N. E. Ponomarev

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXX.1 Correlation Analysis of Solvation Effects in Heterolysis of tert-Butyl Chloride

Quantitative analysis of the effect of solvent parameters on the rate of heterolysis of tert-butyl chloride was performed; the reaction rate is fairly described by the polarity, polarizability, and electrophilicity parameters or by the ionizing ability parameter, while the nucleophilicity of the solvent has no rate effect. A negative effect of nucleophilic solvation was revealed in protic solvents.

Nature of solvation effects and mechanism of heterolysis of tert-alkyl halides

Specificities of heterolysis of tert-alkyl halides in protic and aprotic solvents were analyzed. Values of log k25 for heterolysis of tert-butyl chloride, tert-butyl bromide, tert-butyl iodiede, 1-chloro-1-methylcyclopentane, 1-chloro-1-methylcyclohexane, 1-bromo-1-methylcyclopentane, 1-bromo-1-methylcyclohexane, 2-chloro-2-phenylpropane, 1-iodoadamantane, and 2-bromo-2-methyladamantane in 19 to 44 solvents, determined mostly by the verdazyl technique were collected. Correlation analysis of solvation effects was performed in terms of multiparameter equations based on the linear free energy relationship principle, as well as in the logk-ET coordinates. The nature of solvation effects and mechanism of heterolysis of a covalent C-Hlg bond were discussed.

Special salt effect in monomolecular heterolysis reactions

Data on the special salt effect in monomolecular heterolysis reactions (Sn1, E1, solvolysis) are summarized and critically analyzed. The mechanisms suggested by Ingold, Winstein, Dannenberg, Okamoto, and the authors are discussed. The special salt effect is due to the effect of a salt on the contact ion pair of a substrate. Quadrupoles and ion triplets are formed. In the limiting step of the heterolysis, a contact ion pair interacts with a solvent cavity. Association of salts with a contact ion pair increases the lifetime of the cationoid and the probability of its contact with the solvent cavity. A spatially separated ion pair is formed, which rapidly transforms into a solvation-separated ion pair, which, also rapidly, yields reaction products.

Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLIII. Solvent effect on activation parameters of dehydrochlorination of 3-chloro-3-methylbut-1-ene. Correlation analysis of solvation effects

The influence of temperature on the rate of dehydrochlorination of 3-chloro-3-methylbut-1-ene in 17 aprotic and 13 protic solvents, ν = k[C5H9Cl], was studied by the verdazyl method. In aprotic solvents, the electrophilicity, ionizing power, and cohesion of solvents decrease ΔG≠ by increasing ΔS≠. The nucleophilicity and polarizability increase both ΔH≠ and ΔS≠ to equal extent and therefore do not affect ΔG≠. In protic solvents, the solvent nucleophilicity increases ΔH≠ to a greater extent than ΔS≠, and the overall effect of the nucleophilic solvation is small and negative.

Isokinetic relationships in unimolecular heterolysis. Mechanism of ionization of covalent bond

Various types of isokinetic (isoparametric) relationships in heterolytic reactions were summarized and critically analyzed. It was presumed that the series of substrate reactivity is reversed after passing the isoparametric point, and the bimolecular reaction mechanism changes to unimolecular: SN2-SN1, SN2-E1, SE2-SE1, SE2-SN1, and SN2(SSIP)-SN2(C+). Three particular cases of isoparametric relationships are discussed: (1) isoentropy (ΔS≠ = const) which reflects formation of contact ion pair; (2) isoenthalpy (ΔH≠ = const) which reflects formation of space-separated ion pair; and (3) isoenergy (ΔG≠ = const), when ΔH≠ = ΔG≠ = ΔEr. The rate of heterolysis in cyclohexane does not depend on the substrate nature, and a universal minimal rate of heterolysis exists, k25 ≈ 10−10 s−1, τ1/2 = 220 years. There is no nucleophilic assistance by the solvent in unimolecular heterolysis.

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXIX. Solvent Effects on the Activation Parameters of Heterolysis of tert-Butyl Chloride

The kinetics of heterolysis of t-BuCl in sulfolane, PhCN, PhNO2, acetophenone, cyclohexanone, chloroform, and 1,2-dichloroethane at 30-50°C were studied by the verdazyl method. Quantitative analysis of the effect of solvent parameters on the ΔG≠, ΔH≠, ΔS≠, and log k25 values for heterolysis of t-BuCl in a set of 15 protic and 16 aprotic solvents and separately in either group of solvents was performed. In the above set of solvents, three ΔH≠-ΔS≠ compensation effects are observed, associated with jump changes in the potential energy of the reaction.

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXVIII. Effect of the Nature of the Verdazyl Indicator on Salt Effects in Dehydrobromination of 3-Bromocyclohexene in Nitrobenzene

The salt effect on the rate of dehydrobromination of 3-bromocyclohexene in PhNO2 depends on the nature of the verdazyl indicator. With triphenylverdazyl and its chloro and nitro derivatives in the presence of Et4NClO4, a normal salt effect is observed, in the presence of bromides, a superposition of normal and special salt effects, while in the presence of chlorides, a superposition of normal and special negative salt effects. With the dimethoxy verdazyl derivative, a normal salt effect is always observed.

Specific Features of Solvation Effects in Monomolecular and Bimolecular Solvolysis

A method was suggested for distinguishing monomolecular and bimolecular solvolysis on the basis of reaction kinetics in water, MeOH, EtOH, i-PrOH, cyclohexanol, and t-BuOH. In solvolysis of n-PrBr, CH2 = CHCH2Br, PhCOCl, and MeOClO3 (SN2 reactions), a linear correlation is observed between logk and the solvent ionizing power Z, whereas in solvolysis of t-BuBr, t-BuCl, and 1-AdI (SN1, E1 reactions) this correlation is nonlinear. Deviations from linearity are due to steric hindrance decreasing the negative effect of nucleophilic solvation.