- Reactivity of Superoxide Ion with Ethyl Pyruvate, α-Diketones, and Benzil in Dimethylformamide
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The dominant net reaction of O2(1-) radical with α-dicarbonyls such as ethyl pyruvate, 2,3-butanedione, and 2,3-pentanedione is proton abstraction from their enol tautomer.The rate-limiting step is first order for each reactant, the net products are enolate plus O2 and H2O2, and the second-order rate constants (k) are the same, within experimental error, for the three substrates (k = (4+/-1) x 103 M-1 s-1).For the reaction of benzil (an α-dicarbonyl that cannot enolize) with O2(1-) radical the rate-limiting step is first order for each reactant, and the second- order rate constant (k) is (2+/-1) x 103 M-1 s-1.The process appears to involve an initial nucleophilic addition by O2(1-) radical to carbonyl carbon, followed by a dioxetane closure on the other carbonyl carbon and reductive cleavage by a second O2(1-) radical to give two benzoate ions and O2.
- Sawyer, Donald T.,Stamp John J.,Menton, Kathleen A.
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- Energetics of product formation during anaerobic degradation of phthalate isomers and benzoate
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Methanogenic enrichment cultures grown on phthalate, isophthalate and terephthalate were incubated with the corresponding phthalate isomer on which they were grown, and a mixture of benzoate and the phthalate isomer. All cultures were incubated with bromoethanosulfonate to inactivate the methanogens in the mixed culture. Thus, product formation during fermentation of the aromatic substrates could be studied. It was found that reduction equivalents generated during oxidation of the aromatic substrates to acetate were incorporated in benzoate under formation of carboxycyclohexane. During fermentation of the phthalate isomers, small amounts of benzoate were detected, suggesting that the initial step in the anaerobic degradation of the phthalate isomers is decarboxylation to benzoate. Gibbs free energy analyses indicated that during degradation of the phthalate isomers, benzoate, carboxycyclohexane, acetate and molecular hydrogen accumulated in such amounts that both the reduction and oxidation of benzoate yielded a constant and comparable amount of energy of approximately 30 kJ mol-1. Based on these observations it is suggested that within narrow energetic limits, oxidation and reduction of benzoate may proceed simultaneously. Whether this is controlled by the Gibbs free energy change for carboxycyclohexane oxidation remains unclear. Copyright (C) 1999 Federation of European Microbiological Societies.
- Kleerebezem, Robbert,Hulshoff Pol, Look W.,Lettinga, Gatze
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- Molecular Engineering to Tune the Ligand Environment of Atomically Dispersed Nickel for Efficient Alcohol Electrochemical Oxidation
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Atomically dispersed metals maximize the number of catalytic sites and enhance their activity. However, their challenging synthesis and characterization strongly complicates their optimization. Here, the aim is to demonstrate that tuning the electronic environment of atomically dispersed metal catalysts through the modification of their edge coordination is an effective strategy to maximize their performance. This article focuses on optimizing nickel-based electrocatalysts toward alcohol electrooxidation in alkaline solution. A new organic framework with atomically dispersed nickel is first developed. The coordination environment of nickel within this framework is modified through the addition of carbonyl (CO) groups. The authors then demonstrate that such nickel-based organic frameworks, combined with carbon nanotubes, exhibit outstanding catalytic activity and durability toward the oxidation of methanol (CH3OH), ethanol (CH3CH2OH), and benzyl alcohol (C6H5CH2OH); the smaller molecule exhibits higher catalytic performance. These outstanding electrocatalytic activities for alcohol electrooxidation are attributed to the presence of the carbonyl group in the ligand chemical environment, which enhances the adsorption for alcohol, as revealed by density functional theory calculations. The work not only introduces a new atomically dispersed Ni-based catalyst, but also demonstrates a new strategy for designing and engineering high-performance catalysts through the tuning of their chemical environment.
- Liang, Zhifu,Jiang, Daochuan,Wang, Xiang,Shakouri, Mohsen,Zhang, Ting,Li, Zhongjun,Tang, Pengyi,Llorca, Jordi,Liu, Lijia,Yuan, Yupeng,Heggen, Marc,Dunin-Borkowski, Rafal E.,Morante, Joan R.,Cabot, Andreu,Arbiol, Jordi
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- A Green-LED Driven Source of Hydrated Electrons Characterized from Microseconds to Hours and Applied to Cross-Couplings
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We present a novel photoredox catalytic system that delivers synthetically usable concentrations of hydrated electrons when illuminated with a green light-emitting diode (LED). The catalyst is a ruthenium complex protected by an anionic micelle, and the urate dianion serves as a sacrificial donor confined to the aqueous bulk. By virtue of its chemical properties, this donor not only suppresses charge recombination that would limit the electron yield, but also enables this system to perform cross-couplings through the action of hydrated electrons, the first examples of which are reported here. We have investigated the kinetics of all the steps involving the electron and its direct precursor in a comparative study by means of laser flash photolysis and by monitoring product formation during LED photolysis. Despite the differences in timescales, each approach on its own already gives a complete picture of the reaction over a temporal range spanning ten orders of magnitude. Discrepancies between the kinetic parameters obtained with the two complementary techniques can be rationalized with the slow secondary chemistry of the system; they reveal that the product-based method provides a more accurate description because it also responds to the changes of the system composition during a synthesis; hence, they demonstrate that in complex systems the timescale of the experimental observation should be matched to that of the actual application.
- Naumann, Robert,Goez, Martin
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- Time-resolved RNA SHAPE chemistry
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Selective 2′-hydroxyl acylation analyzed by primer extension (SHAPE) chemistry yields quantitative RNA secondary and tertiary structure information at single nucleotide resolution. SHAPE takes advantage of the discovery that the nucleophilic reactivity of the ribose 2′-hydroxyl group is modulated by local nucleotide flexibility in the RNA backbone. Flexible nucleotides are reactive toward hydroxyl-selective electrophiles, whereas constrained nucleotides are unreactive. Initial versions of SHAPE chemistry, which employ isatoic anhydride derivatives that react on the minute time scale, are emerging as the ideal technology for monitoring equilibrium structures of RNA in a wide variety of biological environments. Here, we extend SHAPE chemistry to a benzoyl cyanide scaffold to make possible facile time-resolved kinetic studies of RNA in~1 s snapshots. We then use SHAPE chemistry to follow the time-dependent folding of an RNase P specificity domain RNA. Tertiary interactions form in two distinct steps with local tertiary contacts forming an order of magnitude faster than long-range interactions. Rate-determining tertiary folding requires minutes despite that no non-native interactions must be disrupted to form the native structure. Instead, overall folding is limited by simultaneous formation of interactions~55 A distant in the RNA. Time-resolved SHAPE holds broad potential for understanding structural biogenesis and the conformational interconversions essential to the functions of complex RNA molecules at single nucleotide resolution. Copyright
- Mortimer, Stefanie A.,Weeks, Kevin M.
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- Indirect detection of intermediate in decarboxylation reaction of phenylglyoxylic acid by hyperpolarized13C NMR
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The decarboxylation reaction of phenylglyoxylic acid with hydrogen peroxide is studied by real-Time hyperpolarized carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy at room temperature. A non-observable reaction intermediate is identified using blind selective saturation pulses in the expected chemical shift range, thereby revealing information on the reaction mechanism. This journal is
- Kim, Jiwon,Kim, Yaewon,Luu, Quy Son,Kim, Jihyun,Qi, Chang,Hilty, Christian,Lee, Youngbok
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p. 15000 - 15003
(2020/12/22)
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- Characterizing Cation Chemistry for Anion Exchange Membranes - A Product Study of Benzylimidazolium Salt Decompositions in the Base
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Imidazolium functionality has played a prominent role in research on anion exchange membranes for use in alkaline electrochemical devices. Base stability and degradation of these materials has been much studied, but in many instances, product pathways have not been thoroughly delineated. We report an NMR study of base-induced decomposition products from three benzylimidazolium salts bearing varying extents of methyl substitution on the imidazolium ring. The major products are consistent with a hydrolytic ring fragmentation pathway as the principal mode of decomposition. We observe several new products not previously reported in the literature on imidazolium salt degradation, including benzilic acid rearrangement products formally derived from intermediate 1,2-dicarbonyl compounds or their equivalents. However, the overall reactions are complex, the yields of observed products do not account for all consumed starting materials, and mechanistic ambiguities remain.
- Pellerite, Mark J.,Kaplun, Marina M.,Webb, Robert J.
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p. 15486 - 15497
(2019/11/19)
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- Oxidation kinetics of ferrocene derivatives with dibenzoyl peroxide
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Chemical oxidation of ferrocene and related derivatives by dibenzoyl peroxide in acetonitrile solution produces ferrocenium and benzoic acid after acidification. The rate law is first order in oxidant and in reductant. Steric effects and activation parameters are consistent with a rate-controlling outer-sphere single-electron transfer (ET) step, and reorganization energies are obtained using Marcus theory with B3LYP calculations. Energetics, optimized structures, and solvent effects indicate that rate is affected more by anion than cation solvation and that oxidation of decamethylferrocene by 3-chloroperoxybenzoic acid does not occur by ET.
- Halstead, Joshua M.,Abu-Saleh, Refaat,Schildcrout, Steven M.,Masnovi, John
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- Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light-Emitting Diode
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We have explored alkyl substitution of the ligands as a means to improve the performance of the title complexes in photoredox catalytic systems that produce synthetically useable amounts of hydrated electrons through photon pooling. Despite generating a super-reductant, these electron sources only consume the bioavailable ascorbate and are driven by a green light-emitting diode (LED). The substitutions influence the catalyst activity through the interplay of the quenching parameters, the recombination rate of the reduced catalyst OER and the ascorbyl radical across the micelle-water interface, and the quantum yield of OER photoionization. Laser flash photolysis yields comprehensive information on all these processes and allows quantitative predictions of the activity observed in LED kinetics, but the latter method provides the only access to the catalyst stability under illumination on the timescale of the syntheses. The homoleptic complex with dimethylbipyridine ligands emerges as the optimum that combines an activity twice as high with an undiminished stability in relation to the parent compound. With this complex, we have effected dehalogenations of alkyl and aryl chlorides and fluorides, hydrogenations of carbon–carbon double bonds, and self- as well as cross-coupling reactions. All the substrates employed are impervious to ordinary photoredox catalysts but present no problems to the hydrated electron as a super-reductant. A particularly attractive application is selective deuteration with high isotopic purity, which is achieved simply by using heavy water as the solvent.
- Naumann, Robert,Lehmann, Florian,Goez, Martin
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p. 13259 - 13269
(2018/09/10)
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- Unexpected resistance to base-catalyzed hydrolysis of nitrogen pyramidal amides based on the 7-azabicyclic[2.2.1]heptane scaffold
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Non-planar amides are usually transitional structures, that are involved in amide bond rotation and inversion of the nitrogen atom, but some ground-minimum non-planar amides have been reported. Non-planar amides are generally sensitive to water or other n
- De Velasco, Diego Antonio Ocampo Gutiérrez,Su, Aoze,Zhai, Luhan,Kinoshita, Satowa,Otani, Yuko,Ohwada, Tomohiko
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- Influences of Micelle Formation and Added Salts on the Hydrolysis Reaction Rate of p-Nitrophenyl Benzoate in Aqueous Buffered Media
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The hydrolysis reaction rate of p-nitrophenyl benzoate (p-NPB) has been examined in aqueous buffer media of pH 9.18, containing surfactants, cetyltrimethylammonium bromide (CTAB) and chloride (CTAC), or sodium dodecyl sulfate (SDS) at 35°C. Although the rate constant [log (k/s?1)] of p-NPB hydrolysis has once decreased slightly below the critical micelle concentration (CMC) value for CTAB and CTAC, it has begun to increase drastically with micellar formation. With increasing concentrations larger than the CMC value, the log (k/s?1) value has reached the optimal value, i.e., a 140- and 200-fold rate acceleration for CTAB and CTAC, respectively, compared to that without a surfactant. Whereas the anionic surfactant, SDS, has caused only a gradual rate deceleration in the whole concentration range (up to 0.03 mol dm?3). Increases in pH of the buffer have resulted in increases of the hydrolysis rate. In the CTAB micellar solution, the remarkably enhanced rate has been retarded significantly by the addition of only 0.10 mol dm?3 bromide salts. The effects of rate retardation caused by the added salts follows in the order of NaBr > Me4NBr > Et4NBr > Pr4NBr > n-Bu4NBr. In the absence of surfactant, however, the addition of the bromide salts has accelerated the hydrolysis rate, except for the metallic salt of NaBr, with the order of Me4NBr 4NBr 4NBr 4NBr. In the CTAC micellar solution, similar rate retardation effects have been observed in the presence of chloride salts (NaCl, Et4NCl, and n-Bu4NCl). The effects of added salts have been interpreted from the viewpoints of the changes in activity of the OH? ion and/or the nucleophilicities of the anions from the added salts.
- Bayissa, Leta Danno,Ohmae, Yoshihito,Hojo, Masashi
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- Identification of an acyl-enzyme intermediate in a meta-cleavage product hydrolase reveals the versatility of the catalytic triad
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Meta-cleavage product (MCP) hydrolases are members of the α/β-hydrolase superfamily that utilize a Ser-His-Asp triad to catalyze the hydrolysis of a C-C bond. BphD, the MCP hydrolase from the biphenyl degradation pathway, hydrolyzes 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to 2-hydroxypenta-2,4-dienoic acid (HPD) and benzoate. A 1.6 A resolution crystal structure of BphD H265Q incubated with HOPDA revealed that the enzyme's catalytic serine was benzoylated. The acyl-enzyme is stabilized by hydrogen bonding from the amide backbone of 'oxyanion hole' residues, consistent with formation of a tetrahedral oxyanion during nucleophilic attack by Ser112. Chemical quench and mass spectrometry studies substantiated the formation and decay of a Ser112-benzoyl species in wild-type BphD on a time scale consistent with turnover and incorporation of a single equivalent of 18O into the benzoate produced during hydrolysis in H218O. Rapid-scanning kinetic studies indicated that the catalytic histidine contributes to the rate of acylation by only an order of magnitude, but affects the rate of deacylation by over 5 orders of magnitude. The orange-colored catalytic intermediate, ESred, previously detected in the wild-type enzyme and proposed herein to be a carbanion, was not observed during hydrolysis by H265Q. In the newly proposed mechanism, the carbanion abstracts a proton from Ser112, thereby completing tautomerization and generating a serinate for nucleophilic attack on the C6-carbonyl. Finally, quantification of an observed pre-steady-state kinetic burst suggests that BphD is a half-site reactive enzyme. While the updated catalytic mechanism shares features with the serine proteases, MCP hydrolase-specific chemistry highlights the versatility of the Ser-His-Asp triad.
- Ruzzini, Antonio C.,Ghosh, Subhangi,Horsman, Geoff P.,Foster, Leonard J.,Bolin, Jeffrey T.,Eltis, Lindsay D.
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supporting information; experimental part
p. 4615 - 4624
(2012/04/23)
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- Catalytic activity of enzyme in water/organic cosolvent mixtures for the hydrolysis of p-nitrophenyl acetate and p-nitrophenyl benzoate
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The dependence of the catalytic activities of α-chymotrypsin on the concentration of organic cosolvents (acetonitrile, dimethyl sulfoxide, dimethyl formamide, ethylene glycol, methanol, ethanol, propan-2-ol and tert-butanol) in mixed aqueous media has bee
- Verma, Santosh Kumar,Ghosh, Kallol K.
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experimental part
p. 1041 - 1046
(2011/01/10)
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- Salivary aldehyde dehydrogenase: Activity towards aromatic aldehydes and comparison with recombinant ALDH3A1
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A series of aromatic aldehydes was examined as substrates for salivary aldehyde dehydrogenase (sALDH) and the recombinant ALDH3A1. Para-substituted benzaldehydes, cinnamic aldehyde and 2-naphthaldehydes were found to be excellent substrates, and kinetic p
- Giebultowicz, Joanna,Wolinowska, Renata,Sztybor, Anna,Pietrzak, Monika,Wroczynski, Piotr,Wierzchowski, Jacek
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body text
p. 2363 - 2372
(2009/12/24)
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- Using toluates as simple and versatile radical precursors
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(Chemical Equation Presented) The viability of the toluate moiety as a radical precursor has been examined by studying deoxygenation and cyclization reactions.
- Lam, Kevin,Marko, Istvan E.
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scheme or table
p. 2773 - 2776
(2009/05/30)
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- A study on a primitive artificial esterase model: Reactivity of a calix[4]resorcinarene bearing carboxyl groups
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The host molecule octacarboxymethyl calix[4]resorcinarene 1 catalyses the hydrolysis of substituted phenyl N-methylpyridinium-4-carboxylate esters 3a-f by complexation followed by intracomplex reaction via an anhydride intermediate. The reactivity in the
- Cevasco, Giorgio,Galatini, Andrea,Pirinccioglu, Necmettin,Thea, Sergio,Williams, Andrew
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scheme or table
p. 498 - 504
(2009/04/11)
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- Structure-activity analysis of base and enzyme-catalyzed 4-hydroxybenzoyl coenzyme A hydrolysis
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In this study, the second-order rate constant k2 of base-catalyzed hydrolysis and the values of kcat, Km and kcat/Km of wild-type Pseudomonas sp. CBS3 4-hydroxybenzoyl coenzyme A (4-HBA-CoA) thioesterase-catalyzed hydrolysis of 4-HBA-CoA and its para-substituted analogs were measured. For the base-catalyzed hydrolysis, the plot of log k2 vs the σ value of the para-substituents was linear with a slope (ρ) of 1.5. In the case of the enzyme-catalyzed hydrolysis, the kcat/Km values measured for the para-substituted analogs defined substrate specificity. Asp32 was shown to play a key role in substrate recognition, and in particular, in the discrimination between the targeted substrate and other cellular benzoyl-CoA thioesters.
- Song, Feng,Zhuang, Zhihao,Dunaway-Mariano, Debra
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- Competitive endo- and exo-cyclic C-N fission in the hydrolysis of N-aroyl β-lactams
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The balance between endo- and exo-cyclic C-N fission in the hydrolysis of N-aroyl β-lactams shows that the difference in reactivity between strained β-lactams and their acyclic analogues is minimal. Attack of hydroxide ion occurs preferentially at the exocyclic acyl centre rather than that of the β-lactam during the hydrolysis of N-p-nitrobenzoyl β-lactam. In general, both endo- and exo-cyclic C-N bond fission occurs in the alkaline hydrolysis of N-aroyl β-lactams, the ratio of which varies with the aryl substituent. Hence, the Bronsted β-values differ for the two processes: -0.55 for the ring-opening reaction and -1.54 for the exocyclic C-N bond fission reaction. For the pH-independent and acid-catalysed hydrolysis of N-benzoyl β-lactam, less than 3% of products are derived from exocyclic C-N bond fission.
- Tsang, Wing Y.,Ahmed, Naveed,Hemming, Karl,Page, Michael I.
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p. 1432 - 1439
(2007/10/03)
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- Kinetics and mechanism of the pyridinolysis of S-4-nitrophenyl 4-substituted thiobenzoates in aqueous ethanol
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The pyridinolysis of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO2; 1, 2, and 3, respectively) is studied kinetically in 44 wt % ethanol-water, at 25.0 °C and an ionic strength of 0.2 M (KCl). The reactions are measured spectrophotometrically (420-425 nm) by following the appearance of 4-nitrobenzenethiolate anion. Pseudo-first-order rate coefficients (kobsd) are obtained throughout, under excess of amine over the substrate. Plots of kobsd vs [free amine] at constant pH are linear with the slope (kN) independent of pH. The Bronsted-type plot (log kN vs pKa of the conjugate acids of the pyridines) for the reactions of thiolbenzoate 1 is curved with a slope at high pK a, β1 = 0.20, and slope at low pKa, β2 = 0.94. The pKa value for the center of the Bronsted curvature is pKa0 = 9.7. The pyridinolysis of thiolbenzoates 2 and 3 show linear Bronsted-type plots of slopes 0.94 and 1.0, respectively. These results and other evidence indicate that these reactions occur with the formation of a zwitterionic tetrahedral intermediate (T±). For the pyridinolysis of thiolbenzoate 1, breakdown of T± to products (k2 step) is rate-limiting for weakly basic pyridines and T± formation (k1 step) is rate-determining for very basic pyridines. The k2 step is rate-limiting for the reactions of thiolbenzoates 2 and 3. The smallest pK a0 value for the reaction of 1 is due to a the weakest electron withdrawal of H (relative to Cl and NO2) in the acyl group, which results in the smallest k-1/k2 ratio. The pK a0 values for the title reactions are smaller than those for the reactions of secondary alicyclic amines with thiolbenzoates 1-3. This is attributed to a lower leaving ability from the T± of pyridines than isobasic alicyclic amines. The lower pKa0 value found for the pyridinolysis of 2,4-dinitrophenyl benzoate (pK a0 = 9.5), compared with that for the pyridinolysis of 1, is explained by the greater nucleofugality from T± of 2,4-dinitrophenoxide than 4-nitrobenzenethiolate, which renders the k -1/k2 ratio smaller for the reactions of the benzoate relative to thiolbenzoate 1. The title reactions are also compared with the aminolysis of similar thiolbenzoates in other solvents to assess the solvent effect.
- Castro, Enrique A.,Vivanco, Melissa,Aguayo, Raul,Santos, Jose G.
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p. 5399 - 5404
(2007/10/03)
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- Kinetics of hydrolysis of 1-benzoyl-1,2,4-triazole in aqueous solution as a function of temperature near the temperature of maximum density, and the isochoric controversy
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At temperatures above and below the temperature of maximum density, TMD, for water at ambient pressure, pairs of temperatures exist at which the molar volumes of water are equal. First-order rate constants for the pH-independent hydrolysis of 1-benzoyl-1,2,4-triazole in aqueous solution at pairs of such isochoric temperatures show no unique features. Taken together with previously published kinetic data for the hydrolysis of a range of simple organic solutes in both water and D2O near their respective TMDs, we conclude that special significance in the context of rates of chemical reactions in aqueous solutions should not be attached to the isochoric condition.
- Blandamer, Michael J.,Buurma, Niklaas J.,Engberts, Jan B.F.N.,Reis, Joao C.R.
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p. 720 - 723
(2007/10/03)
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- Unusual steric effects in sulfonyl transfer reactions
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The hydrolysis of N-acyl-β-sultams generally occurs with ring opening and S-N fission in contrast to the C N fission observed in analogous acyclic N-acyl sulfonamides. Similar to other β-sultams, the N-acyl derivatives are at least 106 more rea
- Hinchliffie, Paul S.,Wood, J. Matthew,Davis, Andrew M.,Austin, Rupert P.,Paul Beckett,Page, Michael I.
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p. 1503 - 1505
(2007/10/03)
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- Preliminary characterization of four 2-chlorobenzoate-degrading anaerobic bacterial consortia
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Dechlorination was the initial step of 2CB biodegradation in four 2-chlorobenzoate-degrading methanogenic consortia. Selected characteristics of ortho reductive dehalogenation were examined in consortia developed from the highest actively dechlorinating dilutions of the original 2CB consortia, designated consortia M34-9, P20-9, P21-9 and M50-7. In addition to 2-chlorobenzoate, all four dilution consortia dehalogenated 4 of 32 additional halogenated aromatic substrates tested, including 2-bromobenzoate; 2,6-dichlorobenzoate; 2,4-dichlorobenzoate; and 2-chloro-5-hydroxybenzoate. Dehalogenation occurred exclusively at the ortho position. Both ortho chlorines were removed from 2,6-dichlorobenzoate. Benzoate was detected from 2-bromobenzoate and 2,6-dichlorobenzoate. 4-Chlorobenzoate and 3-hydroxybenzoate were formed from 2,4-dichlorobenzoate and 2-chloro-5-hydroxybenzoate, respectively. Only benzoate was further degraded. Slightly altering the structure of the parent "benzoate molecule" resulted in observing reductive biotransformations other than dehalogenation. 2-Chlorobenzaldehyde was reduced to 2-chlorobenzyl alcohol by all four consortia. 2-chloroanisole was O-demethoxylated by three of the four consortia forming 2-chlorophenol. GC-MS analysis indicated reduction of the double bond in the propenoic side chain of 2-chlorocinnamate forming 2-chlorohydrocinnamate. None of the reduction products was dechlorinated. The following were not dehalogenated: 3- and 4-bromobenzoate; 3- and 4-chlorobenzoate; 2-, 3-, and 4-fluorobenzoate; 2-, 3-, and 4-iodobenzoate; 2-, 3-, and 4-chlorophenol; 2-chloroaniline; 2-chloro-5-methylbenzoate; 2,3-dichlorobenzoate; 2,5-dichlorobenzoate; 2,4,5-trichlorophenoxyacetic acid; and 2,4-dichlorophenoxyacetic acid. Consortia M34-9, P20-9, P21-9, and M50-7 dechlorinated 2-chlorobenzoate at -9 followed by those of M50-7 with rates declining above 2 and 3 mm 2CB, respectively. The major physiological types of microorganisms in consortia M34-9, P20-9, P21-9, and M50-7 were sulfate-reducing and hydrogen-utilizing anaerobes.
- Genthner, Barbara R. Sharak
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- Elimination reactions of (Z)-thiophene- and (Z)-furan-2-carbaldehyde O- benzoyloximes. Effect of β-aryl group upon the nitrile-forming anti transition state
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Elimination reactions of (Z)-thiophene- and (Z)-furan-2-carbaldehyde O- benzoyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett ρ and k(H)/k(D) values, and an E2 m
- Cho, Bong Rae,Cho, Nam Soon,Song, Sang Hun,Lee, Sang Kook
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p. 8304 - 8309
(2007/10/03)
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- Nucleophilic displacement at the benzoyl centre: A study of the change in geometry at the carbonyl carbon atom
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The second-order rate constants for the reaction between hydroxide ion and phenoxide ion with 4-nitrophenyl esters of substituted benzoic acids in 10% acetonitrile-water (v/v) solution obey Hammett σ correlations. The values of the Hammett ρ of 1.67 (κArO) and 2.14 (κOH) are consistent with a large change in hybridization at the central carbon by comparison with the ρ value for a standard reaction where a full sp2 to sp3 change occurs. The transition state for the concerted reaction thus has a substantially tetrahedral geometry. The observation of the anti-Hammond effect whereby the ρ value for the hydroxide ion exceeds that of the less reactive phenoxide ion is consistent with a concerted, ANDN, mechanism for these reactions. A stepwise mechanism, AN + DN, is unlikely to yield a measurable break in the Hammett correlation for a change in the benzoyl substituent if the partitioning of the putative tetrahedral intermediate involves forward and reverse reactions with Hammett correlations possessing similar ρ values.
- Colthurst, Matthew J.,Williams, Andrew
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p. 1493 - 1497
(2007/10/03)
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- Gas-phase ionic reactions of benzyl and methoxide anions
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Gas-phase reactions of benzyl and methoxide anions with alkyl formate and other esters were compared using Fourier transform io cyclotron resonance spectroscopy. Although these anions have similar basicities, in many cases the reaction pathways differ.
- Gatev, Geo G.,Zhong, Meili,Brauman, John I.
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p. 531 - 536
(2007/10/03)
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- Electron transfer as a possible initial step in nucleophilic addition elimination reactions between (radical) anions and carbonyl compounds in the gas phase
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The reactions of the HO-, CH3S-, CH2S- and CH2=C(CH3)-CH2- ions with three ketones (CF3COR; R=CH3, CF3, C6H5) and three esters of trifluoroacetic acid (CF3CO2R; R=CH3, C2H5 and C6H5) have been studied with use of Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. All four negative ions react exclusively by proton transfer with CF3COCH3. With the other substrates, the HO- ion reacts by various pathways, such as proton transfer, SN2 substitution, E2 elimination and attack on the carbonyl group. The CH3S- ion is unreactive towards CF3COC6H5 but is able to react by hydride transfer, SN2, E2 and/or carbonyl attack with the remaining neutral species. The CH2S- radical anion reacts by electron transfer to afford stable molecular radical anions of CF3COCF3 and CF3COC6H5, whereas the main reaction with the two esters, CF3CO2CH3 and CF3CO2C2H5, is dissociative electron transfer leading to CF3CO2- and CF3- ions. The CH2=C(CH3)-CH2- anion displays a more complex reactivity pattern involving electron transfer, SN2, E2 as well as attack on the carbonyl group. Direct evidence for the occurrence of electron transfer as the initial step in an overall BAC2 type process has not been obtained for the systems studied. The reaction of the CH2S- ion with CF3CO2C6H5 was observed, however, to yield exclusively a CF3COCHS-. radical anion. Based upon the absence of a BAC2 process in the reaction of CH2S- with the methyl and ethyl esters of trifluoroacetic acid in combination with the facile occurrence of electron transfer from this radical anion, it is suggested that the CF3COCHS-. ion is formed by an initial electron transfer followed by coupling between the CH2S molecule and the CF3CO2C6H5- radical anion and subsequent loss of C6H5OH from the collision complex.
- Staneke, Paul O.,Ingemann, Steen,Nibbering, Nico M. M.
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p. 179 - 184
(2007/10/03)
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- Hammett Equation and Micellar Effects upon Deacylation
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Substituent effects upon second order rate constants of reaction of OH- with phenyl p-substituted benzoates at surface of micelles of cetyltrialkylammonium bromide (alkyl = Me, Et, Pr and Bu) and tetradecylquinuclidinium bromide fit the Hammett equation.Values of ρ increase with increasing bulk of the surfactant head group and are considerably larger than in water, corresponding to decreases in micellar surface polarity.The effects of substitution of the propyl group into the phenyl and benzoyl groups show that substrate orientation at micellar surface plays no significant kinetic role.
- Bartoletti, Antonella,Bartolini, Simona,Germani, Raimondo,Savelli, Gianfranco,Bunton, Clifford A.
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p. 723 - 728
(2007/10/02)
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- Cationic Micellar Effect on the Kinetics of the Protolysis of Aromatic Carboxylic Acids studied by the Ultrasonic Absorption Method
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The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the presence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution.The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid derivatives (BAD).The rate constant (γ2kf, kb), the apparent dissociation constant (Ka), and the volume change of the reaction (ΔV) were obtained.Different Ka dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pKa dependences were also observed for ΔV of SAD and BAD.These dependences are larger than those in aqueous solution.This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution.
- Isoda, Teruyo,Yamasaki, Miyuki,Yano, Hiroshige,Sano, Takayuki,Harada, Shoji
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p. 869 - 874
(2007/10/02)
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- Concepts of sterically hindered resonance and buttressing effect: Gas-phase acidities of methyl-substituted benzoic acids and basicities of their methyl esters
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Two classical terms "Steric Hindrance to Resonance" and "Buttressing Effect" are revisited on the basis of the gas-phase acidities of nine methyl-substituted benzole acids and of the gas-phase basicities of their methyl esters, measured using FT- ICR spectrometry. By combining these data with published heats of formation of the neutrals and by using the principle of isodesmic reactions, relative enthalpies of formation were evaluated separately for the acid molecules, their anions (deprotonated), and their protonated cations (substituted by the protonated forms of the corresponding methyl esters). Energies of all species were also calculated at the semiempirical level (AMI). Substituent effects on the gas-phase acidity are similar to those on the acidity in water. All the methyl groups have a stabilizing polar effect, and o-methyl groups have a destabilizing steric effect, both effects increasing from the deprotonated forms to the acid molecules and then to the protonated forms. Separation of the two effects was attempted, assuming equal polar effects in the ortho and para positions. The results are internally consistent: their detailed analysis is in favor of a primary steric effect rather than a steric inhibition of resonance. The latter must be relatively weaker and operating only in 2,6-dimethyl derivatives, which are certainly nonplanar. In the literature this concept has been used too broadly, even for compounds for which the nonplanar conformation has not been proven. The concept of buttressing effect has been confirmed for methyl-substituted benzoic acids, but it is formulated more generally and more exactly. According to the new definition, it can be observed even for nonadjacent substituants.
- Decouzon, Michèle,Ertl, Peter,Exner, Otto,Gal, Jean-Fran?ois,Maria, Pierre-Charles
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p. 12071 - 12078
(2007/10/02)
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- Kinetics of deprotonation of arylnitromethanes by benzoate ions in acetonitrile solution. Effect of equilibrium and nonequilibrium transition-state solvation on intrinsic rate constants of proton transfers
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Second-order rate constants for benzoate ion promoted deprotonation reactions of (3-nitropbenyl)nitromethane, (4-nitrophenyl)nitromethane, and (3,5-dinitrophenyl)nitromethane have been determined in acetonitrile solution at 25 °C. These data were obtained at low benzoate buffer concentrations (a= 21.7; (4-nitromethyl)nitromethane, pKa = 20.6; and (3,5-dinitrophenyl)nitromethane, pKa, = 19.8. A Br?nsted βB value of 0.56 and an αCHlue of 0.79 have been calculated for the benzoate, 3-bromobenzoate, and 4-nitrobenzoate ion promoted reactions of (3,5- dinitrophenyl)nitromethane and for the benzoate ion promoted reactions of (3- nitrophenyl)nitromethane and (3,5-dinitrophenyl)nitromethane, respectively; (4-nitrophenyl)nitromethane deviates negatively from the Bronsted plot due to the resonance effect of the 4-nitro group. The logarithms of the intrinsic rate constants for benzoate promoted deprotonations of (3-nitrophenyl)nitromethane, (4-nitro phenyl)nitromethane, and (3,5-dinitrophenyl)nitromethane are 4.81, 4.58, and 5.27, respectively, and these values are 1.43, 1.70, and 1.30 log units, respectively, higher in acetonitrile than in dimethyl sulfoxide. Transfer activity coefficients from dimethyl sulfoxide (D) to acetonitrile (A) solution, log DγA, for (3-nitropbenyl)nitroimethyl anion (0.28), (4-nitrophenyl)nitromethyl anion (0.56), (3-nitrophenyl)nitromethane (0.18), and (4-nitrophenyl)nitromethane (0.16) have been calculated, and log DγA for benzoic acid (~ 1.9) and the benzoate ion (~0.25) have been estimated. The solvent effects on the intrinsic rate constants are analyzed within the framework of the Principle of Nonperfect Synchronization (PNS) in terms of contributions by late solvation of the arylnitromethyl anion, late solvation of the benzoic acid (produced as a product of the reaction), early desolvation of the benzoate ion and the arylnitromethane, and by a classical solvent effect. The results are also compared with predictions by a theoretical model recently proposed by Kurz. For the comparison of intrinsic rate constants in water and dimethyl sulfoxide there is good agreement between the Kurz model and the experimental results as well as the PNS analysts, but there is a discrepancy between the results and the predictions of the Kurz model for the comparison of intrinsic rate constants in dimethyl sulfoxide and acetonitrile solutions.
- Gandler, Joseph R.,Bernasconi, Claude F.
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p. 631 - 637
(2007/10/02)
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- Micellar Effects Upon the Proton Abstraction and Product Distribution in the Autoxidation of Benzoin: Mechanistic Aspects.
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A study was made of the autoxidateion of benzoin in aqueous sodium hydroxide and in the presence of micells and a quaternary ammonium salt.Repetitive scanning of the u.v. spectrum of the reaction mixture indicates that an enediolate anion 1 is an intermediate, the disappearence of which is the rate-detremining step.In aqueous medium, benzoin is autoxidized to a 1:1 mixture of benzil and benzoate, while in the presence of tetramethylammonium bromide the proportion of benzoate increases.Cationic micelles of cetyltrimethylammonium chloride (CTACl) accelerate the rate of the reaction and sfift the distribution of the product to benzoate only.Anionic micelles of sodium lauryl sulphate (NaLS) inhibit the rate of the reaction and shift the product to benzil only.
- Al-Lohedan, Hamad A.
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p. 3101 - 3115
(2007/10/02)
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- Generation, Thermodynamics, and Chemistry of the Diphenylcarbene Anion Radical (Ph2C.-)
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Dissociative electron attachment with Ph2C=N produced Ph2C.- (m/z 166).The reactions of Ph2C.- with potential proton donors of known gas-phase acidity were used to bracket PA(Ph2C.-) = 380 +/- 2 kcal mol-1 from which ΔHf0(Ph2C.-) = 81.8 +/- 2 kcal mol-1 was calculated.The reactions of Ph2C.- with CH3OH and C2H5OH proceeded with major and minor amounts, respectively, of a H2.+-transfer channel, forming Ph2CH2, RCHO, and an electron.The kinetic nucleophilicity of Ph2C.- in SN2 displacement reactions with CH3X and C2H5X molecules was shown to be medium, which requires a significant intrinsic barrier in these reaction.The reactions of Ph2C.- with various aldehydes, ketones, and esters were fast and established two principal product-forming channels: (1) H+ transfer if the neutral reactant contains activated C-H bonds and (2) carbonyl addition followed by radical β-fragmentation of one of the groups originally attached to the carbonyl carbon.The order for the ease of radical β-fragmentation in the tetrahedral intermediates was RO > alkyl >> H, and CO2CH3 > CH3.Since the reactions of Ph2C.- with the simple esters HCO2CH3 and CH3CO2CH3 were fast, it should now be possible to examine the reactions of carbonyl-containing organic molecules, which are expected to react slower than these esters and obtain their relative reactivities.
- McDonald, Richard N.,Gung, Wei Yi
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p. 7328 - 7334
(2007/10/02)
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- Nonlinear Broensted-Type Plot in the Pyridinolysis of 2,4-Dinitrophenyl Benzoate in Aqueous Ethanol
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From the kinetics of the title reaction in 44 wtpercent aqueous ethanol at 25 deg C, ionic strength 0,2 M (KCl), a curved Broensted-type plot is obtained.The curve is interpreted in terms of a zwitterionic tetrahedral intermediate in the reaction path and a change in the rate-determining step from breakdown to products of this intermediate to its formation as the pyridine becomes more basic.A semiempirical equation based on this hypothesis accounts for the experimental results.The center of the Broensted-type curvature (pKa0) is located at pKa = 9.5, lower than the value found in the pyridinolysis of 2,4-dinitrophenyl p-nitrobenzoate in the same solvent, showing that electron withdrawal from the acyl group of the substrate favors amine expulsion relative to phenoxide from the tetrahedral intermediate.It is claimed that the influence of the acyl group of the substrate on the value of pKa0 cannot be quantified when comparing similar reactions in water and aqueous ethanol since the pKa0 value for a given reaction should be larger in the latter solvent.The activation parameters obtained support the hypothesis that for the title reactions, pKa0 > 9.
- Castro, Enrique A.,Santander, Carlos R.
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p. 3595 - 3600
(2007/10/02)
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- Tin Oxide Surfaces. Part 11.-Infrared Study of the Chemisorption of Ketones on Tin(IV) Oxide
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Infrared spectroscopy has been employed to study the chemisorption of a number of unsymmetrical ketones, RCOMe, onto tin(IV) oxide.In every case, the final product was the surface carboxylate, RCO2-(ads).The data suggested a mechanism involving initial coordination of the ketone to a surface tin site, followed by nucleophilic attack of a neighbouring hydroxyl group at the carbonyl carbon atom.
- Harrison, Phillip G.,Maunders, Barry M.
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p. 1329 - 1340
(2007/10/02)
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- Micellar Effects upon the Hydrolysis of Activated Amides. Mechanistic Aspects
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Reactions of OH- in water with N-acylpyrrole, -indole, and -carbazole (acyl = acetyl or benzoyl) are second order in OH- in 10-3 M NaOH and first order in more concentrated OH-, but reaction in micelles of cetyltrimethylammonium bromide (CTABr) is first order in micellar-bound OH- for both 10-3 and 0.05 M NaOH.Reactions are inhibited by anionic micelles of sodium lauryl sulfate (NaLS).The rate-surfactant profiles for rate enhancements in CTABr, and inhibition in NaLS, are analyzed in terms of the distribution of OH- and amide between aqueous and micellar pseudophases: second-order rate constants for attack of OH- upon amide are lower in the micellar pseudophase than in water.Micellar effects upon reaction of OH- with anilides have been analyzed, giving second-order rate constants in the micellar pseudophase which are very similar to those in water.
- Cipiciani, Antonio,Linda, Paolo,Savelli, Gianfranco,Bunton, Clifford A.
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p. 5262 - 5267
(2007/10/02)
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- Nucleophilic Reactions of F3C- at sp2 and sp3 Carbon in the Gas Phase. Characterization of Carbonyl Addition Adducts
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The reactions of F3C- with CH3Br and CH3Cl established the medium kinetic nucleophilicity of F3C- on Bohme's reactivity scale for gas-phase SN2 reactions.The reactions of F3C- with (CH3)2C=O and CH3CO2CH3 proceeded by competitive bimolecular H+ transfer and termolecular carbonyl addition giving the corresponding adducts anions m/z 127 and 143, respectivaly.F3C- reacted with esters C6H5CO2CH3, CF3CO2CH3, and (CH3O)2C=O both by SN2 displacement forming the corresponding carboxylate anions and by carbonyl addition yielding the adduct anions; with CF3CO2C2H5 and CF3CO2C(CH3)3, the competitive bimolecular reaction channel involved E2 elimination giving CF3CO2-.The major reaction channel of F3C- with HCO2CH3 was the Riveros reaction that produced the series of cluster ions F3C-(HOCH3), F3C-(HOCH3)2, CH3O-(HOCH3), and CH3O-(HOCH3)2, along with a minor amount of carbonyl addition.The fast termolecular reaction of F3C- with (CF3)2C=O exclusively formed the adduct (CF3)3CO- (m/z 235) which was characterized as the bound, tetrahedral structure by bracketing its proton affinity.The reaction of F3C- with CO2 giving CF3CO2- was established as a termolecular process when the "apparent" bimolecular rate constant was shown to be PHe dependent.These results demonstrate unequivocally that the reactions of gas-phase nucleophiles with the carbonyl group of ketones and esters proceed by addition yielding the corresponding adduct oxyanions which is analogous to the related process in the condensed phase.
- McDonald, Richard N.,Chowdhury, A. Kasem
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p. 7267 - 7271
(2007/10/02)
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- INHIBITORY EFFECT OF HYDROXIDE IONS ON THE DEGRADATION OF β-DICARBONYL COMPOUNDS PRODUCED BY THE RING OPENING OF CHROMONES IN ALKALINE MEDIUM
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A kinetic investigation is reported of the degradation of β-dicarbonyl phenolates (1) and enolates (2) produced by ring cleavage reaction of chromone, flavone, isoflavone and the corresponding 2-hydroxychromanones.Hydroxide ions were found to inhibit the cleavage of these compounds, which effect had been unknown in the field of chromonoids and β-dicarbonyl compouns.The inhibitory effect can be expressed by the equation k=(kd1K*+kd22)/(K*+).The reaction rate constants (kd1 and kd2), the equilibrium constants (K* and K), as well as the activation energy and entrophy of the degradation reaction were determined.Based upon the change of the activation energy and entropy, hydratation of the phenolates (1) and enolates (2) was found to play a very important part in the degradation.
- Zsuga, M.,Szabo, V.,Korodi, F.
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p. 111 - 119
(2007/10/02)
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- Untersuchung der Modifikation von Kohlenstoff-Elektroden mittels elektrochemischer Reduktion charakteristischer Depolarisatoren
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The extent of the modification of carbon fibre electrodes during the thermal oxidation is investigated using electrochemical methods.By the above oxidation procedure carboxylic groups are produced on the carbon surface, which cause the formation of pre-waves during the electrochemical reduction of characteristic depolarizers, i. e. of nitrobenzene, p-benzoquinone and azobenzene in non aqueous solvents, such as methanol, acetonitrile and N,N-dimethylformamide.The comparison of the heights of these pre-waves with the height of the obtained pre-waves at non pre-treated electrodes in the presence of carbonic acids constitutes a measure for the carbon surface modification.Benzoic acid has been found to be the most suitable acid for this investigation.Oxidation at 150 deg C for 40 h has been proven to be the most favorable procedure for the production of carboxylic groups on the carbon surface.At higher temperatures the amount of the carboxylic groups is decreased, while at the same time other groups such as hydroxylic or phenolic groups are formed.In this case the extent of the modification can be followed in aqueous solutions by means of electrochemical reduction of organic molecules preferably adsorbed on the hydrophobic electrode sites after the silanization of the surface groups.Nitrophenols and nitronaphthols have been selected as the most suitable substances for this application.Keywords: Carbon fibre electrodes / Thermal oxidation / Reduction prewaves / Silanization / Adsorption
- Theodoridou, E.,Jannakoudakis, A. D.
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p. 175 - 192
(2007/10/02)
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- Kinetics of Epoxidation of α,β-Unsaturated Ketones in Methanol Medium
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The kinetics of epoxidation of α,β-unsaturated ketones by alkaline H2O2 in methanol in the temperature range 25-40 deg C has been studied.The addition follows second order kinetics.The results indicate that the concentration of NaOH has a significant effect on the reactivity.The effect of various substituents in these reactions show that electron-releasing groups attached to the β-carbon atom in the olefin diminish the rate and electron-attracting groups enhance it.The oxidative cleavage of a few substituted epoxychalkones by hydroperoxide anion has also been studied in methanol at 30 deg C.Adherence to second order kinetics is excellent in every case and the rates are slower than those of the corresponding chalkones.The stability of epoxides depends on the substituents; electron-releasing groups diminish the rate and electron-attracting groups enhance it as observed in the case of epoxidation.The reaction obeys the Arrhenius equation and the respective activation parameters have been calculated.The effect of solvent polarity on the rate of epoxidation has been studied.The differences in the rates in methanol and water as solvents have been explained on the basis of solvent-solute interaction.
- Rama Rao, D. Sri
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p. 786 - 789
(2007/10/02)
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