The reversible consecutive mechanism for the reaction of trinitroanisole with methoxide ion.

Although the competitive mechanism for Meisenheimer complex formation during the reaction of 2,4,6-trinitroanisole with methoxide ion in methanol is generally accepted, no kinetic evidence has been presented to rule out a reversible consecutive mechanism. Simulation of the competitive mechanism revealed that a fractional order in [MeO(-)] is predicted by the latter. Conventional pseudo-first-order analysis of the kinetics resulted in cleanly first-order in [MeO(-)], which rules out the competitive mechanism. The kinetic data are consistent with the reversible consecutive mechanism, which is proposed for this important reaction. An intermediate is required for this mechanism, and we propose that a dianion complex (III) is formed reversibly from the initial 1,3-σ complex (I). The trimethoxy complex (III), the (1)H NMR spectrum of which was observed earlier by Servis (Servis, K. L. J. Am. Chem. Soc. 1965, 87, 5495; 1967, 89, 1508), then eliminates methoxide ion reversibly to form the 1,1-σ complex product (II).

Resolution of the non-steady-state kinetics of the elimination of HBr from 2-(p-nitrophenyl)ethyl bromide in alcohol/alkoxide media.

Non-steady-state kinetic studies reveal that the elimination of HBr from 2-(p-nitrophenyl)ethyl bromide in alcohol/alkoxide media, the classical concerted E2 reaction, actually takes place by a two-step mechanism involving the intermediate formation of the carbanion.

Non-steady-state kinetic study of the SN2 reaction between p-nitrophenoxide ion and methyl iodide in aprotic solvents containing water. Evidence for a 2-step mechanism.

Non-steady-state kinetic studies reveal that the SN2 reaction between p-nitrophenoxide ion and methyl iodide in acetonitrile containing water follows a 2-step mechanism involving the formation of a kinetically significant intermediate.

Hydride-exchange reactions between NADH and NAD+ model compounds under non-steady-state conditions. Apparent and real kinetic isotope effects.

The kinetics of the hydride exchange reaction between NADH model compound 10-methyl-9,10-dihydroacridine (MAH) and 1-benzyl-3-cyanoquinolinium (BQCN+) ion in acetonitrile were studied at temperatures ranging from 291 to 325 K. The extent of reaction-time profiles during the first half-lives are compared with theoretical data for the simple single-step mechanism and a 2-step mechanism involving initial donor/acceptor complex formation followed by unimolecular hydride transfer. The profiles for the reactions of MAH deviate significantly from those expected for the simple single-step mechanism with the deviation increasing with increasing temperature. The deviation from simple mechanism behavior is much less pronounced for the reactions of 10-methyl-9,10-dihydroacridine-10,10-d2 (MAD) which gives rise to extent of reaction dependent apparent kinetic isotope effects (KIEapp). Excellent fits of the experimental extent of reaction-time profiles with theoretical data for the 2-step mechanism, in the pre-steady-state time period, were observed in all cases. Resolution of the kinetics of the hydride exchange reaction into the microscopic rate constants over the entire temperature range resulted in real kinetic isotope effects for the hydride transfer step ranging from 40 (291 K) to 8.2 (325 K). That the reaction involves significant hydride tunnelling was verified by the magnitudes of the Arrhenius parameters; Ea D - EaH = 8.7 kcal mol-1 and AD/AH = 8 x 10(4). An electron donor acceptor complex (lambda max = 526 nm) was observed to be a reaction intermediate. Theoretical extent of reaction-time profile data are discussed for the case where a reaction intermediate is formed in a non-productive side equilibrium as compared to the case where it is a real intermediate on the reaction coordinate between reactants and products. The common assumption that the two cases are kinetically indistinguishable is shown to be incorrect.

Resolution of the non-steady-state kinetics of the two-step mechanism for the Diels-Alder reaction between anthracene and tetracyanoethylene in acetonitrile.

The Diels-Alder reaction between anthracene and tetracyanoethylene in acetonitrile does not reach a steady-state during the first half-life. The reaction follows the reversible consecutive second-order mechanism accompanied by the formation of a kinetically significant intermediate. The experimental observations consistent with this mechanism include extent of reaction-time profiles which deviate markedly from those expected for the irreversible second-order mechanism and initial pseudo first-order rate constants which differ significantly from those measured at longer times. It is concluded that the reaction intermediate giving rise to these deviations cannot be the charge-transfer (CT) complex, which is formed during the time of mixing, but rather a more intimate complex with a geometry favorable to the formation of the Diels-Alder adduct. The kinetics of the reaction were resolved into the microscopic rate constants for the individual steps. The rate constants, as shown in equation 1, at 293 K were observed to be 5.46 M(-)(1) s(-)(1) (k(f)), 14.8 s(-)(1) (k(b)), and 12.4 s(-)(1) (k(p)). Concentration profiles calculated under all conditions show that intermediate concentrations increase to maximum values early in the reaction and then continually decay during the first half-life. It is concluded that the charge-transfer complex may be an intermediate preceding the formation of the reactant complex, but due to its rapid formation and dissociation it is not detected by the kinetic measurements.