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 k(S)/k(N) = 2-10 for the reactions of carbocations with SCN(-), values of k(S)/k(N) = 10(3)-10(4) 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.
Loos
Robert
R
Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), D-81377 Münich, Germany.
Kobayashi
Shinjiro
S
Mayr
Herbert
H