Kinetics of the Solvolyses of Benzhydryl Derivatives: Basis for the Construction of a Comprehensive Nucleofugality Scale

Denegri, Bernard; Streiter, André; Jurić, Sandra; Ofial, Armin R.; Kronja, Olga; Mayr, Herbert

doi:10.1002/chem.200500845

Cited by 72 publications

(107 citation statements)

References 92 publications

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“…[5] [b] Previously unpublished electrofugality parameters, based on this work and additional unpublished data.…”

Section: Resultsmentioning

confidence: 99%

Nucleophilicities and Nucleofugalities of Organic Carbonates

Streidl¹,

Branzan²,

Mayr³

2010

Eur J Org Chem

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Keywords: Kinetics / Linear free energy relationships / Solvolysis / Correlation analysis / Leaving groupsThe kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/ Vis spectrophotometry. Substitution of the resulting secondorder rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log k = s(N + E) yielded the nucleophilicity parameters N 25 = 16.03 and s 25 = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The

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“…[5] [b] Previously unpublished electrofugality parameters, based on this work and additional unpublished data.…”

Section: Resultsmentioning

confidence: 99%

Nucleophilicities and Nucleofugalities of Organic Carbonates

Streidl¹,

Branzan²,

Mayr³

2010

Eur J Org Chem

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“…[5] Employing that scale and Equation (1), which represents LFER, abilities of structurally diverse leaving groups have been determined in the range of 12 orders of magnitude in a particular solvent. [3][4][5][6][7] In this equation, the contributions of an electrofuge (alkyl/aryl part of a substrate that in heterolytic process gives carbocation) and a nucleofuge to the overall solvolytic reactivity of a substrate are separated. This approach enables the main practical application of the scales, the estimation of reactivities of substrates constituted from any combination of an electrofuge and a nucleofuge in a given solvent at 25 °C.…”

Section: Introductionmentioning

confidence: 99%

“…[5] The nucleofuge-specific parameters for the certain combination of a leaving group and a solvent can be determined from the log k vs. Ef correlation line using solvolytic rate constants in a given solvent of a series of benzhydrylium derivatives and the reference Ef values. [3][4][5][6][7] In this LFER model, influences of solvation effects are associated to nucleofugality only, defined by two nucleofuge-specific parameters (Nf and sf), which specify the reactivity of some leaving group in a given solvent. [5] In fact, the Nf parameter determines the basic reactivity of the certain leaving group/solvent combination, while the reaction constant (sf parameter) varies within a limited range and slightly decreases as reactivity of leaving groups increases.…”

Section: Introductionmentioning

confidence: 99%

Nucleofugality of Pentafluorophenolate in Various Solvents: Solvolytic Behavior of Phenolates

Matić¹,

Bebek²,

Denegri³

et al. 2016

Croat. Chem. Acta

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Abstract:The leaving group ability (nucleofugality) of the pentafluorophenolate anion has been determined from first order solvolytic rate constants (k) of X,Y-substituted benzhydryl pentafluorophenolates measured in a series of solvents, by using the three parameter LFER equation: log k = sf (Ef + Nf). Comparison with other leaving groups reveals that pentafluorophenolate is a moderate leaving group, whose nucleofugality (Nf value) is between the nucleofugalities of phenyl carbonate and 3,5-dinitrobenzoate leaving groups. However, due to its high reaction constant (sf = 1.29 in 80 % aq. ethanol), relative reactivities of benzhydryl pentafluorophenolates and corresponding benzhydryl carboxylates vary with the electrofugality of a carbocation formed in the heterolytic step, i.e., inversion of the reactivity of benzhydryl series with different leaving groups occurs. The plots of Δ ‡ G° vs. ΔrG° for solvolysis of benzhydryl phenolates and carboxylates reveal that phenolates solvolyze over the lower Marcus intrinsic barrier than corresponding carboxylates.

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“…[11,3] Our philicity [12] and fugality [13] scales can be employed to identify the range where the change of mechanism occurs.…”

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confidence: 99%

“Carbocation Watching” in Solvolysis Reactions

Schaller

Mayr

2008

Angew Chem Int Ed

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The mechanisms of S N 1 reactions were one of the most intensively studied topics in organic chemistry during the middle of the 20th century.[1] These reactions typically proceed with slow formation of the carbocations followed by fast subsequent reactions with the solvent (Scheme 1).Detailed mechanistic studies by Winstein et al. [2] showed that intermediate carbocations can recombine with the leaving group at the ion-pair or free-ion stage before being trapped by the solvent. Since methods for studying fast reaction kinetics were not available at that time, most information on the relative rates of ionization, ion recombination, and reaction of the intermediate carbocation with the solvent was derived indirectly from measurement of the overall solvolysis rates.Recently we have shown that the ionization rates of covalent benzhydryl esters can be observed photometrically if the resulting carbocations are highly stabilized and do not undergo subsequent reactions with the solvent.[3] We have also reported that the trifluoroethanolysis of chlorobis(pmethoxyphenyl)methane proceeds with instantaneous formation of the carbocation, and its subsequent combination with trifluoroethanol could be followed photometrically with a stopped-flow instrument.[4] Based on these results it was predicted that it should be possible to design systems where both the ionization and the following reaction can be measured. We have now realized such systems and report on the first solvolysis reactions in which the formation of carbocations and their subsequent disappearance in aqueous acetone and acetonitrile can be measured directly and fitted by the kinetic model depicted in Scheme 2.When water was added to the colorless solution of 4,4'-bis(morpholino)benzhydryl acetate (1-OAc) in acetonitrile, the solution rapidly turned blue, then faded again within a couple of minutes. A video demonstrating such "carbocation watching" is provided in the Supporting Information (see also Experimental Section).Photometric monitoring of this process showed that the maximum of the carbocation concentration was reached after 7 s in 80 % aqueous acetonitrile. From the absorbance one could derive that at this point, the concentration of the carbocation corresponded to approximately 3 % of the initial substrate concentration. The fact that the blue color appears much faster than it disappears indicates that the small maximum concentration of the carbocation must result from the fast recombination of the carbocations with the acetate anions. Owing to the involvement of several rate and equilibrium constants (partial dissociation of acetic acid) we were not able to find a kinetic model that fits the resulting plot of the carbocation concentration versus time (see Figure S6 in the Supporting Information). However, the corresponding reaction in the presence of diisopropyl-methylamine ((iPr) 2 NMe), which shows a similar absorbance-time correlation (Figure 1), could be fitted to the kinetic model shown in Scheme 1. The resulting rate constants and the Gepasi [5] fi...

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Kinetics of the Solvolyses of Benzhydryl Derivatives: Basis for the Construction of a Comprehensive Nucleofugality Scale

Cited by 72 publications

References 92 publications

Nucleophilicities and Nucleofugalities of Organic Carbonates

Nucleophilicities and Nucleofugalities of Organic Carbonates

Nucleofugality of Pentafluorophenolate in Various Solvents: Solvolytic Behavior of Phenolates

“Carbocation Watching” in Solvolysis Reactions

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