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

Abstract

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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Synthesis and Polymerization of Renewable 1,3‐Cyclohexadiene Using Metathesis, Isomerization, and Cascade Reactions with Late‐metal Catalysts

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