Viscosity B -coefficients of sodium salts of some orthosubstituted benzoic acids in relation to hydrogen bonding and electronegativity

ViscosityB-coefficients of sodium salts of salicylic, thiosalicylic, anthranilic, 2,6-dihydroxy benzoic and ito-anisic acids have been measured at 298, 303 and 308 K respectively. Temperature dependence ofB-values suggest that the salts as well as their anions excepto-anisate behave as structure breakers. This is further supported by energy of activation data. The observedB-coefficients suggest probable involvement of simultaneous intra- and intermolecular hydrogen bonding between an ortho-substituent and the carboxylate group on the one hand and between the ortho-substituent and solvent water on the other. A good correlation betweenB-coefficients and the electronegativities of oxygen, nitrogen and sulphur atoms has been noted. The probable involvement of ringπ-electrons in depolymerising the water structure has been conjectured.     

Deuterium Kinetic Isotope Effect in the Oxidation of Deuteriated Butyric Acid-D7 with Chromium Trioxide in 85% Orthophosphoric Acid

A comparative study of the deuterium kinetic isotope effect in the oxidation of the deuteriated butyric acid-D₇ with Cr(VI) in 85% orthophosphoric acid has been carried out.The (k _HH/k _DD) KIE values between 5.7 and 7.0 indicate that the tunnelling is not involved in oxidation of deuteriated aliphatic carboxylic acids with chromium trioxide in acidic media and the oxidation KIE is determined by the zero point energy difference corresponding to the C-H/C-D stretching vibrations lost in the transition state.The results were compared with the large D-KIE observed in the oxidation of butyrate-D₇ with alkaline manganate caused by the simultaneous operation of zeropoint energy effect and tunnelling.     

Oxidative cleavage of S‐Arylmercaptoacetic acids by pyridinium chlorochromate: Kinetic and correlation analysis

Kinetics of oxidation of 24 S‐Arylmercaptoacetic acids (SAMA) by pyridinium chlorochromate (PCC) have been studied in acidmedium. The product of oxidation is the corresponding thiophenol. The rate data of meta‐ and para‐substituted acids have been correlated well with σ_I, σ_R^o values and the meta‐compounds correlate well with F,R values. The reaction constants are negative and of smaller magnitudes. Further, the ortho‐substituted acids show a good correlation with triparametric equation involving Taft's σ_I and σ_R^o and charton's steric parameter v. There is no considerable steric contribution to the total orthosubstituent effect. Based on these observations, the mechanism involving the formation of protonated arylsulfinylacetic acid intermediate, followed by an intramolecular rearrangement leading to the product thiophenol has been proposed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 683–688, 1999     

Kinetics and mechanism of Os(VIII)-catalyzed hexacyanoferrate(III) oxidation of α amino acids in alkaline medium

The kinetics of the Os(VIII)-catalyzed oxidation of glycine, alanine, valine, phenylalanine, isoleucine, lycine, and glutamic acid by alkaline hexacyanoferrate(III) reveal that these reactions are zero order in hexacyanoferrate(III) and first order in Os(VIII). The order in amino acid as well as in alkali is 1 at [amino acid] ?2.5 × 10^?2M and [OH^?] ?1.3 × 10^?M, but less than unity at higher concentrations of amino acids or alkali. The active oxidizing species under the experimental conditions is OsO₄(H₂O) (OH)^?. The ferricyanide is merely used up to regenerate the Os(VIII) species from Os(VI) formed during the reaction. The structural influence of amino acids on the reactivity has been discussed. The amino acids during oxidation are shown to be degraded through intermediate keto acids. The kinetic data are accommodated by considering the interaction between the conjugate base of the amino acids and the active oxidizing species of Os(VIII) to form a transient complex in the primary rate-determining step. The catalytic effect of hexacyanoferrate(II) has been rationalized.     

Kinetics and mechanism of the oxidation of some α‐hydroxy acids by 2,2′‐bipyridinium chlorochromate

The oxidation of glycolic, lactic, malic, and a few substituted mandelic acids by 2,2′‐bipyridinium chlorochromate (BPCC) in dimethylsulphoxide leads to the formation of corresponding oxoacids. The reaction is first order each in BPCC and the hydroxy acids. The reaction is catalyzed by the hydrogen ions. The hydrogen ion dependence has the form: k_obs = a + b [H⁺]. The oxidation of α‐deuteriomandelic acid exhibited a substantial primary kinetic isotope effect (k_H/k_d = 5.29 at 303 K). Oxidation of p‐methylmandelic acid was studied in 19 different organic solvents. The solvent effect has been analyzed by using Kamlet's and Swain's multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 248–254, 2002     

Trinuclear Non‐Heme Iron Complexes Based on 4‐Substituted 2,6‐Diacylpyridine Ligands as Catalysts in Aerobic Allylic Oxidations

Taking the regio‐ and chemoselectivities of our iron complex precursors with pyridine core in aerobic oxidations into account, we envisioned a more effective influence on catalytic properties by the introduction of different substituents in 4‐position of the pyridine moiety. The synthesis of these novel 4‐substituted (pyridine‐2,6‐diyl)dipropanoic acids 4 is described. Analogously to the unsubstituted derivative, ligands 4 reacted with Fe(ClO₄)₃ to form trinuclear Fe₃(μ₃‐O) complexes 3 , which were tested in the aerobic Gif‐type oxidation of α‐pinene to myrtenol, verbenone, myrtenal, and pinene oxide. The electronic nature of the substituents was found to slightly effect the ratio of allylic oxidation/epoxidation, whereas its influence on the oxidation preference of secondary to primary C? H bonds is negligible as compared to the unsubstituted complex.     

Oxidation of some α‐hydroxy acids by tetraethylammonium chlorochromate: A kinetic and mechanistic study

The oxidation of glycolic, lactic, malic, and a few substituted mandelic acids by tetraethylammonium chlorochromate (TEACC) in dimethylsulfoxide leads to the formation of corresponding oxoacids. The reaction is first order each in TEACC and hydroxy acids. Reaction is failed to induce the polymerization of acrylonitrile. The oxidation of α‐deuteriomandelic acid shows the presence of a primary kinetic isotope effect (k_H/k_D = 5.63 at 298 K). The reaction does not exhibit the solvent isotope effect. The reaction is catalyzed by the hydrogen ions. The hydrogen ion dependence has the following form: k_obs = a + b[H⁺]. Oxidation of p‐methylmandelic acid has been studied in 19 different organic solvents. The solvent effect has been analyzed by using Kamlet's and Swain's multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 50–55, 2010     

Kinetics and mechanism of the liquid-phase oxidation of <Emphasis Type="Italic">n</Emphasis>-carboxylic acids

Short reaction chains participate in the oxidation of C₃-C₆, C₈, C₁₀, and C₁₂ n-carboxylic acids. The quadratic-law recombination of peroxy radicals occurs both without and with chain termination. The ratio of the rate constants of these reactions increases to k′/k _t = 4.5 on passing from propanoic acid to pentanoic acid, and then it decreases almost to zero for dodecanoic acid. The anomalous variation of the k′/k _t ratio is explained by the fact that the radicals resulting from carboxylic acid oxidation at the CH bonds nearest to the functional group make different contributions to the recombination process, depending on the carbon chain length. The cross recombination of secondary hydrocarbon peroxy radicals with the HO₂^· radicals resulting from the oxidation of carboxylic acids at the β-C-H bonds proceeds without chain termination.     

Oxidative cleavage of S-arylmercaptoacetic acids by sodium perborate: Kinetic and correlation study

Kinetics of oxidation of twenty six S-arylmercaptoacetic acids (SAMA) (I) by sodium perborate (PB) have been studied in acid medium. The product of oxidation is the corresponding thiophenol. The rate data of meta-and para-substituted acids have been correlated with DSP equations. While the para-compounds correlate well with σ_I and σ_R° values, the meta-compounds correlate well with σ_I and σ values. The reaction constants are negative and of smaller magnitudes. Further, the ortho-substituted acids show a good correlation with a triparametric equation involving Taft's σ_I and _R° and Charton's steric parameter ν. There is a considerable steric contribution to the total ortho-substituent effect. Based on these observations, mechanism involving the formation of protonated arylsulfinylacetic acid intermediate, followed by an intramolecular rearrangement leading to the product thiophenol has been proposed. © 1995 John Wiley & Sons, Inc.     

Kinetics and mechanism of the oxidation of formic and oxalic acids by quinolinium fluorochromate

Kinetics and mechanism of oxidation of formic and oxalic acids by quinolinium fluorochromate (QFC) have been studied in dimethylsulphoxide. The main product of oxidation is carbon dioxide. The reaction is first-order with respect to QFC. Michaelis-Menten type of kinetics were observed with respect to the reductants. The reaction is acid-catalysed and the acid dependence has the form: k_obs =a +b[H⁺]. The oxidation of α-deuterioformic acid exhibits a substantial primary kinetic isotope effect (k_H/k_D = 6.01 at 303 K). The reaction has been studied in nineteen different organic solvents and the solvent effect has been analysed using Taft’s and Swain’s multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical cyclic transition state in the rate-determining step. Suitable mechanisms have been proposed.     

Hydrogen Peroxide-Responsive Triggers Based on Borinic Acids: Molecular Insights into the Control of Oxidative Rearrangement

Arylborinic acids represent new, efficient, and underexplored hydrogen peroxide-responsive triggers. In contrast to boronic acids, two concomitant oxidative rearrangements are involved in the complete oxidation of these species, which might represent a major limitation for an efficient effector (drug or fluorophore) release. Herein, a comprehensive study of H₂O₂-mediated unsymmetrical arylborinic acid oxidation to investigate the factors that could selectively guide their oxidative rearrangement is described. The o-CF₃ substituent was found to be an excellent directing group allowing a complete regioselectivity on borinic acid models. This result was successfully applied to synthesizing new borinic acid-based fluorogenic probes, which exclusively release the fluorescent moiety upon H₂O₂ treatment. These compounds maintained their superior kinetic properties compared to boronic acids, thus further enhancing the potential of arylborinic acids as valuable new H₂O₂-sensitive triggers.     

Studies on the Air Oxidation of Some Arsenic(III) Compounds

The air oxidation of As(III) oxides [(PhAsO) _x and Ph₂As-O-AsPh₂] and thioesters [Ph-As(SPh)₂, Ph₂As-SPh Me-As(SPh)₂, Me₂As-SPh], in chloroform and in methanol was studied. The air oxidation in chloroform was faster probably because the solubility of dioxygen is greater than in methanol, and it is favored by the electron-withdrawing phenyl groups bound to As(III). The products obtained were the arsonic or arsinic acids and diphenyl disulfide. In one case, diphenyl disulfide and thiophenol were produced. The results can be rationalized by assuming first hydrolysis of the As(III) compounds to arsonous or arsinous acids followed by their oxidation to arsonic and arsinic acids, which should involve the binding of dioxygen to As(III). The other hypothesis assumes first the binding of dioxygen to As(III) of these oxides and thioesters followed by the decomposition of the adducts. The binding of the ground state dioxygen to As(III) may have biochemical implications for toxicity or chemotherapy of arsenic(III) compounds.     

Substituent effects on the kinetics of the cerium(IV) oxidation of mandelic acids in sulfate media. Anomalous kinetic behavior of the methoxy derivative

The kinetics of the cerium(IV) oxidation of p-nitro and p-methoxymandelic acids have been investigated in H₂SO₄-MHSO₄ (M⁺ = Li⁺, Na⁺, K⁺) and H₂SO₄-MClO₄ (M⁺ = H⁺, Na⁺) mixtures at a constant total electrolyte concentration of 2.00 mol/dm³. The oxidation of p+nitromandelic acid proceeds through two [H⁺]-independent paths, as was also observed for some substituted mandelic acids studied previously. The kinetic behavior of the p-methoxy derivative differs from that of the other mandelic acids in that (1) the oxidation occurs via two [H⁺]-dependent paths, (2) the reaction rate is anomalously high, (3) the activation enthalpy and entropy of the overall process are markedly lower. It provides strong support to the suggestion that a different mechanism is operative. The substituent effects and the reaction mechanism are discussed.     

Mechanism of oxidation of phenylthioacetic acids by potassium peroxodiphosphate

The rates of oxidation of phenylthioacetic acid (PTAA) and several substituted phenylthioacetic acids by potassium peroxodiphosphate (PP) in 50% (v/v) aqueous acetic acid have been studied in detail. The rate of oxidation is expressed as An analysis of the dependence of the rate on [H⁺] reveals that H₃P₂O is the active oxidizing species in the oxidation. The effect of ring substituents on the rate gives a ρ⁺ value of -0.45 ± 0.03 (r = 0.998, s = 0.02 at 40°C), pointing to the development of an electron-deficient center in the transition state. The results are discussed in terms of a mechanism involving the rate-determining formation of an intermediate between PP and phenylthioacetic acids, followed by the decomposition of the intermediate. These kinetic results are compared with those obtained in the oxidation of phenylthioacetic acids by peroxodisulfate.     

Reactivity of Tyr–Leu and Leu–Tyr dipeptides: identification of oxidation products by liquid chromatography–tandem mass spectrometry

The exposure of peptides and proteins to reactive hydroxyl radicals results in covalent modifications of amino acid side‐chains and protein backbone. In this study we have investigated the oxidation the isomeric peptides tyrosine–leucine (YL) and leucine–tyrosine (LY), by the hydroxyl radical formed under Fenton reaction (Fe²⁺/H₂O₂). Through mass spectrometry (MS), high‐performance liquid chromatography (HPLC‐MS) and electrospray tandem mass spectrometry (HPLC‐MSⁿ) measurements, we have identified and characterized the oxidation products of these two dipeptides. This approach allowed observing and identifying a wide variety of oxidation products, including isomeric forms of the oxidized dipeptides. We detected oxidation products with 1, 2, 3 and 4 oxygen atoms for both peptides; however, oxidation products with 5 oxygen atoms were only present in LY. LY dipeptide oxidation leads to more isomers with 1 and 2 oxygen atoms than YL (3 vs 5 and 4 vs 5, respectively). Formation of the peroxy group occurred preferentially in the C‐terminal residue. We have also detected oxidation products with double bonds or keto groups, dimers (YL–YL and LY–LY) and other products as a result of cross‐linking. Both amino acids in the dipeptides were oxidized although the peptides showed different oxidation products. Also, amino acid residues have shown different oxidation products depending on the relative position on the dipeptide. Results suggest that amino acids in the C‐terminal position are more prone to oxidation. Copyright © 2009 John Wiley & Sons, Ltd.     

Cr(VI) oxidation using cetylpicolinium dichromate: Kinetics of oxidation of benzaldehydes with a green protocol

Oxidation kinetics of benzaldehyde and some of its ortho- and para-monosubstituted derivatives have been studied using cetylpicolinium dichromates, a class of novel phase transfer oxidants, in dichloromethane medium. The rate of reaction is first order with respect to oxidant and fractional order with respect to the substrates. The Michaelis–Menten type oxidation was observed with respect to the substrates. Benzaldehydes are found to be oxidized to their corresponding acids. The mechanism of oxidation reaction has been suggested based on the solvent isotope effect, Hammett plot, and thermodynamic study. The solvent isotope effect (k_CHCl3/k_CDCl3 = 1.57) indicates the involvement of hydrogen exchange with the medium during oxidation reactions. A strong influence of specific solute–solvent interactions on the rate of the reaction is observed. Both the electron-withdrawing and electron-releasing substituents on the substrates accelerate the rate of reaction.     

Kinetics and mechanism of the oxidation of aliphatic aldehydes by tetrabutylammonium tribromide

The oxidation of six aliphatic aldehydes by tetrabutylammonium tribromide (TBATB) in aqueous acetic acid resulted in the formation of corresponding carboxylic acids. The reaction is first order with respect to TBATB and the aldehydes. The oxidation of deuteriated acetaldehyde (MeCDO) showed the presence of substantial kinetic isotope effect (k_H/k_D = 5.92 at 298 K). Addition of tetrabutylammonium chloride has no effect on the reaction rate. Tribromide ion has been proposed as the reactive oxidizing species. The rate constants correlate well with Taft's σ* values, the reaction constant being negative. A mechanism involving a hydride‐ion transfer from the aldehyde hydrate to the oxidant in the rate‐determining step has been suggested. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 390–395, 2001     

Physicochemical Properties of 1,2-Diacylhydrazines Derived from Aliphatic Carboxylic Acids

The solubility and resistance to oxidation and hydrolysis of 1,2-diacylhydrazines derived from acetic, propionic, butyric, and valeric acids were studied in view of possible application of these compounds for concentrating nonferrous metals. The pK ₁ and pK ₂ values of the 1,2-diacylhydrazines were determined. The stability of conformers and the electron density distribution in the coordination-active moiety were estimated by SCF MO LCAO semiempirical calculations in the MNDO-PM3 approximation.     

Electrochemical characterization of some ferrocenylcarboxylic acids

Summary Several carboxy-substituted ferrocene compounds are prepared and investigated by cyclic voltammetry in acetonitrile solution. The half-wave potentials of most of the acids studied (E _1/2=0.34–0.58 V versus s.c.e.) are more positive than that of ferrocene (0.33 V), reflecting a diminished susceptibility to oxidation of these compounds relative to the parent metallocene. Only -ferrocenylpropionic acid (0.32₅ V) is effectively identical with the latter in its oxidation behaviour, and -ferrocenylbutyric acid (0.31 V) tends to be more readily oxidized. The results are of interest for subsequent chemical oxidation studies of ferrocenylcarboxylic acids.