Diffusion-controlled trapping carbocations

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... 

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 . 

Diffusion-controlled trapping of carbocations benzylic cations... 

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... 

Imidinium ions such as (59), formed from formamidine precursors by C—X bond cleavage, are more reactive than carbocations, being trapped by solvent or nucleophile at not quite diffusion-controlled rates, in general.102 The reaction rates are not... 

---