Rapid Formation and Slow Collapse of a Carbocation−Anion Pair to a Neutral Molecule

The 4,4‘,4‘ ‘-trimethoxytrityl cation (TMT⁺) was observed to react with acetate ion in acetic acid reversibly to give the corresponding ester (TMT-OAc). The rate of the reaction was found to be independent of [NaOAc] over a 25000-fold range. Similar results were observed in the presence of Bu₄N⁺ in acetic acid as well as in HOAc/AN (1/1). It was concluded that {TMT⁺ (HOAc/AcO^-)} is an ion pair that forms essentially completely from free TMT⁺ and HOAc/AcO^- during the time of mixing under stopped-flow conditions. The process which was studied kinetically is the intramolecular collapse of the ion pair to TMT-OAc which takes place in two steps involving a kinetically significant intermediate. The remarkably close resemblance of this reaction to the Winstein scheme for solvolysis reactions is noted. In analogy to the Winstein scheme, it was proposed that the intermediate could be an intimate ion pair formed upon extrusion of solvent from the solvent separated ion pair. The product-forming step could then correspond to the intimate ion pair reacting further to form a covalent bond between the two moieties within the complex. The values of the thermodynamic and the activation parameters as well as the apparent rate constants for the reaction in the presence of either sodium or tetrabutylammonium ions suggest that these counterions play insignificant roles in the reactions. However, the equilibrium constant for the intramolecular step (K₄) was observed to be two times greater in the presence of Bu₄N⁺ than in the presence of Na⁺. The rate of the reaction in HOAc was observed to be about four times as great as that in HOAc/AN (1/1).