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Ambident Reactivity of the Thiocyanate Anion Revisited:  Can the Product Ratio Be Explained by the Hard Soft Acid Base Principle?

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posted on 2003-11-19, 00:00 authored by Robert Loos, Shinjiro Kobayashi, Herbert Mayr
Laser flash photolysis and stopped-flow methods have been employed to determine the kinetics of the reactions of benzhydrylium ions with both termini of the thiocyanate ion. In contrast to previous investigations which reported sulfur/nitrogen ratios of kS/kN = 2−10 for the reactions of carbocations with SCN-, values of kS/kN = 103−104 are now derived from absolute rate constants. This discrepancy is explained by the fact that the data determined in this investigation are the first which refer to activation-controlled attack of carbocations at both termini of the thiocyanate ion, while previous reactivity ratios included diffusion-controlled reactions. It is concluded that the selectivities of the reactions of carbocations with the thiocyanate ion cannot be explained by the hard soft acid base principle.

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