Characteristic features of the change in reactivity of supernucleophilic functional surfactants in acyl group transfer processes

Abstract

We consider the factors responsible for the nucleophilicity and micellar effects of surfactants based on imidazole and pyridine, functionalized by an oximate group. The reactivity of the functional detergents, as for oximes not forming micelles, is described by a nonlinear Brönsted plot with an inflection point at the pK a of the oximate group,~8.5-9.0. The major contribution to the increase in the degradation rate of ecotoxins by supernucleophilic systems based on functional surfactants (by a factor of 10 2 to 10 3 compared with the methyl analogs) comes from the effect of concentration of the substrate. The established characteristics make possible targeted modification of the surfactant structure and obtaining detergents with a specified reactivity level.Design of reagents for fast and irreversible degradation of ecotoxins, including organophosphorus compounds (OPCs), requires designing systems having both high nucleophilicity and high solubilizing ability relative to hydrophobic substrates. Using a-nucleophiles as the basis for such systems makes it possible to provide anomalously high rates of nucleophilic cleavage of organophosphorus compounds [1][2][3]. The special interest in study of the reactivity of typical representatives of this class of reagents, oximate ions (Ox -), is first of all due to the fact that effective antidotes are found among them: re-activators of the acetylcholinesterase inhibited by organophosphorus compounds, the search for which is being vigorously pursued at the moment [4]. There is no doubt that the level of biological activity of the antidotes is directly connected with the high reactivity of Ox -ions [4][5][6][7]. Detailed kinetic analysis of their behavior suggests that the nucleophilicity of Ox -ions cannot be described in terms of a single Brönsted equation [4][5][6][7]. Curvature of the Brönsted plots for reactions of Ox -ions with substrates containing electron-deficient centers and the "leveling off" of the reactivity at p a Ox K -> 8.5-9.0 may be connected with both energetically unfavorable solvation effects of the solvent, the contribution of which becomes more and more significant as the basicity of the nucleophile increases, and also with a change in the structure of the transition state [4][5][6][7]. Nevertheless, for the studied reaction series, it is unlikely that such a considerable change in the structure of the transition state would occur as the basicity of the Oxions increases [7].The similar character of the Brönsted plots for interaction of Ox -ions with different acyl-containing substrates clearly shows that it is hardly possible to modify the structure and synthesize an oxime whose nucleophilicity in aqueous solution 94 0040-5760/10/4602-0094

1-Methyl-3-hexadecyl-2-(oximinomethyl)imidazolium bromide as a new highly efficient, low-basicity reagent for the decomposition of acyl-containing ecotoxicants

“…Analysis of the reactivity data for IIb and IIIb (see Table 1) shows that the points for both these compounds are described satisfactorily by Brönsted equations for non-micelle-forming oximes (Fig. 1a) [3,7], which indicates the predominant effect of basicity on their nucleophilicity and the lack of any special effect of the heterocyclic ring or a "charge effect" [3,7,8].…”

“…However, our experimental data given in Table 1 showed that IIIa has from about five times greater nucleophilicity than IV (NPDEPS, NPDEP) to about ten times more nucleophilicity (NPTS) and is not less reactive than highly basic detergents I (Fig. 1b) [3]. (3) Such a significant difference in reactivity of IIIa and IV may result from differences in the position of the functional group of these compounds relative to micelle surface.…”

“…The reactivity of non-micelle-forming oximes IIb and IIIb was studied in accord with our previous work [3,4,6,7] and described by the following equation:…”

“…In light of this, the prospects for creating detergent IIIa, in which a decrease in basicity by~1.5-2 pK a units relative to IIa is produced by the positive charge on the imidazolium ring, would appear much more attractive [3,4,6,7].…”

“…A quantitative description of the decomposition of the acyl substrates by IIIa/CTAB comicelles was carried out using the pseudophase distributive model The previously established relationship between the basicity of the functional surfactants and their nucleophilicity [3] would suggest a level of reactivity of IIIa close to the level found for functional detergent IV, whose pK a is 7.92 ± 0.06 (sp. m) [6].…”