Nucleophilic Attack of Salicylhydroxamate Ion at CdO and PdO Centers in Cationic Micellar Media Manmohan Lal Satnami,* ,† Sunita Dhritlahre, Rekha Nagwanshi, ‡ Indrapal Karbhal, † Kallol K. Ghosh, † and Faruk Nome § School of Studies in Chemistry, Pandit RaVishankar Shukla UniVersity, Raipur (Chhatisgarh), India 492010, Department of Chemistry, GoVernment College Barnagar (M.P.), India, 456010, and Departamento de Quimica, UniVersidade Federal de Santa Catarina, 88040-900 Floriano’polis, Santa Catarina, Brazil ReceiVed: July 17, 2010; ReVised Manuscript ReceiVed: October 31, 2010 The reaction between the salicylhydroxamate anion (SHA - ) and p-nitrophenyl benzoate (PNPB), tris(3- nitrophenyl)phosphate (TRIS), and bis(2,4-dinitrophenyl)phosphate (BDNPP) have been examined kinetically. The R-nucleophile, SHA - , incorporated into cetyltrimethylammonium bromide (CTAB) micelles accelerates dephosphorylation of tris(3-nitrophenyl)phosphate (TRIS) over the pH range 6.7-11.4. With a 1.0 mM of SHA in CTAB, the nucleophilicity of SHA followed the order of reactivity, PNPB (CdO, carboxylate ester) > TRIS (PdO, triester) > BDNPP (PdO, diester), and monoanionic SHA - and dianionic SA 2- are the reactive species. The critical micelle concentration, cmc, of cetyltrimethylammonium bromide (CTAB) decreases and the fractional ionization constant, R, increases with increasing the concentration of SHA - . Addition of 1 and 10 mM SHA under the reaction conditions (pH 9.2, borate buffer) led to saturation of the micellar surface and provided qualitative information for the micellar incorporation of hydroxamate ion. Plots of the pseudo- first-order rate constant, k obs , log k obs , fraction of hydroxamic acid ionized, R SHA - and R SA 2- , vs pH showed bifunctional nucleophilicity of hydroxamic acid under micellar condition. Plotting k obs vs [SHA] T gave a straight line with intercept k 0 . This indicates that hydroxamate ions are very strong nucleophiles for nucleophilic attack at the C and P center. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. Introduction Esterolytic reactions of carboxylate, phosphate, thiophosphate, and sulfonate have recently received intensive attention due to the importance in chemical and biological processes. 1-5 The phosphotriester-based Paraoxon and Parathion are used as pesticides and are most frequently responsible for the poisoning of agricultural field workers. 6 Due to their potential toxicity, considerable attention has been directed toward detection and methods of facilitating the hydrolysis or decomposition of organophosphates. 7-10 Nucleophile-aided hydrolysis is the most preferred reaction to detoxify them. 11-15 In this regard, nucleo- philes such as peroxides, 11 hypochlorites, 12 oximates, 13 o- iodosylcarboxylate, 14 and hydroxamate 15 have been investigated alone or in concert with surfactants. Considerable research efforts have been focused on the nucleophilic reagents such as monoperoxyphthalates, 16 4-N,N-dialkylaminopyridines, 17 and metallomicelles. 18 Recently, ester cleavage properties and significant turnover catalysis of synthetic hydroxybenzotriaz- oles 19 and 5-alkyl-1H-tetrazoles 20 have been reported. Hydroxamate ions are R-effect nucleophiles, i.e., their reactivity is higher than that predicted by the Bronsted relation- ship between nucleophilicity and basicity. 21 They are effective deacylating and dephosphorylating agents, and reactivities of amphiphilic hydroxamate ions are increased by comicellization with surfactant in water. It is interesting to note that hydroxyl- amine is the parent compound for the three typical classes of the oxygen-containing R-nucleophiles: oximate and hydroxamic acid. 22 Investigation of the R-effect of hydroxamic acids in micellar media is of paramount importance, as compared to those in homogeneous aqueous solvents, since it is currently claimed that solvation/desolvation phenomena are the source of the enhanced reactivity of the R-nucleophiles. 23 Reactivities of hydroxamate ions are increased by incorpora- tion in functional polymers or by comicellization with inert surfactants in water. 24 As expected on the basis of pseudophase treatments of micellar rate effects, 25 high local concentrations of hydroxamate ions in the interfacial micellar region are largely responsible for these rate increases. 24b Considering that (i) cationic micelles of cetyltrimethylammonium bromide, CTAB, accelerate the spontaneous hydrolysis of BDNPP up to 30-fold and high pH accelerates the reaction by concentrating nucleo- philes, 25b,26 e.g., OH - , and (ii) cationic micelles accelerate the reaction of hydroxamates and phosphate triesters. 27 We studied micellar effects on reactions of the p-nitrophenyl benzoate (PNPB), tris(3-nitropheny)phosphate (TRIS), and bis(2,4-dinitrophenyl)phosphate (BDNPP) with salicylhydrox- amic acid (SHA) in cetyltrimethylammonium bromide micellar media (Chart 1). In view of micellar incorporation of SHA - , we reported the critical micelle concentration, cmc, and fractional ionization constant, R, in the presence of hydroxamic acid under buffered condition. Experimental Section Materials. Salicylhydroxamic acid, p-nitrophenyl benzoate, and cetyltrimethylammonium bromide were obtained from * To whom correspondence should be addressed. Phone: +91-99071- 55681. E-mail: manmohanchem@gmail.com. † Pandit Ravishankar Shukla University. ‡ Government College Barnagar. § Universidade Federal de Santa Catarina. J. Phys. Chem. B 2010, 114, 16759–16765 16759 10.1021/jp1066549  2010 American Chemical Society Published on Web 11/24/2010