Role of nucleophilic solvation and the mechanism of covalent bond heterolysis

Дворко, Г. Ф.; Ponomareva, E. A.; Ponomarev, Mykola E.

doi:10.1002/poc.757

Cited by 32 publications

(40 citation statements)

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“…These radicals rapidly and quantitatively react with the ion pair of substrate III after the limiting step to give the verdazylium salt (Z3Vd + Cl 3 ) and verdazyl alkylation product [21]. The reaction follows the equation…”

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confidence: 99%

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

et al. 2005

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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: XXXIX. Salt Effect of LiClO4in Heterolysis of Ph2CHCl in γ-Butyrolactone

et al. 2005

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“…To successfully compete with traces of water, the activity of triphenylverdazyl in unimolecular reaction with diphenylmethyl chloride should exceed the activity of azide ion in the same reaction by almost two orders of magnitude [2], whereas chloride ion (and hence azide ion) under these conditions combines with carbocationic intermediates derived from diphenylmethyl chloride at a diffusion rate [6]. Furthermore, according to the data of [7,8], the rate constant for external return of the solvent-separated ion pair formed from diphenylmethyl chloride in acetonitrile exceeds 10 8 s !1 .…”

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confidence: 99%

“…(2) The relative contributions of reactions (1) and (2) were estimated [2,9] by kinetic experiments in moist acetonitrile ([H 2 O] = 0.1330.3 M). The calculation procedure was based on the fact that addition of an alkaline reducing agent to reaction mixture promotes quantitative transformation of all products, except for alkylverdazyl, into the initial radical.…”

Section: äääääääääääämentioning

confidence: 99%

“…Dvorko and co-workers [1,2] advanced a hypothesis according to which triphenylverdazyl (I) in aprotic solvents completely traps solvent-separated ion pairs formed by heterolytic dissociation of organic substrates; therefore, it can be used as internal indicator for the rate of formation of ion-pair intermediates. This statement was put as the basis for detailed interpretation [3,4] of extensive experimental data on salt effects in heterolytic reactions; in particular, the authors elucidated the mechanism of [negative special salt effect] and substantiated the occur-ÄÄÄÄÄÄÄÄÄÄÄÄ rence on the reaction coordinate of an additional intermediate, space-separated ion pair.…”

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Is Triphenylverdazyl an Indicator for Solvent-Separated Ion Pair?

et al. 2005

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The hypothesis implying complete capture of solvent-separated ion pairs by triphenylverdazyls used as internal indicators for unimolecular heterolysis of organic compounds was critically analyzed. Products of heterolysis of diphenylmethyl bromide in anhydrous acetonitrile in the presence of triphenylverdazyl were identified, and their distribution was determined. Alkylation of verdazyl under kinetic conditions almost does not occur, and verdazyl is consumed as a result of its reaction with hydrogen bromide liberated during the solvolysis.

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“…, and only the iodide ion, thanks to its large size and high polarizability, preserves its activity. However, in the heterolysis in MeCN of such secondary substrates as Ph 2 CHBr [32] and Ph 2 CHCl [33], when the nucleophilic attack from the rear side is possible, additions of chlorides, bromides, iodides, and nitrates strongly decelerate the reaction, with the salt anion being the acting agent.…”

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confidence: 97%

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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Role of nucleophilic solvation and the mechanism of covalent bond heterolysis

Cited by 32 publications

References 51 publications

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: XXXIX. Salt Effect of LiClO4in Heterolysis of Ph2CHCl in γ-Butyrolactone

Is Triphenylverdazyl an Indicator for Solvent-Separated Ion Pair?

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

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