The 11C/14C incoming group and secondary α-deuterium KIEs and Hammett ρ value found by changing the substituent in the leaving group of the SN2 reactions between meta-chlorobenzyl para-substituted benzenesulfonates and cyanide ion in 0.5% aqueous acetonitrile at 0 °C suggest that these reactions occur via an unsymmetrical, product-like transition state. Changing to a better leaving group leads to a transition state with a slightly shorter nucleophile−α-carbon bond and a longer α-carbon−leaving group bond. The changes in transition state structure are consistent with the Bond Strength Hypothesis.
The primary hydrogen−deuterium incoming nucleophile KIEs for the SN2 reactions between para-substituted benzyl chlorides and borohydride ion in DMSO at 30.000 ± 0.002 °C are small (≤1.14) and insensitive to a change in substituent at the α-carbon. The small Hammett ρ (0.51) and ρr (−0.52) values found when the para substituent on the benzene ring of the substrate is altered indicate there is very little charge on the α-carbon in the transition state. The large, constant secondary α-deuterium KIEs of 1.089 ± 0.002 and the large chlorine leaving group KIEs of 1.0076, 1.0074, and 1.0078 found for the p-methyl-, the p-hydrogen-, and the p-chlorobenzyl chloride reactions suggest that the transition states for these reactions are unsymmetric with short H−Cα and long B−Η and Cα−Cl bonds. The decrease in the chlorine leaving group KIE from 1.0076 ± 0.0003 for the p-methylbenzyl chloride reaction to 1.0036 ± 0.0003 for the p-nitrobenzyl chloride reaction indicates the Cα−Cl bond shortens markedly when a strongly electron-withdrawing substituent is on the α-carbon. Unfortunately, the bond strength hypothesis is the only theory that predicts the changes observed in transition-state structure and it only indicates the bond that changes but not how the transition-state structure is altered.
The effect of inert salts on the structure of the transition state has been determined by measuring the secondary alpha deuterium and the chlorine leaving group kinetic isotope effects for the S(N)2 reaction between n-butyl chloride and thiophenoxide ion in both methanol and DMSO. The smaller secondary alpha deuterium isotope effects and very slightly larger chlorine isotope effects found in both solvents when the inert salt is present suggests that the S(N)2 transition state is tighter and more product-like, with a shorter S-C(alpha) and very a slightly longer C(alpha)-Cl bond when the added salt is present. The salt effect on the reaction in methanol where the reacting nucleophile is the solvent-separated ion-pair complex is much greater than the salt effect on the reaction in DMSO where the reacting nucleophile is the free ion. This greater change in transition-state structure found when the inert salt is present in methanol is consistent with the solvation rule for S(N)2 reactions. The greater change in the S-C(alpha) bond is predicted by the bond strength hypothesis. A rationale for the changes found in transition-state structure when the inert salt is present is suggested for both the free-ion and the ion-pair reactions.
The secondary α deuterium and heavy atom kinetic isotope effects found for two different SN2 reactions suggest that the magnitude of secondary α deuterium kinetic isotope effects can be determined by the length of only the shorter (stronger) reacting bond in an unsymmetrical SN2 transition state rather than by the usual nucleophile−leaving group distance. Although this means the interpretation of these isotope effects is more complex than has been recognized, the results suggest that they can be used to determine whether an SN2 transition state is symmetrical or unsymmetrical.
Chlorine leaving group k35/k37, nucleophile carbon k11/k14, and secondary α-deuterium [(kH/kD)α] kinetic isotope effects (KIEs) have been measured for the SN2 reactions between para-substituted benzyl chlorides and tetrabutylammonium cyanide in tetrahydrofuran at 20 °C to determine whether these isotope effects can be used to determine the substituent effect on the structure of the transition state. The secondary α-deuterium KIEs indicate that the transition states for these reactions are unsymmetric. The theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion; i.e., they suggest that the transition states for these reactions are unsymmetric with a long NC−Cα and reasonably short Cα−Cl bonds. The chlorine isotope effects suggest that these KIEs can be used to determine the substituent effects on transition state structure with the KIE decreasing when a more electron-withdrawing para-substituent is present. This conclusion is supported by theoretical calculations. The nucleophile carbon k11/k14 KIEs for these reactions, however, do not change significantly with substituent and, therefore, do not appear to be useful for determining how the NC−Cα transition-state bond changes with substituent. The theoretical calculations indicate that the NC−Cα bond also shortens as a more electron-withdrawing substituent is placed on the benzene ring of the substrate but that the changes in the NC−Cα transition-state bond with substituent are very small and may not be measurable. The results also show that using leaving group and nucleophile carbon KIEs to determine the substituent effect on transition-state structure is more complicated than previously thought. The implication of using both chlorine leaving group and nucleophile carbon KIEs to determine the substituent effect on transition-state structure is discussed.
In this paper the kinetics and mechanism of polyesterification without using foreignstrong acid as catalyst are presented. It is considered that in the process of reaction theremay exist two kinds of reaction mechanism in the system of polyesterification. Owingto the change of dielectric constant and the viscosity of the system, the relative contents ofthe two mechanisms convert into each other with the change of reaction degree.In thelater period of reaction,hydrolysis of polyester produced by the residue water will beconsidered. The kinetic equation deduced in this paper can fully explain the experimental data ofFlory and Wu et al.
The secondary alpha-deuterium, the secondary beta-deuterium, the chlorine leaving-group, the nucleophile secondary nitrogen, the nucleophile (12)C/(13)C carbon, and the (11)C/(14)C alpha-carbon kinetic isotope effects (KIEs) and activation parameters have been measured for the S(N)2 reaction between tetrabutylammonium cyanide and ethyl chloride in DMSO at 30 degrees C. Then, thirty-nine readily available different theoretical methods, both including and excluding solvent, were used to calculate the structure of the transition state, the activation energy, and the kinetic isotope effects for the reaction. A comparison of the experimental and theoretical results by using semiempirical, ab initio, and density functional theory methods has shown that the density functional methods are most successful in calculating the experimental isotope effects. With two exceptions, including solvent in the calculation does not improve the fit with the experimental KIEs. Finally, none of the transition states and force constants obtained from the theoretical methods was able to predict all six of the KIEs found by experiment. Moreover, none of the calculated transition structures, which are all early and loose, agree with the late (product-like) transition-state structure suggested by interpreting the experimental KIEs.
Direct comparisons of the reactivity and mechanistic pathways for anionic systems in the gas phase and in solution are presented. Rate constants and kinetic isotope effects for the reactions of methyl, ethyl, isopropyl, and tert-butyl iodide with cyanide ion in the gas phase, as well as for the reactions of methyl and ethyl iodide with cyanide ion in several solvents, are reported. In addition to measuring the perdeutero kinetic isotope effect (KIE) for each reaction, the secondary α- and β-deuterium KIEs were determined for the ethyl iodide reaction. Comparisons of experimental results with computational transition states, KIEs, and branching fractions are explored to determine how solvent affects these reactions. The KIEs show that the transition state does not change significantly when the solvent is changed from dimethyl sulfoxide/methanol (a protic solvent) to dimethyl sulfoxide (a strongly polar aprotic solvent) to tetrahydrofuran (a slightly polar aprotic solvent) in the ethyl iodide−cyanide ion SN2 reaction in solution, as the "Solvation Rule for SN2 Reactions" predicts. However, the Solvation Rule fails the ultimate test of predicting gas phase results, where significantly smaller (more inverse) KIEs indicate the existence of a tighter transition state. This result is primarily attributed to the greater electrostatic forces between the partial negative charges on the iodide and cyanide ions and the partial positive charge on the α carbon in the gas phase transition state. Nevertheless, in evaluating the competition between SN2 and E2 processes, the mechanistic results for the solution and gas phase reactions are strikingly similar. The reaction of cyanide ion with ethyl iodide occurs exclusively by an SN2 mechanism in solution and primarily by an SN2 mechanism in the gas phase; only ∼1% of the gas phase reaction is ascribed to an elimination process.
Nucleophile 11C/14C [k11/k14] and secondary α-deuterium [(kH/kD)α] kinetic isotope effects (KIEs) were measured for the SN2 reactions between tetrabutylammonium cyanide and ethyl iodide, bromide, chloride, and tosylate in anhydrous DMSO at 20 °C to determine whether these isotope effects can be used to determine the structure of SN2 transition states. Interpreting the experimental KIEs in the usual fashion (i.e., that a smaller nucleophile KIE indicates the Nu−Cα transition state bond is shorter and a smaller (kH/kD)α is found when the Nu−LG distance in the transition state is shorter) suggests that the transition state is tighter with a slightly shorter NC−Cα bond and a much shorter Cα−LG bond when the substrate has a poorer halogen leaving group. Theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion. The results show that the experimental nucleophile 11C/14C KIEs can be used to determine transition-state structure in different reactions and that the usual method of interpreting these KIEs is correct. The magnitude of the experimental secondary α-deuterium KIE is related to the nucleophile−leaving group distance in the SN2 transition state (RTS) for reactions with a halogen leaving group. Unfortunately, the calculated and experimental (kH/kD)α's change oppositely with leaving group ability. However, the calculated (kH/kD)α's duplicate both the trend in the KIE with leaving group ability and the magnitude of the (kH/kD)α's for the ethyl halide reactions when different scale factors are used for the high and the low energy vibrations. This suggests it is critical that different scaling factors for the low and high energy vibrations be used if one wishes to duplicate experimental (kH/kD)α's. Finally, neither the experimental nor the theoretical secondary α-deuterium KIEs for the ethyl tosylate reaction fit the trend found for the reactions with a halogen leaving group. This presumably is found because of the bulky (sterically hindered) leaving group in the tosylate reaction. From every prospective, the tosylate reaction is too different from the halogen reactions to be compared.
