Structure of the head group, nucleophilicity, and micellar effects of functional detergents in acyl transfer reactions

Abstract

The micellar effects of 1-cetyl-2-methyl-3-(2-hydroxyiminopropyl)imidazolium and 1-cetyl-3-hydroxyiminomethylpyridinium halides in acyl transfer reactions (phosphoryl, phosphonyl, and toluenesulfonyl) were investigated. Variation of the nature of the head group does not lead to change in the reactivity of the oximate group, while the nucleophilicity follows the basicity of the functional fragment. The increase of the observed reaction rates during transfer of the disintegration of the substrates from water to the micellar pseudophase is due both to concentration of the reagents and to change in the reactivity of the oximate group. The new detergent 1-cetyl-2-methyl-3-(2-hydroxyiminopropyl)imidazolium chloride is one of the most effective functional surfactants in the decomposition of organophosphorus compounds. Key words: functional surfactants based on pyridine and imidazole, nucleophilicity, micellar effects.Functional surfactants containing an a-nucleophilic fragment in the head group are usually characterized both by high nucleophilic reactivity and by effective solubilization of electrically neutral polar substrates, including excotoxicants of organophosphorus type [1-10]. During the design of new functional detergents an important role is played by the true solubility of the surfactant in water, the basicity of the functional group, and its reactivity. However, the introduction of the functional fragment often leads to a decrease in the solubility of the surfactant in water [1,2,5,8]. Therefore, during study of the micellar effects of such detergents, as during their use in systems for the decomposition of excotoxicants, it is necessary to introduce an inert surfactant -a co-detergent, and this makes the system multicomponent and less attractive from the practical standpoint. The nucleophilicity of functional detergents can be predicted on the basis of the reactivity of analogs not forming micelles [1,2,[7][8][9][10]. This opens up the possibility of specific modification of the structure of the surfactant with the aim of producing compounds with the required level of nucleophilicity. This is the approach that we used during the creation of functional detergents based on imidazole [7][8][9]. With variation of the position of the oximate group in the series of hydroxyiminomethyl-1-cetylpyridinium halides the variation of the observed rates of decomposition of 4-nitrophenyl diethyl phosphate corresponds likewise to the variation of the basicity of the a-nucleophilic fragment in hydroxyimino-1-methylpyridinium halides [1]. It is important to note that for oximes that are not micelle-forming compounds there is a single Brönsted relationship onto which the points for oximes containing both an imidazole ring and a pyridinium ring fit [11]. In so far as analysis of the experimental results in [1] in terms of the existing models for description of micellar effects was not made 0040-5760/08/4402-0093

Reactivity of functional detergents with a pyridine ring and an α-nucleophile fragment in the head group

“…We first determined the acid ionization constant by the spectrophotometric method (C 0 £ CMC) or from the kinetic data (C 0 > CMC) [8]. In the last case the values of K a app (the apparent acid ionization constant of the functional group) were calculated by means of the equation [4] created such a high concentration of functional detergents; we were only able to investigate the decomposition of the substrates in functional surfactant/CTAB comicelles with the former at mole fraction c £ 0.5. b) The k ob m values given in [1] are low, and consequently the t 1/2 values are substantially higher than the values we determined in the present work and in [7]. c) The k ob m values were determined at pH 7.2, when the fraction of the oximate ion does not exceed 0.01 [6,15]; the t 1/2 values we determined with allowance for the oximate ion fraction are given in parentheses.…”

“…and a fragment of an a-nucleophile in the head group are effective supernucleophilic reagents in the decomposition of phosphate esters -model analogs of pesticides, toxic warfare agents, etc. [1][2][3][4][5][6][7][8][9]. Modifications of the surfactants and the creation of new compounds were directed at the production of substances that make it possible to achieve the highest observable rates of decomposition of ecotoxicants under "mild" experimental conditions and in particular in media with acidity close to neutral.…”

“…The direction of the investigation was chosen for the following reasons: The oximate and hydroxamate anions are typical a-nucleophiles that react anomalously quickly with organophosphorus compounds (OPC) [1][2][3][4][5][6][7][8]; oximes are widely used as antidotes, and the creation and investigation of inhibited cholinesterase reactivators based on them are currently being 292 0040-5760/08/4405-0292…”

“…The choice of direction for the modification of the functional detergents was based on the fact that change in the structure of the surfactant only affects the head group. In this case it is not impossible that the basicity of the functional fragment will determine the reactivity of the surfactant [7]. At the same time the micellar effects of the detergents must depend not only on the reactivity of the functional group but also on the effectiveness of solubilization of the substrate [7,8,10].…”

“…represents the spectrophotometric method, k.m. the kinetic method, and p.m. the potentiometric method for the determination of pK a ; c is the mole fraction of the functional detergent in the comicelle with CTAB; during calculation of k 2 m /V m the bonding constants K S were taken as equal to 200 (for NPDEP and NPDEPS) [7] and 3000 L/mol (for NPTS) [7]; during determination of the k 2 m values the partial molar volume was taken as equal to 0.4 L/mol [7]; *the k 2 m /V m value was calculated from the pH dependence.…”