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Ponomarev, N. E.; Stambirskii, M. V.; Дворко, Г. Ф.

doi:10.1023/a:1015349429876

Russian Journal of General Chemistry

2002

DOI: 10.1023/a:1015349429876

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N. E. Ponomarev

M. V. Stambirskii

Г. Ф. Дворко

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“…In the previous studies of this series [14,15], we examined by the verdazyl method [16] the effect of various salts on the rate of heterolysis of 3-bromocyclohexene IV in PhNO 2 . As internal triphenylverdazyl indicators ZVd .…”

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“…Data obtained in PhNO 2 [14,15] show that a salt can interact simultaneously with both the covalent substrate and various cationoid intermediates; the intensity of this interaction is determined by the principle of hard and soft acids and bases [12]. The polyfunctonal effect of a salt is revealed in some cases when studying the normal salt effect.…”

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“…In going from MeOH to toluene, the rate of the heterolysis of IV decreases by 5 orders of magnitude [29]. In dipolar aprotic solvents, various solvates can form, which react at different rates and differently interact with salts.…”

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“…The rate of the heterolysis of both 3-bromocyclohexene and diphenylbromomethane depends on the polarity and electrophilicity of the solvent, or on its ionizing power [29,34]; the solvent effect is considerably stronger with Ph 2 CHBr. For 9 protic and 14 aprotic solvents, the following correlation was obtained: log k IV = 1.71 + 0.719log k Ph 2 CHBr , R 0.989, S 0.24, n 23.…”

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“…rolactone [14,15,22,35,36] are listed in the table. In all the solvents, the rate of the heterolysis of Ph 2 CHBr is slightly lower than that of IV, although the activation enthalpy in heterolysis of Ph 2 CHBr is always lower than that of IV.…”

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Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XL. Nature of Salt Effects in Dehydrobromination of 3-Bromocyclohexene in γ-Butyrolactone. Role of Solvation Effects of Dipolar Aprotic Solvents

2005

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The influence of neutral salts on the rate of heterolysis of 3-bromocyclohexene at 31oC in g-butyrolactone was studied by the verdazyl method; v = k[C 6 H 9 Br], E1 mechanism. Additions of lithium picrate do not affect the reaction rate; those of LiClO 4 and Et 4 NClO 4 increase it; and those of LiCl, Et 4 NCl, and KNCS decelerate the reaction. The nature of salt and solvation effects in the heterolysis of 3-bromocyclohexene in g-butyrolactone, MeCN, and PhNO 2 is discussed. The rate of monomolecular heterolysis (SN1, E1, solvolysis) is controlled by the ionization of a covalent bond, occurring via successive formation of three ion pairs: contact I, spacially separated II, and solvation-separated III [2]: RX 76 47 R+X ! 76 47 R + ...X ! 76 47 R + |Solv|X ! I II III 76 Reaction products.(1)In the limiting step, pair I interacts with a solvent cavity (cavities account for~10% of the volume of a liquid [3]). Ion pair II is thus formed, which rapidly transforms into ion pair III which, in turn, rapidly yields the reaction products.Neutral salts (ammonium or alkali metal salts) affect the heterolysis rate strongly and specifically [4 37]. The nature and intensity of the salt effect depend on the substrate structure, salt, and solvent. The observed diversity of the salt effects is due to the fact that an ion pair of a salt or its ions can interact with a covalent substrate or various ion-pair intermediates [6,7].When a salt acts on a covalent substrate, the reaction rate linearly grows with the salt concentration (normal salt effect). The value of this effect (b, l mol !1 ) is calculated by Winstein equation (2) [8], where k s and k are the rate constants of the reaction in the presence of a salt and without it.ÄÄÄÄÄÄÄÄÄÄÄÄ 1 For communication XXXIX, see [1].The linear character of the normal salt effect is due to the low constant of association of a salt with a covalent substrate; the parameter b reflects this association constant.When a salt acts on pair I, the reaction rate first sharply increases, but further additions of the salt do not longer affect the reaction rate, and dk/d[salt] = 0 (special salt effect). The association constant of the salt with pair I is high; therefore, even at low concentrations of the additive (10 !2 310 !4 M), the ion pair is virtually fully bound by the salt to form a quadrupole or a ternary ionic associate, and further additions of the salt exert no effect on the reaction rate.Usually a superposition of the normal and special salt effects is observed: The reaction rate first sharply increases at low salt concentrations, and then the increase becomes more gradual. Extrapolation of the linear portion of the curve to [salt] = 0 gives the maximal value of the special salt effect (k ext ). The parameters b 1 and b 2 characterizing the association of a salt with pair I and covalent substrate are calculated from the first and second portions of the curve by Eq. (2).Usually b 1 : b 2 ; 10 310 2 .When a salt acts on intermediates formed after the limiting step (pairs II , III), the reaction ra...

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confidence: 99%

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confidence: 99%

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confidence: 99%

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confidence: 99%

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confidence: 99%

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Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XL. Nature of Salt Effects in Dehydrobromination of 3-Bromocyclohexene in γ-Butyrolactone. Role of Solvation Effects of Dipolar Aprotic Solvents

2005

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Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXIX. Salt Effect of LiClO4in Heterolysis of Ph2CHCl in γ-Butyrolactone

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Additions of LiClO 4 accelerate the heterolysis of Ph 2 CHCl in g-butyrolactone; v = k[Ph 2 CHCl], SN1 mechanism. The salt effect increases with an increase in the electron-acceptor properties of the verdazyl indicator. A superposition of three salt effects (normal, special, and negative special) is observed.Neutral (ammonium and alkali metal) salts strongly and specifically affect the rate of monomolecular heterolysis reactions (SN1, E1, solvolysis) [23 4]. The rate of these reactions is controlled by ionization of the covalent bond, which occurs via successive formation of three ion pairs: contact (I), spacially separated (II), and solvation-separated (III) [5] R3X 76 47 R + X 3 76 47 R + ...X 3 76 47 R + 9Solv9X 3 I II III 76 Reaction products.In the limiting step, ion pair I interacts with a solvent cavity (cavities occupy~10% of the liquid volume [6]). Ion pair II is formed, which rapidly transforms into ion pair III, which, in turn, rapidly yields the reaction products. The free carbocation is formed only in exceptional cases [5], usually in water or in mixtures with a high water content [7].The rate of monomolecular heterolysis is independent of the nucleophile concentration and is described by a first-order kinetic equation v = k [RX].Salts [M + Y 3 ] can both increase and decrease the reaction rate (Fig. 1). The salt effect depends on particular species (cationoid intermediate or covalent substrate) subject to the action of the salt ions or ion pairs. A study of the kinetics and mechanism of monomolecular heterolysis using the verdazyl method [8] showed that, when a salt acts on the covalent substrate, the reaction rate linearly increases with the salt concentration (normal salt effect, Fig. 1, line 1); when ÄÄÄÄÄÄÄÄÄÄÄÄ 1 For communication XXXVIII, see [1].a salt acts on ion pair I of the substrate, the reaction rate initially sharply increases and then flattens out (dk/d[salt] = 0, special salt effect, curve 2). In the first case, the salt catalyzes formation of ion pair I, and in the second case it facilitates formation of ion pair II. Usually a superposition of the normal and special salt effects is observed: The reaction rate initially sharply grows with increasing salt concentration, and then the dependence becomes linear with a smaller slope (curve 3) [3]. When a salt acts on ion pair III or II, the reaction rate first sharply decreases with an increase in the salt concentration and then flattens out (dk/d[salt] = 0, negative special salt effect, curve 4). In this case, the salt catalyzes the external return of 3 k 2 1 4 [Salt] k 0 extr k extr Fig. 1. Salt effect on the heterolysis rate: (1) normal salt effect, (2) special salt effect, (3) superposition of normal and special salt effects, and (4) negative special salt effect; (k extr , k 0 extr ) extrapolated values.

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Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XLII. Effect of Aprotic Solvent on the Rate of 3-Methyl-3-chloro-1-butene Dehydrochlorination. Correlation Analysis of Solvation Effects

2005

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The kinetics of 3-methyl-3-chloro-1-butene dehydrochlorination in propylene carbonate, g-butyrolactone, sulfolane, acetone, MeCN, PhNO 2 , PhCN, PhCOMe, MeCOEt, cyclohexanone, o-dichlorobenzene, PhCl, PhBr, 1,2-dichloroethane, dioxane, and AcOEt were studied; v = k[C 5 H 9 Cl], E1 mechanism. The reaction rate is satisfactorily described by the parameters of the polarity, electrophilicity, and cohesion of the solvent; the solvent nucleophilicity and polarizability exert no effect on the reaction rate.

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Untitled

Cited by 5 publications

References 8 publications

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XL. Nature of Salt Effects in Dehydrobromination of 3-Bromocyclohexene in γ-Butyrolactone. Role of Solvation Effects of Dipolar Aprotic Solvents

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XL. Nature of Salt Effects in Dehydrobromination of 3-Bromocyclohexene in γ-Butyrolactone. Role of Solvation Effects of Dipolar Aprotic Solvents

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXIX. Salt Effect of LiClO4in Heterolysis of Ph2CHCl in γ-Butyrolactone

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XLII. Effect of Aprotic Solvent on the Rate of 3-Methyl-3-chloro-1-butene Dehydrochlorination. Correlation Analysis of Solvation Effects

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