The verdazyl method for the study of the kinetics and mechanisms of the unimolecular heterolysis reactions of organic compounds

Дворко, Г. Ф.; Ponomareva, E. A.

doi:10.1070/rc1991v060n10abeh001130

Cited by 25 publications

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“…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].…”

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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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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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“…To gain insight into this phenomenon, we studied the effect of LiClO 4 on the heterolysis of Ph 2 CHCl in g-butyrolactone. The kinetic experiments were performed at 35oC by the verdazyl method [8] using as internal indicators triphenylverdazyl radicals Z3Vd . : 1,3,5-triphenylverdazyl (H3Vd .…”

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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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“…This substituent, e.g., an alkyl group, is either present in the precursor or is introduced during the reaction. The accessibility of the initial formazans and the high stability of the verdazyl radicals have attracted and are still attracting the attention of chemists, biologists, and physicists [2][3][4][5][6][7][8][9][10][11][12][13][14][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. New verdazyls [15-17, 19, 33] and their complexes with metals [16, 21-25, 27, 28, 30] and with π-acceptors [26,29,31,32] are regularly being synthesized, new characteristics of known verdazyls are being studied, and new aspects of their application are being sought, as in the creation of linear and helical polymers, organic magnetic materials, semiconductors, luminophores, etc.…”

Section: 234-tetrahydro-1245-tetrazinesmentioning

confidence: 99%

Formazans in the synthesis of heterocycles. II. Synthesis of azines (Review)

Бузыкин

2010

Chem Heterocycl Comp

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The availability of various types of formazans, the prospects for their use in the synthesis of heterocycles, and the principles of the construction of various heterocyclic systems on the basis of the formazan fragment have already been discussed in general features in the first part of our review devoted to the synthesis of azoles on the formazans basis [1]. We will therefore only present a more detailed scheme of the possible paths to the formation of six-membered heterocyclic systems from formazans. As in the case of azoles, the principles of the construction of azine rings are based on the ability of the formazan fragment to enter completely or partially into the heterocycle that forms (Scheme 1) [2][3][4][5][6][7][8][9][10][11]. Most of the described paths (a-g in Scheme 1) involve intramolecular cyclization quite often without isolation of the precursor. There are only two examples of intermolecular [4+2] cycloaddition of formazans as 1,2,4-triazabuta-1,3-dienes with only one type of dienophile (path h) [12], since it has been demonstrated that the formation of 1,4-diaryl-3,6-bis(arylazo)-1,4-dihydro-1,2,4,5-tetrazines from 1,5-diaryl-3-chloroformazans (path i) is a two-step process and can be regarded as a variant of the intramolecular cyclization of path d (Scheme 1).The N-1 atom and one of the atoms of the substituents at the N-5 atom of the formazan fragment are most often involved in the intramolecular cyclization of formazans to azine derivatives according to a [6+0] scheme (paths a-c, described for the C and B atoms). In other cases the formation of the six-membered ring according to the [6+0] scheme results from nucleophilic substitution by a fragment of the meso substituent (from R 3 ) at the N-5 atom of the formazan fragment (paths d-f and, possibly, i). The cyclization of formazans with loss of one of the nitrogen atoms has only been described for cases of the formation of benzo-1,2,4-tria-zines (path h) [2-5, 10, 11].

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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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“…Proceeding with studies of salt effects in the heterolysis of bromide IV, we examined by the verdazyl method [16] . The reaction follows stoichiometric equation (3).…”

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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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The verdazyl method for the study of the kinetics and mechanisms of the unimolecular heterolysis reactions of organic compounds

Cited by 25 publications

References 3 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

Formazans in the synthesis of heterocycles. II. Synthesis of azines (Review)

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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