- Kinetic study on alkaline hydrolysis of Y-substituted phenyl picolinates: Effects of modification of nonleaving group from benzoyl to picolinyl on reactivity and reaction mechanism
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Second-order rate constants (kOH-) for alkaline hydrolysis of Y-substituted phenyl picolinates (6a-6i) have been measured spectrophotometrically. A linear Bronsted-type plot is obtained with βlg = -0.34, which is typical for reactions reported previously to proceed through a stepwise mechanism with formation of an addition intermediate being the rate-determining step (RDS). However, σYo constants result in a much poorer Hammett correlation than σY- constants. Furthermore, the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρY = 0.82 and r = 0.72, indicating that a negative charge develops partially on the O atom of the leaving group in the RDS. Thus, the reactions have been concluded to proceed through a forced concerted mechanism with a highly unstable intermediate 7. Comparison of the current kinetic data with those reported previously for the corresponding reactions of Y-substituted phenyl benzoates has revealed that modification of the nonleaving group from benzoyl to picolinyl causes not only an increase in reactivity but also a change in the reaction mechanism (i.e., from a stepwise mechanism to a forced concerted pathway).
- Kim, Myung-Joo,Kim, Min-Young,Um, Ik-Hwan
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p. 1138 - 1142
(2015/07/15)
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- Reactions of [aryloxy(phenyl)carbene]pentacarbonylchromium(0) complexes with thiolate ions. Decreasing reactivity with increasing basicity of the nucleophile
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(Chemical Equation Presented) A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]-pentacarbonyl chromium(0) complexes in 50% MeCN-50% water (v/v) is reported. Bronsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (βnuc ≈ 1.0) followed by a slightly descending leg with a negative slope (βnuc ≈ -0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pKaRSH to rate-limiting nucleophilic attachment at high pKaRSH. The negative βnuc values result from a combination of minimal progress of C-S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.
- Bernasconi, Claude F.,Perez-Lorenzo, Moises,Codding, Sara J.
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p. 9456 - 9463
(2008/03/15)
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- Kinetics and mechanistic study of the reaction of cyclic anhydrides with substituted phenols. Structure-reactivity relationships
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(Chemical Equation Presented) The kinetic of the reactions of phthalic and maleic anhydrides with different substituted phenols (Z-PhOH with Z = H, m-CH3, p-CH3, m-Cl, p-Cl, and m-CN) were studied in aqueous solution. Two kinetic processes well separated in time were observed. The fast one is attributed to the formation of the aryl ester in equilibrium with the anhydride and allows the determination of the rate of nucleophilic attack of the phenol on the anhydride (k-A). From the slow kinetic process, the equilibrium constant for this reaction was determined. The Broensted-type plots for the nucleophilic attack of substituted phenols on the anhydrides were linear with slopes βNuc of 0.45 and 0.56 for phthalic and maleic anhydride, respectively. The results are consistent with a mechanism involving rate-determining nucleophilic attack and also with a concerted mechanism. The calculated effective charge on the atoms involved in the reactions and the Broensted β values are consistent with a mechanism involving a concerted or enforced concerted mechanism where a tetrahedral intermediate with significant lifetime is not formed along the reaction coordinate. The latter mechanism is preferred over the stepwise process.
- Andres, Gabriel O.,De Rossi, Rita H.
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p. 1445 - 1449
(2007/10/03)
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- Mechanistic change in the reactivity of substituted phenyl acetates over phenyl thiolacetates toward imidazole in aqueous phase
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The kinetics of aminolysis of several substituted phenyl acetates by imidazole is studied in aqueous medium at 20°C and an ionic strength of 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 272-401 nm), under excess free imidazole, pseudo-first-order rate constants (κobs) are obtained. For the esters with good nucleofuges, the reaction follows clean second-order kinetics and the plots of (κobs - κH) against free imidazole concentration are linear at constant pH. The macroscopic nucleophilic substitution rate constants (κN) are obtained as the slopes of these plots and found to be pH independent. For the esters with poor nucleofuges, a rate dependence on more than first power of the free imidazole and a linear dependence of κ′2 on free imidazole is observed. Accordingly, the microscopic rate constants for the assisted paths viz. κga and κgb have been disseminated besides for simple bimolecular attack. The Broensted-type plots and Hammett plots were constructed whose slope values are consistent with a stepwise mechanism through a bipolar tetrahedral addition intermediate whose formation or breakdown being rate determining for various paths. Comparison of this reaction of oxyesters with the earlier reported works on similar reaction of analogue thiolesters under identical reaction conditions showed remarkable mechanistic differences which are discussed in detail. The discussion is extended to include the details on previously studied ammonolysis of these two types of esters wherein thiolesters showed differed reactivity than that reported in the present study.
- Rajarathnam,Jeyakumar,Nadar, P. Ananthakrishna
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p. 211 - 221
(2007/10/03)
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- Curved Hammett Plot in Alkaline Hydrolysis of O-Aryl Thionobenzoates: Change in Rate-Determining Step versus Ground-State Stabilization
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Second-order rate constants have been measured for alkaline hydrolysis of O-aryl thionobenzoates (X-C6H4-CS-OC6H 4-Y) in 80 mol % H2O-20 mol % DMSO at 25.0 ± 0.1 °C. The Hammett plot for the reaction of O-4-nitrophenyl X-substituted thionobenzoates (X-C6H4-CS-OC6H 4-NO2, 1a-e) exhibits a downward curvature. However, a possible traditional explanation in terms of a change in the rate-determining step (RDS) has been considered but rejected. The proposed explanation involves stabilization of the ground-state (GS) through-resonance interaction between the electron-donating substituent X and the thionocarbonyl functionality on the basis of the linear Yukawa-Tsuno plot obtained for the same reaction. The Bronsted-type plot for the reaction of O-aryl thionobenzoates (C 6H5-CS-OC6H4-Y, 2a-i) is linear but exhibits many scattered points with a small βlg, (-0.35). The Hammett plot for the same reaction shows rather poor correlation with σ- constants but much better correlation with σ° constants. The alkaline hydrolysis of O-aryl thionobenzoates (1a-e and 2a-i) has been proposed to proceed through an addition intermediate in which bond formation is the RDS.
- Um, Ik-Hwan,Lee, Ji-Youn,Kim, Han-Tae,Bae, Sun-Kun
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p. 2436 - 2441
(2007/10/03)
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- Reinterpretation of curved hammett plots in reaction of nucleophiles with aryl benzoates: Change in rate-determining step or mechanism versus ground-state stabilization
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A kinetic study is reported for the reaction of the anionic nucleophiles OH-, CN-, and N3- with aryl benzoates containing substituents on the benzoyl as well as the aryloxy moiety, in 80 mol % H2O-20 mol % dimethyl sulfoxide at 25.0 °C. Hammett log k vs σ plots for these systems are consistently nonlinear. However, a possible traditional explanation in terms of a mechanism involving a tetrahedral intermediate with curvature resulting from a change in rate-determining step is considered but rejected. The proposed explanation involves ground-state stabilization through resonance interaction between the benzoyl substituent and the electrophilic carbonyl center in the two-stage mechanism. Accordingly, the data are nicely accommodated on the basis of the Yukawa-Tsuno equation, which gives linear plots for all three nuceophiles. Literature reports of the mechanism of acyl transfer processes are reconsidered in this light.
- Um, Ik-Hwan,Han, Hyun-Joo,Ahn, Jung-Ae,Kang, Swan,Buncel, Erwin
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p. 8475 - 8480
(2007/10/03)
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- Kinetics and mechanism of phosphate-catalyzed hydrolysis of benzoate esters: Comparison with nucleophilic catalysis by imidazole and o-iodosobenzoate
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Phosphate-catalyzed hydrolysis of 2,4-dinitrophenyl 4-X-benzoate, and 3- or 4-Y-phenyl 3,5-dinitrobenzoates, where X and Y are substituents, has been studied spectrophotometrically. The following conclusions are based on catalytic rate constants, solvent
- El Seoud, Omar A.,Ruasse, Marie-Francoise,Rodrigues, Wagner A.
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p. 1053 - 1058
(2007/10/03)
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- Acid dissociation constants of phenols and reaction mechanism for the reactions of substituted phenyl benzoates with phenoxide anions in absolute ethanol
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Acid dissociation constants of 10 substituted phenols have been measured by a kinetic method together with second-order rate constants for the reactions of aryl benzoates (X-C6H4CO-OC6H4-Y) with Z-substituted phenoxide (Z-C6H4O-) and EtO- in absolute ethanol at 25.0 ± 0.1°C. The kinetic results support a stepwise mechanism for the present acyl-transfer reaction.
- Um, Ik-Hwan,Hong, Yeon-Ju,Kwon, Dong-Sook
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p. 5073 - 5082
(2007/10/03)
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- Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals: Comparisons with phenoxyl radicals
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The reduction potentials relative to the standard hydrogen electrode (SHE) for a number of para-substituted phenylthiyl radicals (Eo(p-XC6H4S./p-XC6H 4S-)) have been derived from pulse radiolytic studies of electron transfer equilibria which compare their values to those of radicals of known reduction potentials. A ladder combining the reduction potentials for both phenylthiyl and phenoxyl radicals has been established. These reduction potentials have been shown to be self-consistent and are intermediate between those of p-benzosemiquinone radical anion at 0.02 V and phenoxyl radical at 0.79 V. The reduction potential decreases as the electron donating power of the para substituent rises. The substituent effect is, however, much weaker for the phenylthiyl radicals than for their oxygen analogs. These observations demonstrate that the electronic interaction between the sulfur atoms and the aromatic ring system is much less than that which occurs with oxygen atoms. Examination of the rates of electron transfer in terms of the Marcus theory indicates that the reorganization energies of both p-XC6H4O. and p-XC6H4S. radicals are similarly affected by H, CH3, and CH3O substitution. However, the reorganization energies increase substantially for H2N and O- para substituents with the effect being much less for the p-XC6H4S. radicals than for the p-XC6H4O. radicals. These observations are in accord with structural information from spectroscopic and theoretical studies of the radicals which show that in the latter system the substituent groups interact strongly with the aromatic π system.
- Armstrong,Sun, Qun,Schuler
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p. 9892 - 9899
(2007/10/03)
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- Redox and acidity properties of 4-substituted aniline radical cations in water
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By means of pulse radiolysis the one-electron reduction potentials of twelve 4-substituted aniline radical cations and the pKa values of eight 4-substituted aniline radical cations have been determined. Both the potentials and the pKa values were shown to be linearly dependent on the Brown σp+ values for the 4-substituents. The one-electron reduction potentials were also compared to previously reported electrochemically determined E1/2 values resulting in good agreement between the two sets of data. By combining the one-electron reduction potentials and the pKa values the N-H bond dissociation energies of some 4-substituted anilines were calculated, also showing a linear substituent dependence. The absolute N-H bond dissociation energy of aniline found was close to a previously reported value.
- Jonsson, Mats,Lind, Johan,Eriksen, Trygve E.,Merényi, Gábor
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p. 1423 - 1427
(2007/10/02)
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- ALKALINE HYDROLYSIS OF SUBSTITUTED PHENYL ACETATES CATALYZED BY QUATERNARY AMMONIUM SALTS
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Alkaline hydrolysis of the phenyl acetates CH3COOC6H4X (X = 4-NO2, 3-NO2, 3-Cl, H, 4-CH3, 3-CH3, and 4-OCH3) in the presence of hexadecyltris(2-hydroxyethyl)ammonium chloride, bis(2-hydroxyethyl)hexadecyl(methyl)ammonium bromide, and hexadecyltrimethylamm
- Kulic, Jiri,Sussenbek, Jiri,Ptacek, Ales
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p. 2891 - 2895
(2007/10/02)
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- The Effect of Hydrogen Bonding between Methyl-Substituted Phenols and Dipolar Aprotic Solvents on the Rate Constants for Protonation of Anthracene Anion Radical
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The kinetics for the protonation of anthracene anion radical (A-anion radical) by phenol and 13 methyl-substituted phenols (ArOH) have been studied by derivative cyclic voltammery in dimethyl sulfoxide, N,N-dimethylformamide, propylene carbonate and acetonitrile.The observed rate constants were corrected for the stoichiometric effects of formation of the homoconjugation complexes, ArOH/ArO- and Ar/OH/ArOH/ArO- and the kinetic contributions from protonation of A-anion radical by ArOH dimers.Available data for equilibrium (1) show that ArOH exists in solution predominantly as the ArOH/solvent hydrogen bond complex with only minor amounts of the 'free' ArOH present.It is suggested that the protonation of A-anion radical involves only the free ArOH in the proton transfer step (II).The unique rate constants corrected for the effect of equilibrium (I) were between two and three orders of magnitude higher than those usually referred to as the second-order rate constants for protonation of A-anion radical.ArOH/solvent ArOH + solvent (I) A-anion radical + ArOH AH-radical + ArO- (II).The kinetic effect of methyl substituents in the 2,6-positions of the phenol was observed to depend on the solvent.It was found that these effects could be satisfactority accounted for by the influence of the 2,6-methyl substituents on the magnitude of K1.The 'best' values of the substituent constants, ?b2 and ?b2.6, defined by the protonation of A-anion radical were found to be between those defined by equilibrium (1) and those defined by the unique equilibrium acidity of the phenols.This is taken as an indication that the phenol O-H bond is stretched considerably in the transition state for protonation of A-anion radical.From the relation between kII and ΔG deg it was found that the Broensted α value for the process is close to 0.5.
- Nielsen, Merete Folmer,Hammerich, Ole
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p. 883 - 896
(2007/10/02)
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- The one-electron reduction potential of 4-substituted phenoxyl radicals in water
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By means of pulse radiolysis the one-electron reduction potentials of twelve 4-substituted phenoxy radicals have been determined. The main reference used was the ClO2./ClO2- couple. By combining the redox potentials of phenoxyl radicals with the aqueous acidities of phenols the bond strength of the phenolic O-H bond was calculated. These values were found to be in good agreement with O-H bond dissociation enthalpies measured in the gas phase.
- Lind,Shen,Eriksen,Merényi
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p. 479 - 482
(2007/10/02)
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- Effective Charge Development in Ester Hydrolysis Catalyzed by Cationic Micelles
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Reaction of hydroxide ion with substituted phenyl laurates is catalyzed by micelles containing CTAB (cetyltrimethylammonium bromide).The observed rate constants for ester consumption obey a Michaelis-Menten-like rate law, kobsd = (kOHKeq + kOHcat)/(Keq + ), where kOHcat is the second order rate constant for reaction of hydroxide ion with micelle-bound ester, Keq is a partition coefficient for dissociation of ester from the micelle-bound ester, and kOH is the second order rate constant for reaction of free ester with hydroxide ion.The value of the parameter Keq is independent of the nature of the substituent on the ester.The ratio kOHcat/Keq represents the free energy change from ester in aqueous solution to the transition state of the ester reaction in the micellar pseudophase.The dependence of log kOHcat/Keq on the pKa of the leaving phenol obeys a linear Bronsted equation with β1g = -0.51, and log kOH obeys a linear Bronsted equation with β1g = -0.56.Since both Bronsted parameters refer to catalyzed and uncatalyzed reactions starting from the same standard state (aqueous solution), the effective charge in the transtition state of both reactions is almost the same.The microscopic medium is thus similar for both cases, consistent with the COOAr moiety's residing in an aqueous-like region of the pseudophase in the catalytic process.
- Al-Awadi, Nouria,Williams, Andrew
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p. 2001 - 2004
(2007/10/02)
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- A systematic entropy relationship for the general-base catalysis of the deprotonation of a carbon acid. A quantitative probe of transition-state solvation
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The general-base-catalyzed deprotonation of a carbon acid, the l-methyl-4-(phenylacetyl)pyridinium cation (pKa = 9.02 at 25 °C), has been investigated for 32 general-base catalysts (25 amines and seven phenoxide ions) in aqueous solution. Amines give a generally scattered Bronsted plot; ring-substituted benzylamines have ?= 0.52, and ring-substituted phenoxides have ?= 0.60, with the phenoxides being more reactive than amines of similar basicity. The temperature dependences of the general-base-catalyzed deprotonation of this carbon acid have been measured over the range 15-45 °C for 12 base catalysts (eight primary, secondary, and tertiary amines; 4-(dimethylamino)pyridine; two phenoxide ions; hydroxide ion). The entropies of activation for these deprotonations show a clean curvilinear dependence upon the entropies of protonation of these base species, with the hydroxide ion being the only significant deviant from this relationship. This observation quantitatively establishes the importance of solvation effects as the major source of deviations that are commonly observed in Bronsted relationships for general-base-catalyzed processes.
- Bunting, John W.,Stefanidis, Dimitrios
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p. 779 - 786
(2007/10/02)
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- Aminopyrine and Antipyrine Free Radical-cations: Pulse Radiolysis Studies of One-electron Transfer Reactions
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Absolute rate constants for the reaction of a variety of electrophilic free radicals with the pyrazoline derivatives aminopyrine and antipyrine have been measured by pulse radiolysis.In the case of aminopyrine the resulting radical cation is a particularly stable species ε325 5.35*103 dm3 mol-1 cm-1).Both compounds are readily oxidised to their respective radical-cations with the one-electron oxidation potential of antipyrine (E0 1.1-1.6 V) being higher than that of aminopyrine (E0 0.26-0.5 V).Studies of the reaction of the radical-cations with reducing agents suggest that aminopyrine in particular may prove to be a useful reference compound in studies of free radical one-electron oxidations.
- Forni, Luigi G.,Mora-Arellano, Victor O.,Packer, John E.,Willson, Robin L.
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p. 1579 - 1584
(2007/10/02)
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- Thermodynamics of Proton Transfer in Phenol-Acetate Hydrogen Bonds with Large Proton Polarizability and the Conversion of Light Energy into Chemical Energy in Bacteriorhodopsin
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Phenol-acetate solutions in CCl4 are studied by IR spectroscopy as a function of the pKa of the phenols.The (I) Ar-OH...-OC Ar-O-...HOC (II) hydrogen bonds formed show large proton polarizability as indicated by continua in the IR spectra.The percent proton transfer (PT) increases from the p-cresol-acetate to the pentachlorophenol-acetate system from 0percent to 56percent.The Gibbs free energy, ΔG0PT, values of the PT equilibria at 295 K are determined as well as the standard enthalpy values, ΔH0PT, and the standard entropy values, ΔS0PT.These data are given in Table II.The shape of the intensity of the continuum as a function of the ΔH0PT value changes.In the classical approximation the average difference between the two minima of the proton double-minimum potential is given by ΔH0PT.With the decreasing amount of ΔH0PT, i.e., decreasing degree of asymmetry, the intensity of the continua decreases at higher and increases at lower wavenumbers.This result is in good agreement with the predictions from calculated line spectra.In the photocycle of bacteriorhodopsin a tyrosine-aspartate hydrogen bond is probably of importance for the conversion of light energy into chemical energy.On the basis of the obtained data it is shown that 9.5 kJ/mol can be converted into chemical energy due to a proton transfer induced by a local electrical field in a Tyr-Asp hydrogen bond.Furthermore, if the Ar-O-...HOC structure is broken afterwards by a conformational change, at least 25 kJ/mol of conformational energy is converted.Thus, altogether 34.5 kJ/mol of Gibbs free energy may be converted into chemical energy and stored by these processes.
- Merz, Helmut,Tangermann, Ulrike,Zundel, Georg
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p. 6535 - 6541
(2007/10/02)
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- A Mechanistic Change in the Alkaline Hydrolysis of Esters of Fluorene-9-carboxylic Acid
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The Broensted leaving group (βlg) plot for the alkaline hydrolysis of fluorene-9-carboxylate esters in the pH region corresponding to the existence of the ester anion shows a minimum flanked by a region of high negative βlg (-1.01), corresponding to an E1cb pathway and, for esters of pKlg >9.5, a region of positive βlg (+0.11).
- Alborz, Manoochehr,Douglas, Kenneth T.
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p. 728 - 729
(2007/10/02)
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