Experimental and Theoretical Evaluation of Energetics for Nucleophilic Solvent Participation in the Solvolysis of Tertiary Alkyl Chlorides on the Basis of Gas Phase Bridgehead Carbocation Stabilities

Abstract

The specific rates of solvolysis in 80% ethanol and 97% 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) for various tertiary alkyl chlorides having different steric requirement and experimental (FT ICR) gas-phase stabilities of the corresponding carbocations were determined. The experimental gas-phase stabilities were in good agreement with theoretical values computed at the MP2/6-311G(d,p) or the MP2/6-311G(d,p)//MP2/ 6-31G(d) level. The relation of differential activation Gibbs energy changes for solvolysis [δ∆G ‡ ) -RT ln(k/k o )] (relative to 1-chloroadamantane) vs the experimental gas-phase cation-stabilities ∆G°(relative to 1-adamantyl cation) was compared with the previously established similar relation for bridgehead systems. It was revealed that the solvolysis of tert-butyl chloride in 80% ethanol is nucleophilically assisted by 4-8 kcal mol -1 . The δ∆G ‡ vs ∆G°relation for heavily crowded 4-chloro-2,2,4,6,6-pentamethylheptane was found to be comparable to that of bridgehead compounds. The reversal of the ranking of stabilities of the tert-butyl cation and 1-adamantyl cation on going from the gas phase to aqueous solution was computationally assessed. The results agree with the fact that larger substituents around a cationic center increase the stability of the ion in the gas phase thanks to their larger polarizability, and that this effect is either offset or even dramatically reversed in solution.

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

“…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 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 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 electrophilically pair A.…”

Concurrent Phenomena at the Reaction Path of the SN2 Reaction CH3Cl + F−. Information Planes and Statistical Complexity Analysis

“…The double-well feature for S N 2 reactions has been the subject of a great deal of research on the theoretical side by use of powerful computers coupled with modern techniques of molecular electronic structure. Recently gas-phase S N 2 reactions have been widely investigated by kinetic experiments [3][4][5], ab initio quantum and semiclassical dynamical methods and trajectory simulations [6,7], statistical mechanical studies [8,9], ab-initio and density functional structural analyses [10][11][12][13][14]. The importance of S N 2 studies in the chemical literature has made these reactions to constitute a paradigm for quantitative understandings of ion molecule reactions in general [15].…”