Um et al.
toxicity toward mammalian organisms.9,10 Their mode of action
is on the enzyme acetylcholinesterase, involved in the transmis-
sion of nerve impulses, through phosphorylation and subsequent
inactivation. Thus, paraoxon and parathion both act as nerve
agents; hence, their destruction under mild conditions is an
important endeavor.6,9,10
In continuation of these works, we have examined the
selectivity among alkali-metal ions of their effect on the
ethanolysis of ethyl and methyl paraoxon (1a, 1b) and ethyl
We have reported in a series of studies on the catalytic
efficiency of alkali-metal ions on the decomposition of various
OP models, including phosphinate, phosphonate, and phosphate
derivatives, shown below.7,8a These studies have revealed
different interesting selectivity patterns among the alkali-metal
ions.
and methyl parathion (2a, 2b). Our study has revealed an
unusual range of catalysis and inhibition as well as reactivity/
selectivity patterns and contrasting behaviors of the ubiquitous
alkali-metal cations toward PdO and PdS systems.
Results
The ethanolysis of ethyl and methyl paraoxon and ethyl and
methyl parathion was found to proceed through P-OAr bond
fission according to eq 1 with quantitative liberation of
4-nitrophenoxide ion as determined spectrophotometrically.
Selectivity among alkali-metal ions is a feature of a number
of important biological processes, a well-known case being that
of potassium and sodium ions, with high K+ and low Na+
concentrations maintained in mammalian cells by Na+-K+
pumps.11
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First-order kinetics was observed under the reaction conditions
with MOEt (M ) Li, Na, K) concentration in large excess.
Pseudo-first-order rate constants (kobsd) were obtained from plots
of ln(A∞ - At) vs t, which were linear over 90% reaction. It is
estimated from replicate runs that the uncertainty in the kobsd
values is less than (3%. Tables S1 and S2 in Supporting
Information present the kobsd data as a function of [MOEt] (M
) Li, Na, K) for ethyl and methyl paraoxon (1a, 1b) and ethyl
and methyl parathion (2a, 2b), respectively. The kobsd values as
a function of [MOEt] for the reactions of 1a, 1b, 2a, and 2b
are also shown in Figures 1 and 2.
Discussion
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