Unimolecular Reaction Mechanism Research Articles

Structural and mechanistic studies of co-ordination compounds. Part II. Preparation, aquation, and base hydrolysis of trans-chloronitro- and trans-bromonitro-tetra-amminecobalt(III) cations

The preparation and characterization of trans-[Co(NH3)4NO2Cl]+ and trans-[Co(NH3)4NO2Br]+ cations are described. The assignment of geometric configuration is based on 1H n.m.r. spectroscopy. The kinetics of the aquation and base hydrolysis of these two complex cations have been studied over a range of temperature. The first-order aquation rate constants are greater than those of the corresponding bis(ethylenediamine) and 1,4,8,11-tetra-azacyclotetradecane complexes, whereas the second-order base hydrolysis rate constants are less. These results are consistent with the kinetic nephelauxetic effect of these amine ligands on the central cobalt(III) ion. Unimolecular reaction mechanisms are assigned to these reactions.

A Solution of the LindemannMechanism of Unimolecular Reactions

A Solution of the L<scp>indemann</scp>Mechanism of Unimolecular Reactions

THE ACID HYDROLYSIS OF GLYCOSIDES: I. GENERAL CONDITIONS AND THE EFFECT OF THE NATURE OF THE AGLYCONE

First-order rate coefficients and energies and entropies of activation have been determined for the acid-catalyzed hydrolysis of a number of methyl D-glycopyranosides and disaccharides. The relation between the logarithm of the rate coefficients and values for Hammett's acidity function was linear, although different for different acids. All compounds had entropies of activation indicating a unimolecular reaction mechanism. Glucosides of tertiary alcohols were hydrolyzed very rapidly, triethylmethyl β-D-glucopyranoside, for example, 30,000 times taster than the corresponding methyl compound.Increase in size of the aglycone caused a slight increase in the rate of hydrolysis of β-D-glucopyranosides, steric hindrance thus being of no significance. Electron-attracting substituents in the aglycone had little or no influence on the rate of hydrolysis, obviously because they would tend to lower the equilibrium concentration of the conjugate acid, while facilitating the subsequent heterolysis, the two opposing effects more or less cancelling out. These results were discussed in connection with recent studies on the acid hydrolysis of various phenyl glycopyranosides and with reference to the postulated occurrence of an activating inductive effect in oligo- and poly-saccharides containing carboxyl or other electronegative groups at C-5. It was concluded that there is little evidence for the existence of any such effect and that, for example, pseudoaldobiouronic acids should be hydrolyzed at the same rate as corresponding neutral disaccharides.

The Mechanism of Unimolecular Reactions

The general theory of unimolecular reactions is re-examined to determine what type of information about rate constants for specific activated states may be determined from the experimental reaction rate-pressure curves. It is found that the possibilities in this respect are very limited. Contrary to a recent statement by Johnston, data on decomposition of nitrogen pentoxide in the presence of nitric oxide are adequately fitted by the Kassel theory. Data on decomposition of nitrous oxide, however, cannot be fitted by this theory. Failure of the theory in this case is the result of too small a range of decomposition rate constants for various activated states.

On the Mechanism of Unimolecular Reactions.

On the Mechanism of Unimolecular Reactions.