CONCERTED MECHANISM OF THE REACTIONS OF SECONDARY ALICYCLIC AMINES WITH O-ETHYL S-(2,4,6-TRINITROPHENYL) THIOCARBONATE

The title reactions are subjected to a kinetic study in water, 25.0 de grees C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first- order coefficients (k(obsd)) are obtained. Plots of k(obsd) vs free am ine concentration at constant pH are linear, with the slope (k(N)) ind ependent of pH. The Bronsted-type plot (log k(N) vs pK(a) of the amine ) found is linear, with slope beta=0.48, indicating a concerted mechan ism. If the present reactions were stepwise, the hypothetical pK(a) va lue at the Bronsted break (pK(a)(0)) would be 7;9, which is within the pK(a) range of the amines (6.4-11.5). The fact that the Bronsted brea k is not observed confirms that the present reactions are concerted. W ith the data for the present reactions together with those for the sam e aminolysis of O-ethyl S-(2,4-dinitrophenyl) thiocarbonate, an equati on relating log k(N) with the pK(a) of both the amine and the leaving group of the substrate is deduced. Comparison of the present reactions with similar aminolyses show that a tetrahedral addition intermediate is kinetically destabilized by the changes of the following groups at tached to its central carbon: ii) methyl by ethoxy, (ii) S- by O-, and (iii) pyridino by an isobasic secondary alicyclic amino group.