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Dipolar Micelles. 9. The Mechanism of Hydrolysis of Cationic Long Chained Benzoate Esters in Choline and Homocholine‐type Micelles
Author(s) -
Pillersdorf A.,
Katzhendler J.
Publication year - 1979
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.197900051
Subject(s) - chemistry , micelle , ammonium bromide , bromide , cationic polymerization , hydrolysis , micellar solutions , medicinal chemistry , aqueous solution , reaction rate constant , substituent , ammonium , organic chemistry , inorganic chemistry , kinetics , pulmonary surfactant , biochemistry , physics , quantum mechanics
Abstract An investigation was carried out on the hydrolytic reactions of the para ‐substituted long chain esters N‐decyl‐N, N‐dimethyl‐N‐(2‐benzoyloxyethyl) ammonium bromide (CBE), N‐decyl‐N, N‐dimethyl‐N‐(3‐benzoyloxypropyl) ammonium bromide (HCBE) and decyl benzoates (DBE) in the cholinium type detergent N‐decyl‐N, N‐dimethyl‐N‐(2‐hydroxyethyl) ammonium bromide ( I ), and in the homocholinium type detergent, N‐decyl‐N, N‐dimethyl‐N(3‐hydroxypropyl) ammonium bromide ( II ). The first order rate constants ( k c ) of the trans‐benzoylation stage for HCBE in micelle I , CBE in micelle II and DBE in micelles I and II as catalysts were determined. The second order rate constants ( k ' OH ) for the alkaline hydrolysis of these esters, were also evaluated. The results showed that the partition ratio ( k −n / k o ) of the tetrahedral intermediate formed during the reaction of I + HCBE, to the reactants and products, is relatively small when compared to the known values for oxygen esters in non‐micellar media. This phenomenon is discussed in terms of: (a) the stabilization of the anionic transition state by the positively charged micellar surface; (b) the electrophilic assistance to the expulsion of the leaving group; and (c) the favourable entropy of the substrate‐micelle intracomplex‐formation. Substituent effects on the rates ( k c , k ' OH ) of CBE, HCBE and DBE were analyzed using the Hammett equation. When compared to the ρ values displayed in aqueous solution, the increased ρ values observed in micellar and dioxane‐water systems indicate that the hydrolytic reactions of HCBE and CBE in micelles proceed close to the less polar surface area. This requires a folded conformation of the head groups of the above mentioned esters. The low effectiveness of the hydroxylic micelles ( I, II ) in the hydrolysis of esters is also discussed.