The effect of medium on the alpha-effect: Nucleophilic substitution reaction of p-nitrophenyl diphenyl phosphinate with butane-2,3-dione monoximate and substituted phenoxides in cationic micelles

Abstract

Pseudo-first-order rate constants (k(obs)) have been determined for the nucleophilic substitution reactions of p-nitrophenyl diphenyl phosphinate (PNPDPP) with substituted phenoxides (XC6H4O-) and butane-2,3-dione monoximate (Ox(-)) in 0.1 M borate buffer (pH = 10.0) at 25.0 +/- 0.1 degreesC. The k(obs) value increases sharply upon addition of cethyltrimethylammonium bromide (CTAB) to the reaction medium up to near 7 x 10(-4) M CTAB and then decreases smoothly upon further addition of CTAB. The rate enhancement upon the addition of CTAB is most significant for the reaction with -O2CC6H4O- and least significant for the one with C6H5O-, indicating that the reactivity of these aryloxides in the presence of CTAB cannot be determined by the basicity alone. The strength of the interaction of these anionic aryloxides with the positively charged micellar aggregates has been suggested to be an important factor to determine the reactivity in the presence of CTAB. The k(obs) value for the reaction with Ox(-) increases also upon the addition of CTAB. However, the increase in the k(obs) value is much more significant for the reaction with Ox(-) than for the one with ClC6H4O-, indicating that Ox(-) is less strongly solvated than ClC6H4O- in H2O. The alpha-effect shown by Ox(-) in H2O has been attributed to the ground-state solvation difference between Ox(-) and ClC6H4O-.