pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in aqueous micellar solutions: relative contributions of hydrophobic and electrostatic interactions

The pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate (NPDC P) in the presence of aqueous micelles of sodium dodecyl sulfate, sodium do decylbenzene sulfonate, alkyltrimethylammonium chlorides, alkyldimethylbenz ylammonium chlorides (alkyl = cetyl and dodecyl) and polyoxyethylene(9) non ylphenyl ether was studied spectrophotometrically. The observed rate consta nts, k(obs), decrease in the following order: bulk water > cationic micelle s > anionic micelles > non-ionic micelles. This order is different from tha t observed for pH-independent hydrolysis of 4-nitrophenyl chloroformate (NP CF), whose reaction is faster in cationic micelles than in bulk water. A pr oton NMR study on solubilization of a model ester, 4-nitrophenyl 2-chloropr opionate, showed that the methylene groups in the middle of the surfactant hydrophobic chain are most affected by the solubilizate. Lower polarity and high ionic strength of interfacial water decrease the rates of hydrolysis of both NPCF and NPDCP, but the fraction of the former ester that diffuses to the interface is probably higher than that of the latter. Therefore, whe reas the (negatively charged) transition state of NPCF is stabilized by cat ionic interfaces and destabilized by anionic interfaces. that of NPDCP is n egligibly affected by ionic interfaces. which explains the observed rate re tardation by all ionic micelles. Calculated activation parameters corrobora te our explanation. Copyright (C) 2001 John Wiley & Sons, Ltd.