Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XLI. Solvent Effect on the Rate of 3-Methyl-3-chloro-1-butene Solvolysis. Correlation Analysis of Solvation Effects and Role of Solvent Nucleophilicity

Abstract

The kinetics of 3-methyl-3-chloro-1-butene solvolysis at 25oC in MeOH, EtOH, BuOH, PentOH, HexOH, OctOH, cyclohexanol, and allyl alcohol was studied by the verdazyl method; v = k[C 5 H 9 Cl], SN1 + E1 mechanism. The reaction rate shows a satisfactory correlation with the parameter of the solvent ionizing power E T and is independent of the solvent nucleophilicity.Studies of the solvent effect on the rate of monomolecular heterolysis (SN1, E1, solvolysis) are usually performed with such weakly active reference substrates as t-BuCl and 1-AdCl [2,3]. In such a polar solvent as MeOH, the conversion half-time t 1/2 at 25oC is~21 h for t-BuCl and~150 years for 1-AdCl; in dipolar aprotic MeCN, t 1/2 of t-BuCl is about 5 years [3,4]. Therefore, the kinetics of the heterolysis of t-BuCl in aprotic solvents is studied insufficiently, and for 1-AdCl data are few for protic solvents and lacking at all for aprotic solvents.Monomolecular heterolysis involves successive formation of three ion pairs: contact (A), spacially separated (B), and solvation-separated (C) [5]: RX $ " R + ...X ! $ " R + ³Solv³X ! 76 Reaction products. AB CIn the limiting step, pair A interacts with a solvent cavity (cavities occupy about 10% of the volume of a liquid [6]) to form pair B, which rapidly transforms into pair C and then, also rapidly, into the reaction products.The effect of the solvent nucleophilicity on the rates of these reactions was discussed in numerous papers [335,7,8]. It is usually believed that, in heterolysis of adamantyl substrate, the solvent renders no nucleophilic assistance, because the nucleophilic solvation from the rear side in this case is impossible ÄÄÄÄÄÄÄÄÄÄÄÄ 1 For communication XL, see [1].because of steric factors; in heterolysis of tert-butyl halides, weak nucleophilic assistance is assumed, although the nucleophilic solvation from the rear side is sterically hindered [3,7,8]. A study of the solvation effects in heterolysis of adamantyl and tert-butyl substrates by the verdazyl method [9] revealed no nucleophilic assistance of the solvent in both cases [5]. The nucleophilic solvation of tertiary substrates occurs in the step of formation of pair A formed before the limiting step. It stabilizes the intermediate and complicates removal of the nucleofuge by the SN1 mechanism. The reaction rate decreases, and with adamantyl substrates this effect is stronger than with tert-butyl substrates. Since only the relative effect of the solvent nucleophilicity was determined, it was erroneously concluded that a solvent renders nucleophilic assistance in the heterolysis of tert-butyl substrates [4,7,8].The negative effect of the nucleophilic solvation is observed only in the heterolysis of tertiary substrates; it is indicative of the nonequilibrium solvation of the transition state when the solvation shells of the ground and excited states have different structures. This effect is usually pronounced in protic solvents, which is apparently associated with hydrogen bonding between molecules solvating nucleophilically and electr...

Nucleophilic Aliphatic Substitution

“…84 The weak negative correlation with solvent nucleophilicity and the first-order kinetics was taken as evidence that the reaction occurs via an S N 1 mechanism.…”