CATION-ANION COMBINATION REACTIONS .17. REACTIVITIES OF ALKYLTHIOLATE IONS IN AQUEOUS-SOLUTION

Rate constants and, where possible, equilibrium constants for the reactions of several alkylthiolate ions with aseries of triarylmethyl cations, P-dimethylaminophenyltropylium ion, and 2,4-dinitroiodobenzene have been determined in aqueous solution at 25 °C. Bronsted β values for these reactions are very small, 0.0-0.3, although the Bronsted a value for variation of the triarylmethyl cation is ca. 0.5. Quite generally, alkyl thiolate ions are 104-105 more reactive than hydroxide ion toward all of the electrophiles studied here. The present data togetherwith literature data for reactions of alkylthiolates with other electrophiles clearly show that the carbonyl group is unique as an electrophilic center in exhibiting very low thiolate/hydroxide rate constant ratios. Neither Hard-Soft Acid-Base concepts nor extended Marcus theory is capable of rationalizing the extensive data. We suggest that the development of a highly localized negative charge on oxygen in the carbonyl-nucleophile adducts is the controlling factor, perhaps through a solvent effect, for the unique behavior of the carbonyl group in these reactions. © 1979, American Chemical Society. All rights reserved.