n and the Thiele modulus ϕ . Graphs are plotted for these parameters and discussed.

Kinetics and mechanism of oxidation of aniline and substituted anilines by isoquinolinium bromochromate in aqueous acetic acid

Oxidation of anilines by isoquinolinium bromochromate (IQBC) in aqueous acetic acid leads to the formation of corresponding azobenzenes. The reaction is first order with respect to both aniline and IQBC and is catalyzed by hydrogen ion. The rate of oxidation decreases with increasing dielectric constant of solvent, indicating the presence of an ion-dipole interaction. The rate of oxidation decreases with increase in concentration of KCl, possibly due to the formation of less reactive species by interaction of Cl- and protonated IQBC. The specific rate of oxidizing species anilines reaction correlates with substituents constant affording a negative reaction constant. Hammett plot is found to be valid and the correlation between enthalpies and free energies of activation is reasonably linear with an isokinetic temperature of 401 K.

Kinetics and correlation analysis of reactivity in the oxidation of organic sulfides by butyltriphenylphosphonium dichromate

The oxidation of a number of monosubstituted aryl methyl, alkyl phenyl, dialkyl, and diphenyl sulfides by butyltriphenylphosphonium dichromate (BTPPD), to the corresponding sulfoxides, is first order with respect to BTPPD and is second order with respect to sulfide. The reaction is catalysed by hydrogen ions and the dependence is of second order. The oxidation of meta- and para-substituted aryl methyl sulfides correlated best in terms of Hammett equation, the reactions exhibited negative polar reaction constant. The ortho-substituted compounds correlated best in terms of Charton’s triparametric equation with negative polar constant and a small degree of steric hindrance. The oxidation of alkyl phenyl sulfides exhibited a good correlation in terms of Pavelich-Taft equation confirming that the electron-donating power of the alkyl group increases the rate, however, the reactivity is not markedly controlled by the bulkiness of the alkyl group. The rates of oxidation of sulfides were determined in nineteen organic solvents. An analysis of the solvent effect by multi-parametric equations indicated the relatively greater importance of the cation-solvating power of the solvents. A mechanism involving a single-step electrophilic oxygen transfer from BTPPD to the sulfide leading to polar transition state has been proposed.

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

Electrochemical oxidation of substituted benzylamines in aquo-acetic acid medium: substituent and solvent effects

Electrochemical oxidation of nine para- and meta-substituted benzylamines in varying mole fractions of acetic acid in water has been investigated in the presence of 0.1 M sulphuric acid as supporting electrolyte. The oxidation potentials correlate well with Hammett’s substituent constants affording negative reaction constants. The correlation of potential values with macroscopic solvent parameters is non-linear suggesting that the operation of both specific and non-specific solvent-solvent-solute interaction mechanisms. Multiple correlation analysis of the experimental data with Kamlet-Taft solvatochromic parameters is employed.

New nitrophenyl-substituted polyperoxotungstate catalyst: A more active and selective for the oxidation of sulfides by hydrogen peroxide

Kinetic studies on the catalytic activity of phenylphosphopolyperoxotungstate complexes [C 21H 37N] 2[(PhPO 3){WO(O 2) 2} 2{WO(O 2) 2H 2O}], ( A) and [C 21H 37N] 2[( p-NO 2Ph(O)PO 3){WO(O 2) 2} 2{WO(O 2) 2H 2O}], ( B) toward oxidation of organic sulfides by hydrogen peroxide were carried out in CH 3OH/H 2O mixed solvents. Compound B has been prepared for the first time as a modified form of A. The presence of a nitro group in B increases the electrophilicity of the catalyst, and leads to a higher activity and selectivity for the oxidation of sulfides to sulfoxides by H 2O 2. The reaction rate is found to be first-order in the [catalyst], the [sulfide] and the [H 2O 2] at low [H 2O 2] (<0.01). At higher [H 2O 2] (>0.06 M), the rate becomes almost zero-order in [H 2O 2]. The rate of oxidation increases with the sulfide nucleophilicity. Hammett correlations of the rate constants with σ for different substituted diaryl sulfides give negative reaction constants ( ρ ∼ −1.2). The effects of temperature variation, the ratio of water in the solvent, and the solution acidity on the reaction rate are also investigated. A three-membered-ring transition state has been proposed, in which the sulfide attacks the electrophilic O-atom of the W-peroxo species prior to a complete oxygen transfer, as a key step in the reaction mechanism. A radical mechanism has been ruled out.

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 &lt; ρ &gt; −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 &gt; 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

Solvent and substituent effects on the electrochemical oxidation of para‐ and meta‐substituted anilines

AbstractElectrochemical oxidation of 15 para‐ and meta‐substituted anilines in different mole fractions of water in 2‐methylpropan‐2‐ol has been investigated in the presence of 0.1 M sulfuric acid as a supporting electrolyte. The oxidation potential data of anilines correlate well with the Brown–Okamoto's substituent constants affording a negative reaction constant. The effect of para‐ and meta‐substituents on the oxidation potential confirms to Swain's F and R, affording negative reaction constants. The oxidation potential values also correlate satisfactorily with macroscopic solvent parameter such as relative permittivity, εr. The results of Kamlet–Taft multiple correlation analysis show that specific solute–solvent interactions play a dominant role in governing the reactivity. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 289–297, 2007

Preferential solvation effect on the kinetics of oxidation of parasubstituted anilines by tetramethylammonium fluorochromate

Tetramethylammonium fluorochromate (TMAFC) oxidation of para-substituted anilines, in varying mole fractions of benzene in 2-methylpropan-2-ol, in the presence ofp.toluenesulfonic acid (PTS) is first-order in TMAFC and PTS and is zero-order with respect to substrate. The TMAFC oxidation of six para-substituted anilines at 298-308 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 correlates with Brown-Okamoto's substituent constants affording negative reaction constant. The correlation of rate data with macroscopic solvent parameters such as e r and E T N is satisfactory while correlation with Kamlet-Taft's solvatochromic parameters (a, β, π*) suggests that the specific solute-solvent interactions play a major role in governing the reactivity.

Substituent and Solvent Effects on the Electrochemical Oxidation of para- and meta-Substituted Anilines

Electrochemical oxidation of fifteen para- and meta-substituted anilines in different mole fractions of acetic acid in water has been investigated in the presence of 0.1 M sulphuric acid as supporting electrolyte. The oxidation potential data of anilines correlates well with Hammett’s and Brown- Okamoto’s substituent constants affording negative reaction constants. The effect of para- and metasubstituents on the oxidation potential conforms to Swain’s F and R parameters, affording negative reaction constants. The oxidation potential values correlate well with macroscopic solvent parameters such as relative permittivity, εr, and polarity, ET N. Multiple correlation analysis also shows an excellent correlation with the oxidation potential values.

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

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.

Kinetics and mechanism of the oxidation of substituted benzyl alcohols by [bis(trifluoroacetoxy)iodo]benzene

The oxidation of substituted benzyl alcohols by bis(trifluoroacetoxy)iodo]benzene in aqueous acetic acid solution results in the formation of the corresponding benzaldehydes. The reaction is first order with respect to each of the alcohol, TFAIB and hydrogen ions. The oxidation of [1,1-2H2]benzyl alcohol exhibited the presence of a substantial primary kinetic isotope effect, indicating the cleavage of the α-C–H bond in the rate-determining step. Increase in the amount of water, in the solvent mixture of acetic acid and water, results in a decrease of the reaction rate. The analysis of the substituent effect in terms of Charton's LDR equation yielded an excellent correlation with negative reaction constants. A mechanism involving a hydride-ion transfer in the rate-determining step has been proposed.