- Nucleophilic Addition to Olefins. 6. Structure-Reactivity Relationships in the Reactions of Substituted Benzylidene Meldrum's Acids with Water, Hydroxide Ion, and Aryl Oxide Ions
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The addition of water to p-methoxybenzylidene Meldrum's Acid (1-OMe), to form an adduct (TOH-, is subject to weak general base catalysis, just as water addition to carbocations studied by Ritchie.This implies a mechanism in which the
- Bernasconi, Claude F.,Leonarduzzi, Gianni D.
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- Kinetic Study on Nucleophilic Substitution Reactions of O-Phenyl O-Y-substituted-Phenyl Thionocarbonates with 1,8-Diazabicyclo[5.4.0]undec-7-ene in Acetonitrile
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Second-order rate constants (kN) for nucleophilic substitution reactions of O-phenyl O-Y-substituted-phenyl thionocarbonates (4a–4k) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in MeCN at 25.0 ± 0.1°C are reported. The reactivity of 4a–4k decreases as basicity of the leaving group increases except O-2,4-dinitrophenyl O-phenyl thionocarbonate (4a), which is less reactive than O-3,4-dinitrophenyl O-phenyl thionocarbonate (4b) although the former possesses 2 pKa units less basic nucleofuge than the latter. The Br?nsted-type plot for the reactions of 4b–4k is linear with βlg = ?0.50, a typical βlg value for reactions reported to proceed through a concerted mechanism. The Hammett plot correlated with σY? constants for the reactions of 4b–4k results in a better linear correlation than that correlated with σYo constants. Besides, the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρY = 2.12, r = 0.68 and R2 = 0.990, indicating that a negative charge develops partially on the O atom of the leaving group in the rate-determining step (RDS). Thus, the reactions have been concluded to proceed through a forced concerted mechanism. Effects of steric hindrance on reactivity and reaction mechanism are also discussed in detail.
- Um, Ik-Hwan,Park, Kyoung-Ho
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p. 1169 - 1173
(2017/09/13)
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- Kinetic study on alkaline hydrolysis of Y-substituted phenyl X-substituted benzenesulfonates: Effects of changing nucleophile from azide to hydroxide ion on reactivity and transition-state structure
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Second-order rate constants (kOH-) for alkaline hydrolysis of 2,4-dinitrophenyl X-substituted benzenesulfonates (1a-1f) and Y-substituted phenyl 4-nitrobezenesulfonates (2a-2g) have been measured spectrophotometrically. Comparison of kOH/
- Moon, Ji-Hyun,Kim, Min-Young,Han, So-Yeop,Um, Ik-Hwan
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p. 1563 - 1568
(2015/08/11)
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- 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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- Kinetic and theoretical study on nucleofugality in the phenolysis of 3-nitrophenyl and 4-nitrophenyl 4-cyanophenyl thionocarbonates
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The phenolysis of 3-nitrophenyl 4-cyanophenyl thionocarbonate (1) and 4-nitrophenyl 4-cyanophenyl thionocarbonate (2) are subjected to a kinetic investigation in order to evaluate the nucleofugality of the corresponding leaving groups. For the reaction of 2 only 4-nitrophenoxide is obtained as leaving group. For the reaction of 1 the nucleofugality ratio 3-nitrophenoxide/4-cyanophenoxide is 1/3 from the corresponding T- intermediate. Theoretical calculations confirm the experimental results. From these results it can be concluded that the non-leaving group affects the nucleofugality ratio.
- Castro, Enrique A.,Ca?ete, Alvaro,Campodónico, Paola R.,Cepeda, Marjorie,Pavez, Paulina,Contreras, Renato,Santos, José G.
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p. 130 - 135
(2013/07/11)
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- Pitfalls in assessing the α-effect: Reactions of substituted phenyl methanesulfonates with HOO-, OH-, and substituted phenoxides in H2O
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Toward resolving the current controversy regarding the validity of the α-effect, we have examined the reactions of Y-substituted phenyl methanesulfonates 1a-1l with HOO-, OH-, and Z-substituted phenoxides in the gas phase versus solu
- Um, Ik-Hwan,Im, Li-Ra,Buncel, Erwin
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p. 8571 - 8577
(2011/03/20)
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- Analysis of linear free-energy relationships combined with activation parameters assigns a concerted mechanism to alkaline hydrolysis of X-substituted phenyl diphenylphosphinates
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A kinetic study is reported for alkaline hydrolysis of X-substituted phenyl diphenylphosphinates (1a-i). The Bronsted-type plot for the reactions of 1a-i is linear over 4.5 pKa units with βlg= -0.49, a typical βlg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σo and σ- constants are linear but exhibit many scattered points, while the corresponding Yukawa-Tsuno plot results in excellent linear correlation with ρ= 1.42 and r=0.35. The r value of 0.35 implies that leaving-group departure is partially advanced at the rate-determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO - ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (DN + AN) mechanism is also ruled out on the basis of the small βlg value. As the substituent X in the leaving group changes from H to 4-NO2 and 3,4-(NO2)2, ΔH≠ decreases from 11.3 kcal mol-1 to 9.7 and 8.7 kcal mol-1, respectively, while ΔS≠ varies from -22.6 cal mol-1 K-1 to -21.4 and -20.2 cal mol-1 K-1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1a-i.
- Um, Ik-Hwan,Han, Jeong-Yoon,Hwang, So-Jeong
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supporting information; experimental part
p. 7324 - 7330
(2009/08/07)
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- Aminolyses of aryl diphenylphosphinates and diphenylphosphinothioates: Effect of modification of electrophilic center from P=O to P=S
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(Chemical Equation Presented) A kinetic study is reported for aminolysis of aryl diphenylphosphinothioates (2a-i). The phosphinothioates 2a-i are less reactive than aryl diphenylphosphinates (1a-i), the oxygen analogues of 2a-i, regardless of the basicity
- Um, Ik-Hwan,Akhtar, Kalsoom,Shin, Young-Hee,Han, Jeong-Yoon
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p. 3823 - 3829
(2008/02/02)
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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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- Transition-state variation in the nucleophilic substitution reactions of aryl bis(4-methoxyphenyl) phosphates with pyridines in acetonitrile
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The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-MeOC6H4O)2P(=O) OC6H4Z, with pyridines (XC5H4N) are investigated in acetonitrile at 55.0 °C. In the case of more basic phenolate leaving groups (Z = 4-Cl, 3-CN), the magnitudes of βx (βnuc) and βz (βlg) indicate that mechanism changes from a concerted process (βx = 0.22-0.36, βz = -0.42 to -0.56) for the weakly basic pyridines (X = 3-Cl, 4-CN) to a stepwise process with rate-limiting formation of a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate (βx = 0.09-0.14, βz = -0.08 to -0.28) for the more basic pyridines (X = 4-NH2, 3-CH3). This proposal is supported by a large negative cross-interaction constant (ρxz = -1.98) for the former and a positive ρxz(+0.97) for the latter processes. In the case of less basic phenolate leaving groups (Z = 3-CN, 4-NO2), the unusually small magnitude of βz values is indicative of a direct backside attack TBP-5C TS in which the two apical sites are occupied by the nucleophile and leaving group, ap(NX)-ap(LZ). The instability of the putative TBP-5C intermediate leading to a concerted displacement is considered to result from relatively strong proximate charge transfer interactions between the π-lone pairs on the directly bonded equatorial oxygen atoms and the apical bond (no(eq) - ?*(ap)). These are supported by the results of natural bond orbital (NBO) analyses at the NBO-HF/6-311+G**//B3LYP/6-311+G** level of theory.
- Lee, Hai Whang,Guha, Arun Kanti,Kim, Chang Kon,Lee, Ikchoon
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p. 2215 - 2222
(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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- Electrochemistry of electron transfer probes. α-aryloxyacetoveratrones and implications for the mechanism of photo-yellowing of pulp1
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Standard potentials (Eo) of a series of substituted α-aryloxyacetoveratrone derivatives have been determined from a correlation of the 13C NMR chemical shifts of the carbonyl group and a similar correlation (Eo vs. 13C NMR shifts) within a series of α-anilinoacetoveratrones. Using these potentials the rate constants for fragmentation of the radical anions were determined by digital simulation of the voltammetric waves. The rate contants for C O cleavage in the radical anions correlate with the pKa of the corresponding phenols. The fragmentations are all in the activated region of a general free energy relationship for this class of compound (α = 0.5). The standard potential and rate constant for fragmention of the α-guaiacoxyacetoveratrone radical anion also were determined. This species is a model compound for one of the lignin substructures. The implication of these results on the currently accepted mechanism for photo-yellowing of lignin rich paper is discussed.
- Andersen, Mogens L.,Wayner, Danial D. M.
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p. 830 - 836
(2007/10/03)
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- Photochemical generation of the N-isopropylbenzonitrilium ion and unusually slow trapping by azide ion
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The imidate ester 4-cyanophenyl N-isopropylbenzimidate undergoes efficient photoheterolysis in water producing the 4-cyanophenoxide ion and the N-isopropylbenzonitrilium ion. The latter is detected as a transient intermediate in flash photolysis experiments and has a relatively long lifetime (1/kw) in water of 2.5 ms. The rate constants for hydroxide and azide are respectively 5.2 × 106 and 3.9 × 106 dm3 mol-1 s-1. This pattern is not seen with sp2 hybridized cations, where kaz is normally significantly greater than kOH. Further indications that azide ion is unexpectedly poor as a trap for this cation is seen in considering the kaz:kw value in the context of ratios directly measured for sp2 hybridized cations. For the latter a kaz:kw of less than 105 can be taken to mean that the reaction with azide is at (or certainly very close to) the diffusion limit (i.e. 5-10 × 109 dm3 mol-1 s-1). The nitrilium ion 3 has kaz:kw equal to only 104 mol-1 dm3 and yet its azide reaction is three orders of magnitude slower than diffusion.
- Ruane, Patrick H.,McClelland, Robert A.,Hegarty, A. Frank,Steenken, Steen
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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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- Electrochemistry of electron-transfer probes. The role of the leaving group in the cleavage of radical anions of α-aryloxyacetophenones
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The formal reduction potential (E°) of α-phenoxyacetophenone has been determined from the voltammetric peak potential obtained by linear sweep voltammetry in combination with the rate constant for fragmentation of the radical anion which had been determin
- Andersen, Mogens L.,Mathivanan,Wayner, Danial D. M.
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p. 4871 - 4879
(2007/10/03)
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- Highly Sterically hindered Carbon Acids: The Intrinsic Reactivity of 5,5',5''-Trimethyl- and 3,3',3'',5,5',5''-Hexamethyl-2,2',2'',4,4',4''-Hexanitrotriphenylmethanes
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Rate constants (kpB,kpBH) for the reversible deprotonation of 5,5',5''-trimethyl- and 3,3',3'',5,5',5''-hexamethyl-2,2',2'',4,4',4''-hexanitrotriphenylmethanes (2 and 3) by primary aliphatic amines, piperidine and morpholine as well as by phenoxide anions and hydroxide anion have been measured in H2O-Me2SO (20:80) at 25 deg C.Comparison of the results obtained with those for 2,2',2'',4,4',4''-hexanitrotriphenylmethane (1a) shows that the introduction of methyl groups in positions adjacent to the nitro groups decreases markedly the thermodynamic acidity of theexocyclic CH group: ΔpK2a1a = 1.68; ΔpK3a1a = 6.48.It is suggested that these decreases are very likely the reflection of a twisting of the nitro groups out of their attached aromatic planes and therefore of a reduced resonance stabilization of the conjugated carbanions C-2 and C-3.Other important steric effects are operating in the ionization of 2 and 3.These arise from the accumulation of ortho-nitro groups in the triphenylmethane system which makes the approach of the base reagents from the exocyclic carbon of 2 and 3 very difficult.The finding of extremely low intrinsic reactivities for 2 and 3 and the observation of a much greater catalytic efficiency of primary amines than of secondary amines in assisting the proton transfers are the two most striking manifestations of these F-strain effects.
- Terrier, Francois,Xiao, Lan,Farrell, Patrick G.,Moskowitz, Danielle
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p. 1259 - 1263
(2007/10/02)
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- Heterolysis and homolysis energies for some carbon-oxygen bonds
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Methods described previously for obtaining heterolysis (ΔHhet) and homolysis (ΔHhomo) enthalpies for bonds that can be cleaved to produce resonance-stabilized carbenium ions, anions, and radicals are extended to the study of carbon-oxygen bonds through the reactions of resonance-stabilized carbenium ions with substituted phenoxide ions. Titration calorimetry was used to obtain the heat of heterolysis, and the second-harmonic ac voltammetry (SHACV) method was used to obtain reversible oxidation potentials for the anions. In several cases, the electrode reactions were so fast that reversible potentials were obtained only with the greatest difficulty. Nonetheless, there is remarkably good agreement between these oxidation potentials for phenoxide ions obtained by electrochemical methods in sulfolane solution and those reported by others using entirely different techniques in different media. Such agreement provides unprecedented evidence for the soundness of the various methods used to study redox potentials of organic ions and radicals. As before, a wide variety of correlations was tested between ΔHhet and ΔHhomo. These two properties showed little correlation with each other, but ΔHhet gave good correlations between many properties for which neutral species are converted into ions or vice versa, such as redox potentials of both types of ions, the pKas of the anions, or the free energies of electron transfer. In contrast to the earlier study of cleavage to carbanions and carbenium ions, the present ΔHhet values are predicted well by a general equation that employs the pKR+ of the carbenium ion (without modification) and the pKa of the phenol. The improvement is consistent with the fact that the cleavage of carbon-oxygen bonds of the triarylcarbinols used to establish the pKR+ stability scale is a more appropriate model for the heterolysis of carbon-oxygen bonds in sulfolane at 25°C than it is for the cleavage of carbon-carbon bonds under the same conditions.
- Arnett, Edward M.,Amarnath, Kalyani,Harvey, Noel G.,Venimadhavan, Sampath
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p. 7346 - 7353
(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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- 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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- 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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- Transfer of the Diethoxyphosphoryl Group between Imidazole and Aryloxy Anion Nucleophiles
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Rate constans have been measured for reaction of imidazole with aryl diethyl phosphate (k1) and of aryloxy anions with N-(diethoxyphosphoryl)imidazolium ion (k-1) in aqueous solution at 25 deg C; they obey the following linear Broensted equations: log k1 = -1.02pKArOH + 1.83 (n = 6, r = 0.989); log k-1 = 0.85pKArOH - 0.48 (n = 10, r = 0.957).The value of βeq (1.87) obtained from βlg and βnuc supports a previously determined value (1.83) for the transfer of the neutral phosphoryl group from phenolate ion nucleophiles.The pKaof (diethoxyphosphoryl)imidazolium ion is 6.04.The equilibrium constant for reaction of 4-nitrophenyl diethyl phosphate with imidazole is 5.9 x 10-6; in the case of the aryl ester from phenol with pKArOH = 4.34 the equilibrium constant is calculated to be unity.The Broensted βeq data are used to calibrate effective charges derived from previously measured βlg values for attack of nucleophiles at phosphorus bearing phenolate ion leaving groups.
- Ba-Saif, Salem,Williams, Andrew
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p. 2204 - 2209
(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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- Nucleophilic Addition to Olefins. 2. Reaction of Benzylidene Meldrum's Acid with Water, Hydroxide Ion, and Aryloxide Ions. Complete Kinetic Analysis of the Hydrolytic Cleavage of the C=C Double Bond
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The kinetics of reversible nucleophilic attack by water, hydroxide ion, phenoxide ion, and p-bromophenoxide ion on benzylidene Meldrum's acid was measured in aqueous solution by the stopped-flow method.The rate of protonation on carbon of the anionic addition complex between benzylidene Meldrum's acid and hydroxide ion was also measured.The combination of our results with literature data on the rate of hydrolytic cleavage of benzylidene Meldrum's acid allowed us to map out a detailed mechanism of the cleavage reaction and assign rate-limiting step in different pH ranges.At high pH, protonation on carbon of the anionic addition complex is rate limiting; at low pH, deprotonation of the OH group in the neutral addition complex is rate limiting, which implies a rate constant for the breakdown of the tetrahedral intermediate by carbanion departure on the order of, and probably exceeding, 1010 s-1.
- Bernasconi, Claude F.,Leonarduzzi, Gianni D.
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p. 1361 - 1366
(2007/10/02)
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