para- and ortho-Pyridinium aldoximes in reaction with acetylthiocholine

In the oximolysis reaction para-aldoximes K027 and TMB-4 react faster with ATCh than ortho-aldoximes HI-6 and K033. The reaction rate constants at 25 °C were 22 M−1 min−1 for HI-6 and K033, 230 M−1 min−1 for TMB-4 and 306 M−1 min−1 for K027. Semi-empirical calculations showed that differences in rates do not origin from different electron density on the oxygen of the oxime group, but can be explained by the steric hindrance of the oxime group within the molecule. Thermodynamic parameters, ΔG#, ΔH# and ΔS#, were also determined for oximolysis reaction.