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Дворко, Г. Ф.; Koshchii, I. V.; Ponomareva, E. A.

doi:10.1023/a:1025640520072

Russian Journal of General Chemistry

2003

DOI: 10.1023/a:1025640520072

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Г. Ф. Дворко

I. V. Koshchii

E. A. Ponomareva

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“…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.…”

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“…(2). At a normal salt effect, a salt catalyzes transformation of a covalent substrate into pair I, and under the conditions of a special salt effect a salt facilitates the ion separation in pair I; the negative special salt effect is associated with the catalysis of the return of the external pair (III3II3I3RX) [6,7].…”

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confidence: 99%

“…The negative salt effect is accounted for by the action of a salt on pair III or II [6,7]. In the first case, the reaction rate should depend on the concentration and structure of the verdazyl indicator, since ZVd .…”

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confidence: 99%

“…and ZVd . act on pair III can transform along two pathways: into the reaction products and back into the covalent substrate [6,7].…”

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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%

“…and ZVd . act on pair III can transform along two pathways: into the reaction products and back into the covalent substrate [6,7].…”

mentioning

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: XXXVIII. Correlation analysis of solvent effects on the activation parameters of heterolysis of t-BuBr and t-BuI

2004

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Heterolysis of t-BuBr and t-BuI in aprotic solvents involves a DH 3DS compensation effect.The DG of t-BuBr heterolysis in aprotic solvents decreases with increasing solvent polarity and cohesion, whereas the respective value for t-BuI heterolysis decreases with increasing solvent polarity, nucleophilicity, and polarizability. In protic solvents, a negative effect of nucleophilic solvation is observed.The correlation analysis of the effects of solvent parameters (ionizing ability, polarity, polarizability, electrophilicity, nucleophilicity, cohesion) on the rates of monomolecular heterolysis reactions (SN1, E1, solvolysis) in terms of many-parameter linear free energy equations has given insight into the nature of solvation effects [237]. Most commonly, solvent effects on log rate constants or, what is the same, activation Gibbs energies are studied. However, these values are strongly temperature-dependent [438]. The activation Gibbs energy is determined by the activation enthalpy and entropy (DG = DH 3 TDS ), which are almost invariable in the experimental temperature range. Studying solvent effects on these activation parameters makes it possible to understand the nature of solvation effects and to reveal the mechanism of transition state solvation [9313]. However, such studies have been few in number.

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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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Untitled

Cited by 5 publications

References 7 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: XXXVIII. Correlation analysis of solvent effects on the activation parameters of heterolysis of t-BuBr and t-BuI

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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