• Bulletin of the Korean Chemical Society
• /
• v.33 no.3
• /
• pp.1089-1092
• /
• 2012

• Bulletin of the Korean Chemical Society
• /
• v.33 no.3
• /
• pp.1042-1046
• /
• 2012

The kinetics and mechanism of the pyridinolysis ($XC_5H_4N$) of diethyl isothiocyanophosphate are investigated in acetonitrile at $55.0^{\circ}C$. The Hammett and Bronsted plots for substituent X variations in the nucleophiles exhibit the two discrete slopes with a break region between X = 3-Ac and 4-Ac. These are interpreted to indicate a mechanistic change at the break region from a concerted to a stepwise mechanism with rate-limiting expulsion of the isothiocyanate leaving group from a trigonal bipyramidal pentacoordinated intermediate. The relatively large ${\beta}_x$ values with more basic and less basic pyridines imply much greater fraction of frontside nucleophilic attack TSf than that of backside attack TSb.