Diffusion control, azide trapping

Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s ), with decreasing stability of the carbocation intermediate, relative to the constant value of az (M s ) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k Now, the nucleophile adduct must form through a... 

Azide ion is a modest leaving group in An + Dn nucleophilic substitution reactions, and at the same time a potent nucleophile for addition to the carbocation reaction intermediate. Consequently, ring-substituted benzaldehyde g m-diazides (X-2-N3) undergo solvolysis in water in reactions that are subject to strong common-ion inhibition by added azide ion from reversible trapping of an o -azido carbocation intermediate (X-2 ) by diffusion controlled addition of azide anion (Scheme... 

A semiquantitative procedure used to estimate the lifetimes of carbocations and oxocarbenium ions by using diffusion-controlled trapping of the cations by nucleophiles . Ions of intermediate stability react with azide ions at a constant, diffusion-controlled rate and react with water by an activated process. The ratio of the products obtained from the azide path and the water path is dependent on the electronic characteristics of the cation. 

An important question is whether nucleophilic substitution at tertiary carbon proceeds though a carbocation intermediate that shows a significant chemical barrier to the addition of solvent and other nucleophiles. The yield of the azide ion substitution product from the reaction of 5-Cl is similar to that observed for the reactions of X-2-Y when this product forms exclusively by conversion of the preassociation complex to product. Therefore the carbocation 5 is too unstable to escape from an aqueous solvation shell and undergo diffusion-controlled trapping by azide ion. This result sets a lower limit of w fcj > -d 1.6 x 10 ° s (Scheme 2.4) " for addition of solvent to the ion pair intermediate 5" C1 . 

Extensive experiments have been carried out on the effect of impurity ions on the kinetics of decomposition, the optical properties, and the temperature dependence of ionic conductivity of several azides in an attempt to determine the nature and concentration of the species in the material. Torkar and colleagues studied the kinetics and conductivity of pure and doped sodium azide [97] and observed that cationic impurities and anionic vacancies speed up decomposition by acting as electron traps which facilitate the formation of nitrogen from N3. They also found that the activation energy for ionic conductivity was close to that for decomposition, implying a diffusion-controlled mechanism of decomposition. These results are qualitatively in accord with the microscopic observations of decomposition made by Secco [25] and Walker et al. [26]. 

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