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

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 <i>k</i><sub>S</sub>/<i>k</i><sub>N</sub> = 2−10 for the reactions of carbocations with SCN<sup>-</sup>, values of <i>k</i><sub>S</sub>/<i>k</i><sub>N</sub> = 10<sup>3</sup>−10<sup>4</sup> 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.