Carbocations reactivity

The substitution of an a-oxygen by an a-sulfur or an a-selenium results in a decrease in ks/kp for partitioning of oxocarbenium ions between addition of solvent and deprotonation. The largest effect is observed for the a-selenium substitution. These changes are probably the result of several different effects on carbocation reactivity, each of which contributes to the observed substituent effect on kjkp. 

In the presence of solvent alone, the lifetime of the intermediate of the stepwise reaction of X-l-Y in the narrow borderline between the S l and Sn2 substitution reactions of azide ion (—0.32 < a" " < —0.08, Fig. 2.2) is 1/ = 10 ° s. Azide ion is 10°-10 -fold more reactive than water toward triarylmethyl carboca-tions and related electrophiles, and this selectivity is independent of carbocation reactivity, so long as the reactions of both azide ion and solvent are limited by... 

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