The Rate-Product Criterion for Carbonium Ion Intermediates

As befits their status as compounds well-known to be in equilibrium with carbonium ions in suitable solvents, triphenylmethyl halides and related compounds give particularly unambiguous evidence of reaction involving ionic intermediates. In polar solvents they give 

The rate of the reaction is the same in the solvent alone as it is in the presence of sodium fluoride (up to 3 x 10-3 M), sodium hydroxide (up to 5 x 10-3 M), or sodium azide (up to 3 x 10-3 M). Yet sodium azide diverts the product from carbinol to a 90% yield of trityl azide even when sodium azide is present in as low a concentration as 1.3 X 10-3 M. 

The addition of chloride ion to the medium in the case of the trityl chloride reaction reduces the net rate by virtue of the back reaction to regenerate trityl chloride. The product composition in the presence of various added reagents is a quantitative measure of the relative reactivity of the added reagents towards the carbonium ion. The results are summarized in Table VI. 

Relative Reactivities of Various Reagents toward Carbonium Ions214 

Nucleophile Trityl benzhydryl Benzhydryl tertiary Butyl 

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