Cationic Transition State Research Articles

Mutational, Structural, and Kinetic Evidence for a Dissociative Mechanism in the GDP-Mannose Mannosyl Hydrolase Reaction

GDP-mannose hydrolase (GDPMH) catalyzes the hydrolysis of GDP-alpha-d-sugars by nucleophilic substitution with inversion at the anomeric C1 atom of the sugar, with general base catalysis by H124. Three lines of evidence indicate a mechanism with dissociative character. First, in the 1.3 A X-ray structure of the GDPMH-Mg(2+)-GDP.Tris(+) complex [Gabelli, S. B., et al. (2004) Structure 12, 927-935], the GDP leaving group interacts with five catalytic components: R37, Y103, R52, R65, and the essential Mg(2+). As determined by the effects of site-specific mutants on k(cat), these components contribute factors of 24-, 100-, 309-, 24-, and >/=10(5)-fold, respectively, to catalysis. Both R37 and Y103 bind the beta-phosphate of GDP and are only 5.0 A apart. Accordingly, the R37Q/Y103F double mutant exhibits partially additive effects of the two single mutants on k(cat), indicating cooperativity of R37 and Y103 in promoting catalysis, and antagonistic effects on K(m). Second, the conserved residue, D22, is positioned to accept a hydrogen bond from the C2-OH group of the sugar undergoing substitution at C1, as was shown by modeling an alpha-d-mannosyl group into the sugar binding site. The D22A and D22N mutations decreased k(cat) by factors of 10(2.1) and 10(2.6), respectively, for the hydrolysis of GDP-alpha-d-mannose, and showed smaller effects on K(m), suggesting that the D22 anion stabilizes a cationic oxocarbenium transition state. Third, the fluorinated substrate, GDP-2F-alpha-d-mannose, for which a cationic oxocarbenium transition state would be destabilized by electron withdrawal, exhibited a 16-fold decrease in k(cat) and a smaller, 2.5-fold increase in K(m). The D22A and D22N mutations further decreased the k(cat) with GDP-2F-alpha-d-mannose to values similar to those found with GDP-alpha-d-mannose, and decreased the K(m) of the fluorinated substrate. The choice of histidine as the general base over glutamate, the preferred base in other Nudix enzymes, is not due to the greater basicity of histidine, since the pK(a) of E124 in the active complex (7.7) exceeded that of H124 (6.7), and the H124E mutation showed a 10(2.2)-fold decrease in k(cat) and a 4.0-fold increase in K(m) at pH 9.3. Similarly, the catalytic triad detected in the X-ray structure (H124- - -Y127- - -P120) is unnecessary for orienting H124, since the Y127F mutation had only 2-fold effects on k(cat) and K(m) with either H124 or E124 as the general base. Hence, a neutral histidine rather than an anionic glutamate may be necessary to preserve electroneutrality in the active complex.

Computational Study of the Stone−Wales Transformation in C36

The transition of the D6h neutral and charged isomers to D2d isomers of C36 via Stone-Wales transformation has been studied by means of the hybrid density functional method (B3LYP). The results show that the transition state (TS) and reaction pathway could be identified for the rearrangement from C36-D6h to C36-D2d on the potential energy surface. We found that the neutral and charged transition states all have C2 molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other kind of possible TS with a carbene-like structure along the stepwise reaction path does not exist as a stationary point with the density functionals utilized here. The classical barriers are 6.23 eV through the neutral TS, 6.37 eV through the anionic TS, and 6.29 eV through the cationic TS at the B3LYP/6-31G level of theory.

Role of Rare Earth Cations in Y Zeolite for Hydrocarbon Cracking

Reaction kinetics data were collected for isobutane conversion over a series of ultra stable Y (USY) zeolite catalysts with and without rare earth cations and subjected to various extents of dealumination by steaming. We conducted these reaction studies at low temperatures (523-573 K) using isobutane feed streams containing known levels of isobutylene (100-400 ppm) so that the kinetics were controlled by bimolecular hydride transfer and oligomerization/beta-scission processes with little or no participation of monomolecular initiation reactions. These experimental conditions led to stable catalyst performance with the main products of isobutane conversion being propane, n-butane, and isopentane, with smaller amounts of propylene, trans-2-butene, and cis-2-butene. The rates of formation of these products per Brønsted acid site (as counted by pyridine adsorption) depended exponentially on Brønsted acid site density, regardless of whether the catalyst contained rare earth cations. Kinetic modeling showed an exponential dependence of hydride transfer and oligomerization/ beta-scission reaction rates on Brønsted acid site density which translated into composite activation energies for these reactions having a linear relationship with site density. Based on results in the literature from theoretical calculations, we suggest that increasing Brønsted acid site density in zeolite Y leads to larger zeolite elasticity, increased stabilization of cationic transition states, and lower composite activation barriers for hydride transfer and beta-scission steps. The role of rare earth cations, therefore, is to ensure the retention of high Brønsted acid site density under hydrothermal conditions, such as in fluid catalytic cracking (FCC) regenerators, where steam would dealuminate the Y zeolite framework and reduce this site density. It is for this reason that hydride transfer reaction rates are high in the presence of rare earth cations and lead to higher yields of less olefinic gasoline during FCC.

Molecular orbital studies on the Wagner-Meerwein migration in some acyclic pinacol—pinacolone rearrangements

The semi-empirical PM3 SCF-MO method is used to investigate the Wagner-Meerwein migration of various groups during the pinacol-pinacolone rearrangement of some acyclic systems. Pinacol first protonates and dehydrates to form a carbocation that undergoes a 1,2-migration to form a protonated ketone, which then deprotonates to yield the pinacolone product. We study the Wagner-Meerwein migration of hydride, methyl, ethyl, isopropyl,t- butyl, phenyl and heterocylic 2-, 3-and 4-pyridyl groups in various acyclic 1,2-diol (pinacol) systems as they rearrange to pinacolones. This 1,2-migration involves a three-centred moiety in the cationic transition state. The migratory aptitude predicted here follows the order: hydride >t-butyl > isopropyl > ethyl > methyl > phenyl, which accords well with available experimental data and/or chemical intuition, reflecting also on the ability of the group involved to carry positive charge in the transition state. The structure of the migrating group (whether aliphatic or aromatic) within the transition state also supports the stabilising role of delocalisation of positive charge for reaction feasibility. Geometrical and thermodynamic considerations coincide in assigning the following order to relative “earliness” of the transition state along the reaction pathway:t-butyl > isopropyl > phenyl > methyl > 2-pyridyl > 4-pyridyl

Roles of Individual Enzyme−Substrate Interactions by α-1,3-Galactosyltransferase in Catalysis and Specificity,

The retaining glycosyltransferase, alpha-1,3-galactosyltransferase (alpha3GT), is mutationally inactivated in humans, leading to the presence of circulating antibodies against its product, the alpha-Gal epitope. alpha3GT catalyzes galactose transfer from UDP-Gal to beta-linked galactosides, such as lactose, and in the absence of an acceptor substrate, to water at a lower rate. We have used site-directed mutagenesis to investigate the roles in catalysis and specificity of residues in alpha3GT that form H-bonds as well as other interactions with substrates. Mutation of the conserved Glu(317) to Gln weakens lactose binding and reduces the k(cat) for galactosyltransfer to lactose and water by 2400 and 120, respectively. The structure is not perturbed by this substitution, but the orientation of the bound lactose molecule is changed. The magnitude of these changes does not support a previous proposal that Glu(317) is the catalytic nucleophile in a double displacement mechanism and suggests it acts in acceptor substrate binding and in stabilizing a cationic transition state for cleavage of the bond between UDP and C1 of the galactose. Cleavage of this bond also linked to a conformational change in the C-terminal region of alpha3GT that is coupled with UDP binding. Mutagenesis indicates that His(280), which is projected to interact with the 2-OH of the galactose moiety of UDP-Gal, is a key residue in the stringent donor substrate specificity through its role in stabilizing the bound UDP-Gal in a suitable conformation for catalysis. Mutation of Gln(247), which forms multiple interactions with acceptor substrates, to Glu reduces the catalytic rate of galactose transfer to lactose but not to water. This mutation is predicted to perturb the orientation or environment of the bound acceptor substrate. The results highlight the importance of H-bonds between enzyme and substrates in this glycosyltransferase, in arranging substrates in appropriate conformations and orientation for efficient catalysis. These factors are manifested in increases in catalytic rate rather than substrate affinity.

Transition state analogues for enzymes of nucleic acid metabolism.

Knowledge of enzymatic transition states permits the logical design of transition state analogues. Kinetic isotope effects have been used to solve transition state structures for several N-ribosyltransferases. Nucleoside hydrolases from protozoan parasites show ribooxacarbenium ion character at their transition states, but with different extents of activation at the leaving group and the oxacarbenium ion. Transition state analogues are designed to capture these interactions and provide isozyme-specific inhibitors. Ricin A-chain is an RNA N-ribohydrolase for a single site on 28S rRNA. Its transition state resembles a fully dissociated ribooxacarbenium ion. A transition state analogue for ricin A-chain mimics the fully dissociated purine and cationic ribosyl transition state. The transition state for human purine nucleoside phosphorylase (PNP) is more dissociated than for the bovine enzyme. Immucillin-H, a powerful transition state inhibitor for human PNP, has entered clinical trials as an anti T-cell agent.

Ionic states of substrates and transition state analogues at the catalytic sites of N-ribosyltransferases.

Purine nucleoside phosphorylase (PNP) and hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) catalyze N-ribosidic bond cleavage in purine nucleosides and nucleotides, with addition of phosphate or pyrophosphate to form phosphorylated alpha-D-ribose products. The transition states have oxacarbenium ion character with a positive charge near 1'-C and ionic stabilization from nearby phosphoryl anions. Immucillin-H (ImmH) and Immucillin-H 5'-PO(4) (ImmHP) resemble the transition state charge when protonated at 4'-N and bind tightly to these enzymes with K(d) values of 20 pM to 1 nM. It has been proposed that Immucillins bind as the 4'-N neutral form and are protonated in the slow-onset step. Solution and solid-state NMR spectra of ImmH, ImmHP, guanosine, and GMP in complexes with two PNPs and a HGPRTase have been used to characterize their ionization states. Results with PNP*ImmH*PO(4) and HGPRTase*ImmHP*MgPP(i) indicate protonation at N-4' for the tightly bound inhibitors. The 1'-(13)C and 1'-(1)H resonances of bound Immucillins showed large downfield shifts as compared to Michaelis complexes, suggesting distortion of 1'-C toward sp(2) geometry. The Immucillins act as transition state mimics by binding with neutral iminoribitol groups followed by 4'-N protonation during slow-onset inhibition to form carbocationic mimics of the transition states. The ability of the Immucillins to mimic both substrate and transition state features contributes to their capture of transition state binding energy. Enzyme-activated phosphoryl nucleophiles bound to PNP and HGPRTase suggest enhanced electrostatic stabilization of the cationic transition states. Distortion of the oxacarbenium ion mimic toward transition state geometry is a common feature of the three distinct enzymatic complexes analyzed here. Substrate complexes, even in catalytically cycling equilibrium mixtures, do not reveal similar distortions.

Powering DNA repair through substrate electrostatic interactions.

The reaction catalyzed by the DNA repair enzyme uracil DNA glycosylase (UDG) proceeds through an unprecedented stepwise mechanism involving a positively charged oxacarbenium ion sugar and uracil anion leaving group. Here we use a novel approach to evaluate the catalytic contribution of electrostatic interactions between four essential phosphodiester groups of the DNA substrate and the cationic transition state. Our strategy was to substitute each of these phosphate groups with an uncharged (R)- or (S)-methylphosphonate linkage (MeP). We then compared the damaging effects of these methylphosphonate substitutions on catalysis with their damaging effects on binding of a cationic 1-azadeoxyribose (1-aza-dR(+)) oxacarbenium ion analogue to the UDG-uracil anion binary complex. A plot of log k(cat)/K(m) for the series of MeP-substituted substrates against log K(D) for binding of the 1-aza-dR(+) inhibitors gives a linear correlation of unit slope, confirming that the electronic features of the transition state resemble that of the 1-aza-dR(+), and that the anionic backbone of DNA is used in transition state stabilization. We estimate that all of the combined phosphodiester interactions with the substrate contribute 6-8 kcal/mol toward lowering the activation barrier, a stabilization that is significant compared to the 16 kcal/mol catalytic power of UDG. However, unlike groups of the enzyme that selectively stabilize the charged transition state by an estimated 7 kcal/mol, these phosphodiester groups also interact strongly in the ground state. To our knowledge, these results provide the first experimental evidence for electrostatic stabilization of a charged enzymatic transition state and intermediate using the anionic backbone of DNA.

Influence of nonbonded interactions in the kinetics of formation of chalcogenol esters from chalcogenoacetylenes

AbstractThe influence of nonbonded interactions in the kinetics of formation of chalcogenol (thiol and selenol) esters from chalcogenoacetylenes was studied by molecular modeling. Using semiempirical and density functional theory methods it was possible to explain the differences between the reaction rates for the analogous sulfur and selenium chalcogenoacetylenes as well as evaluate the structural and electronic effects (nonbonded interactions) on the formation of the esters. The differences in the reaction rates can be explained in terms of the carbocation stabilization by the chalcogen atom. It is proposed that these differences are due to the differences in the intensity of the dominant interaction π/nY between the nonbonding orbitals of sulfur and selenium with the vacant orbital of carbon in the cationic transition state. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 267–273, 2003

Small carbon clusters (C n0, C n+, C n−) from acyclic and cyclic precursors : Neutralization–reionization and theory

Dissociative ionization of several linear, cyclic, and branched molecules was used to generate C 3 + and C 4 + carbon clusters. However, mass spectra from collisionally activated dissociation (CAD) and neutralization–reionization (NR) under a wide variety conditions were indistinguishable, indicating only one isomer or same mixture of isomers. Likewise, CAD and NR spectra of C 4 + and C 3 + from 13 CH 2 CHCH 13 CH 2 and CH 3 13 C CH 2 show complete 12 C / 13 C scrambling. CAD cross sections are consistent with C 4 +–C 6 + ions as mainly linear isomers and C 7 + ions from cyclic precursors as mainly cyclic. CCSD(T) ab initio and B3LYP density functional theory calculations with large basis sets yielded structures and energies for a variety of cationic, neutral, and anionic carbon clusters and transition states for some cationic rearrangements. The calculated enthalpies of formation, dissociation energies, ionization energies, and electron affinities mostly agree with currently accepted experimental data. However, CCSD(T) calculations indicate adiabatic ionization energies for C 3 (11.79 eV) and C 4 isomers (10.48, 10.82, and 10.86 eV) that differ >1 eV from recent measurements.

Solvolysis of 1‐tert‐Butyl‐1‐(4‐phenoxyphenyl) Methyl Bromide and Chloride. Further Example for Possible Contribution of Non‐canonical Resonance Structure in the Transition State

AbstractThe solvolysis of 1‐tert‐butyl‐1‐(4‐phenoxy)methyl bromide (5b) and chloride (5c) in a variety of solvents were carried out. Limiting SN 1 mechanisms were suggested from the observation of linear relation ships using single‐parameter Winstein‐Grunwald type correlation between logarithm of rate constants with YxBnBr and YxBnCl scales, respectively. This result indicated positive charge delocalization over both arylrings and provided further evidence for possible contribution of non‐canonical resonance structure in the cationic transition state.

Extended charge delocalization to 4‐phenoxy substituent in benzhydryl solvolysis: possible contribution of non‐canonical resonance structure in the cationic transition state

AbstractSolvolytic reactivities of 4‐nitrobenzhydryl bromides (2b–5b) and chlorides (2c–5c) were studied using single‐ and dual‐parameter Grunwald–Winstein‐type correlation analyses with YBnX and Y xBnX scales, respectively. Extended charge delocalization over two aryl rings at cationic transition states were found for 3 and 5, but not for 2 or 4. Calculations of the charge distributions in 3c and in the corresponding cation 3a were performed using a Hartree–Fock approximation (RHF/6‐31G* basis set) and density functional models (pBP/DN** and other basis sets), respectively, on Mulliken population analysis and on electrostatic potential analysis. The possible contribution of non‐canonical resonance structure is discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Stereochemical control factors in the Hantzsch thiazole synthesis: a Hammett substitution correlation analysis.

[reaction--see text] It is possible to correlate the distribution of stereochemical products produced during a Hantzsch thiazole synthesis according to the Hammett free-energy equation. This analysis confirms the presumed control of the rate of epimerization during thiazole formation due to stabilization of a cationic transition state intermediate during dehydration of the thiazoline ring system. In the chemical system under study, the stereochemical outcome of the reaction also appears to occur according to a kinetically controlled protonation of a thiazoline tautomer.

Solvolytic Reactions in Fluorinated Alcohols. Role of Nucleophilic and Other Solvation Effects

Rate constants and products for solvolyses of chlorodiphenylmethane (Ph2CHCl) and p-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)/water and TFE/ethanol are reported, along with additional kinetic data for solvolyses of tert-butyl and other alkyl halides (RX) in 97% w/w TFE/water and in 97% w/w hexafluoropropan-2-ol/water (HFIP). Results are discussed in terms of the solvent ionizing power (Y) and the solvent nucleophilicity (N), and contributions from other solvation effects are considered. Comparisons with other SN1 solvolyses show that solvolyses of Ph2CHCl in TFE mixtures are unexpectedly fast, but product ratios are unexceptional. An additional solvation effect influences solvolyses leading to delocalized cations, and a delocalized cationic transition state for concerted elimination may explain the recent results of Takeuchi et al., (J. Org. Chem. 1997, 62, 4904) without the need to postulate additional specific solvation effects for adamantyl systems, such as Bronsted-base solvation of α- and β-hydrogen atoms; concerted elimination may occur because simple tertiary alkyl cations are too unstable to form in predominantly aqueous media. Iodide/bromide and bromide/chloride rate ratios are very similar for 1-adamantyl halides and corresponding solvolyses of tert-butyl halides; these ratios decrease in the order aq EtOH > TFE > HFIP, as expected for an electrophilic solvation effect (this effect can readily be incorporated into Y values). From kinetic data for a series of tertiary alkyl chlorides in 97% TFE/water, it is shown that the susceptibility of rates of solvolyses of RCl to N decreases with an increase in steric hindrance or with an increase in charge stabilization. Also, the small kinetic solvent isotope effects for typical solvolyses (e.g., methyl tosylate) indicate that nucleophilic attack lags behind heterolysis of the C−X bond.

Solvolysis of benzylic chlorides with extended charge delocalization. ?-tert-butyl(2-naphthyl)methyl, 9-fluorenyl and monosubstituted benzhydryl chlorides

Rate constants of solvolysis of α-tert-butyl(2-naphthyl)methyl chloride (1), 9-fluorenyl chloride (2) and a series of monosubstituted benzhydryl chlorides (3) in a wide range of solvents were measured. Grunwald–Winstein-type correlation analysis of log k for 2 and 3 against YBnCl, with or without nucleophilicity N, yielded less satisfactory linear correlations than that against log k(1). A new scale of solvent ionizing power, YxBnCl, for the correlation of solvolytic reactivities of benzylic chlorides with extended charge delocalization based on log k(1) was developed. Application to the mechanistic study suggested the solvolysis of 2 and 4-nitrobenzhydryl chloride were non-limiting. Hammett plots against σ+ constants exhibited more negative ρ values in less nucleophilic solvents. In a benzhydryl chloride containing a strong deactivating substituent, such as 4-nitro, the positive charge delocalizes mainly over the unsubstituted ring in the cationic transition state. The uneven charge distribution was also confirmed by Mulliken population analysis at the level of the RHF/6–31G*//RHF/3–21G(*) basis set for cations. Comparison of the results of correlation analysis using the equation log(k/k0) = mY vs the equation log(k/k0) = mY+hI, and using the equation log(k/k0) = mY+lN vs the equation log(k/k0) = mY+lN+hI indicated the use of YBnCl or YxBnCl could give a better understanding of solvolytic mechanisms than the combinatorial use of YCl and I. © 1998 John Wiley & Sons, Ltd.

Solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes. Nucleophilic solvent intervention and extendedYBnCl scale

The solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes (4a–d) in aqueous acetone, aqueous ethanol, aqueous methanol and ethanol–trifluoroethanol was studied. Grunwald–Winstein-type correlation analysis using the YBnCl scale suggests significant nucleophilic solvent intervention in the case of 1-chloro-1-(4-methyl)phenylethane (4a). Increasing bulkiness of the 1-alkyl substituent from methyl (4a) to ethyl (4b), to isopropyl (4c) and to tert-butyl (4d) resulted in a gradual change to limiting SN1 mechanisms. The observed excellent linear correlations with YBnCl and the good solubility in high-water-containing binary solvents made 4d a suitable reference standard for deriving more YBnCl values. A positive azide salt effect was realized in the solvolysis of 4a but not 4d. A small decrease in the β-deuterium kinetic isotope effect from 4a to 1-chloro-1-(4-methoxyphenyl)propane (5) suggested the presence of additional stabilization of the benzylic cationic transition state. Howver, no relationship between k(CH3)/k(CD3) and the solvent effect was found. The superiority of employing the YBnCl scale over the combination of YCl and I scales in the mehanistic study was observed. © 1997 John Wiley & Sons, Ltd.

Gas-Phase Substituent Effects in Highly Electron-Deficient Systems. II. Stabilities of 1-Aryl-2,2,2-trifluoroethyl Cations Based on Chloride-Transfer Equilibria

Abstract The relative stabilities of 1-aryl-2,2,2-trifluoroethyl cations were determined based on the chloride ion-transfer equilibria in the gas phase. An application of the Yukawa–Tsuno equation to this substituent effect on the equilibrium constants gave a remarkably larger r+ of 1.53 and a ρ of −10.6, supporting our previous conclusion that the highly electron-deficient benzylic carbocation systems are characterized by extremely high resonance demands. This r+ value, furthermore, conformed a linear relationship between the r+ value and the relative stability of the unsubstituted member of the respective benzylic carbocations, clearly demonstrating a continuous spectrum of varying resonance demands characteristic of the stabilities of carbocations. The π-delocalization of the positive charge into the aryl π-system increases with the destabilization of a carbocation by the α-substituent(s) linked to the central carbon. In addition, the r+ value of 1.53 for 1-aryl-2,2,2-trifluoroethyl cations was found to be in complete agreement with that for the solvolysis of 1-aryl-2,2,2-trifluoroethyl tosylates in 80% aq acetone. This reveals that the r+ value observed for this solvolysis must be the intrinsic resonance demand of a highly electron-deficient cationic transition state in the SN1 ionizing process. The identity of the r+ value was consistent with our previous observation for other benzylic carbocation systems, indicating that the degree of the π-delocalization of the positive charge is identical between the cationic transition state and an intermediate cation for all benzylic systems, which cover a wide range of reactivity and stability of the carbocation. This leads us to the conclusion that the geometry of the transition state in the ionizing process of the SN1 solvolysis, which is a highly endothermic reaction, closely resembles the high-energy product, an intermediate cation.

Solvolysis of 9-fluorenyl bromide and tosylate. Nucleophilic solvent intervention and insignificance of antiaromaticity

Grunwald-Winstein type correlation analysis on the solvolysis of 9-fluorenyl bromide and tosylate suggests extensive charge delocalization throughout the whole system at cationic transition states in both cases, and significant nucleophilic solvent intervention for the bromide.

Kinetics and mechanism of hydrogen peroxide oxidation of chromone‐3‐carboxaldehydes in aqueous acid and micellar media

Oxidation of chromone-3-carboxaldehyde (CCA) and substituted analogues by H2O2 has been carried out in aqueous acid (HCl and H2SO4) and micellar media. Reaction kinetics indicated order in [CCA] as well as [H2O2] to be unity while it is a fraction (1 > n > O) in [acid]. Reaction rates were found to be faster in the solvents of low-dielectric constant (D). Added salt (KCl or (NH4)2SO4) increased the rate of oxidation marginally. On the basis of observed linearity of Amis plot and marginal positive salt effect, protonated CCA (enol form of CCA, a cation) and H2O2 (neutral molecule) were considered as reactive species in the rate limiting step. Reaction rates were found to be enhanced significantly in anionic and nonionic micellar (sodium dodecylsulfate (SDS) and Triton X-100 (Tx), respectively) media. However, cationic micelles [cetyl trimethyl ammonium bromide (CTAB)] indicated marginal retardation effect. Effect of anionic and cationic micelles has been interpreted in terms of electrostatic interactions, while that of nonionic micelles in terms of hydrophobic interactions. Structure-reactivity correlations have been interpreted by Hammett's equation. Negative “ρ” (reaction constant) values indicated cationic transition state. © 1996 John Wiley & Sons, Inc.

Kinetics and mechanism of hydrogen peroxide oxidation of chromone-3-carboxaldehydes in aqueous acid and micellar media

Oxidation of chromone-3-carboxaldehyde (CCA) and substituted analogues by H2O2 has been carried out in aqueous acid (HCl and H2SO4) and micellar media. Reaction kinetics indicated order in [CCA] as well as [H2O2] to be unity while it is a fraction (1 > n > O) in [acid]. Reaction rates were found to be faster in the solvents of low-dielectric constant (D). Added salt (KCl or (NH4)2SO4) increased the rate of oxidation marginally. On the basis of observed linearity of Amis plot and marginal positive salt effect, protonated CCA (enol form of CCA, a cation) and H2O2 (neutral molecule) were considered as reactive species in the rate limiting step. Reaction rates were found to be enhanced significantly in anionic and nonionic micellar (sodium dodecylsulfate (SDS) and Triton X-100 (Tx), respectively) media. However, cationic micelles [cetyl trimethyl ammonium bromide (CTAB)] indicated marginal retardation effect. Effect of anionic and cationic micelles has been interpreted in terms of electrostatic interactions, while that of nonionic micelles in terms of hydrophobic interactions. Structure-reactivity correlations have been interpreted by Hammett's equation. Negative “ρ” (reaction constant) values indicated cationic transition state. © 1996 John Wiley & Sons, Inc.