Number and structure of solvolysis intermediates. Part 3.SN1 solvolysis of 2,2‐Dimethyl‐1‐(p‐Methoxyphenyl)Propyl p‐Nitrobenzoate: Mechanism of the common ion salt effects arising at the stage of the second ion‐pair intermediate

Abstract

Abstract2,2‐Dimethyl‐1‐(p‐methoxyphenyl)propyl p‐nitrobenzoate (ROPNB) was subjected to solvolysis in phenol in the presence of tetrabutylammonium [carboxy‐13C]‐p‐nitrobenzoate, in which the ‘common ion rate depression’ was confirmed to arise at the stage of the second ion‐pair intermediate (Int‐2). The unchanged substrate recovered at 46% reaction contained the isotopically labelled leaving group, indicating the occurrence of common ion exchange to the extent of 41–46%. In the solvolysis of the optically active substrate under identical conditions, the unchanged substrate was recovered with 51·4% racemization and ROPh was produced with slightly (1·56%) retained configuration, similarly to the solvolysis in the absence of the common ion salt. These isotope‐tracer and stereochemical outcomes indicate that the common ion exchange in this solvolysis system should be attributable to the retentive nucleophilic attack on Int‐2 by the common ion salt via a quadrupole (four‐centre ion pair) transition state, accompanying the common ion rate depression, and they suggest that the special salt effect also should proceed by an analogous anion‐exchange mechanism to that for the common ion effects.