Showing papers on "Transition state published in 1969"

Mutarotation of sugars in solution. II. Catalytic processes, isotope effects, reaction mechanisms, and biochemical aspects.

Abstract: Publisher Summary This chapter describes the mutarotations of α and β-D-glucose. The mutarotations of all reducing sugars are catalyzed by acids and bases. Differences in the ionization for α and β-D-glucose and possible differences in the transition states for the anomers generally make detailed investigations of the individual velocity constants k1 and k2 desirable. The chapter focuses on the velocity constant k1 + k2. The concerted mechanism is of no significance in the mutarotation of sugars in aqueous solution with the possible exception of reactions catalyzed by the water molecule. However, in solvents of low dielectric constant, the formation of ionic intermediates becomes less favored, and the concerted mechanism can be applied. In the concerted process, both the acid catalyst and the base catalyst take part in the transition state, with addition of a proton at one point in the molecule and with elimination of a proton at another point. However, in a stepwise process, the acid catalyst and the base catalyst act separately.

The kinetics of the gas phase intramolecular elimination of isobutene from 2-d1-triisobutylaluminum and the deuterium isotope effect in reactions involving cyclic transition states

Abstract: The intramolecular elimination of isobutene from 2-d1-triisobutylaluminum has been studied in the gas phase for temperatures ranging between 102.4 and 184.6°C. The reaction is apparently homogeneous and obeys the first order rate law, yielding the following Arrhenius relationship: Excess ethylene was added to the starting material in order to avoid complications from the backreaction. The cyclic 4-center nature of the transition state proposed earlier has been unequivocally demonstrated by deuterium labelling. Mass-spectral analyses show that the isobutene formed contains no deuterium. The hydrolyses products of the mixed trialkylaluminum formed during the reaction consist of monodeuteroethane and 2-d1-isobutane. The observed negative entropy of activation of ∼12 cal/°-mole agrees with prediction and implies a reasonably tight transition state structure. Combined with the corresponding data for the non deuterized Al(i-bu)3 reported earlier, these data result in a primary kinetic deuterium isotope effect of kH/kD = 1.3 × 100.6/θ corresponding to a ratio of the isotopic rate constants of 3.7 at 25°C. This result is in excellent agreement with a predicted value of 1.4 × 100.7/θ and it is in line with literature data on similar reactions involving cyclic transition state complexes.

Solvent effects on the free energies of the reactants and transition states in the Menschutkin reaction of trimethylamine with alkyl halides

Abstract: In the reaction of trimethylamine with p-nitrobenzyl chloride, the variation of the free energy of the transition state with solvent is generally larger than the corresponding variation in the sum of the free energies of the reactants, but in the reaction of trimethylamine with methyl iodide the accelerating effect of solvent water is entirely due to a large increase in the free energy of the reactants.

The Role of Vibrational, Excitation in the Formation of 4‐Center Transition States

Abstract: The experimental results describing the kinetics of the interaction between hydrogen and deuterium obtained by Farkas and Farkas are discussed in the light of the recently derived rate constants of the reactions: H2 + M H + H + M and H + D2 HD + D. It is suggested that in the early study of this exchange reaction, the dissociation of hydrogen occurred under heterogeneous conditions, so that the conclusions regarding the relative rates of the various steps in the exchange scheme are not entirely correct. A technique which provides purely homogeneous conditions at high temperatures is briefly described, and recent experimental results obtained in various exchange studies are presented. It is shown that under homogeneous conditions, at the temperature range over which exchange readily occurs, dissociation processes are slow, and the free radicals are present in too small concentrations for a free radical mechanism to contribute to the observed rate of exchange. The experimental results suggest that exchange occurs via a 4-center intermediate, but not as a result of a hard collision between the exchanging molecules. A reaction may occur only if one of the molecules is excited to a critical vibrational state before colliding with its exchange partner. In a few diatomic molecules where the efficiency of translation vibration energy exchange is not very high, the rate of vibrational excitation determines the rate of exchange. For one typical study, an evaluation of the rate of vibrational excitation from available data on translation vibration transition probabilities is described. The agreement between the calculated and the experimental rates is excellent.

Investigation of the transition state in the diene condensation of trans-piperylene with methyl acrylate

Abstract: 1. The influence of pressure and temperature on the isomeric composition of adducts of diene condensation of trans-piperylene with methyl acrylate was investigated. The cis-transition states possess lower values of the volume, entropy, and energy in comparison with the corresponding trans-states; the ortho-isomsrs are characterized by lower values of the entropy and energy of the transition states in comparison with the meta-isomers. 2. The differences in the volumes and entropies of the transition states were used to consider the structure of the latter; the results obtained agree with a cyclic structure of the transition states. It was concluded that the transition state apparently has a structure closer to the “plane parallel” model of the prereaction complex than to the structure of the adduct. 3. It was concluded that the determining influence on the structural and steric direction of the reaction is exerted by electronic, rather than steric factors.