Hammett and Taft Substituent Constants for the Mesylate, Tosylate, and Triflate Groups

Abstract

The Hammett σp values for the mesylate (1), tosylate (2), and triflate groups (3) were determined by titration of the appropriate benzoic acids and found to be +0.33, +0.29, and +0.47, respectively. Taft σ1 values were determined by 19F NMR of the appropriately substituted 3-fluorophenylsulfonate esters and found to be 1, +0.61; 2, +0.54, 3, +0.84. By interpolation using pKa data for a number of substituted acetic acids, the value of σ* for the CF3SO2OCH2- group was found to be +1.98. The possible nature and origin of these values are discussed and applied to the relative leaving group ability of the various sulfonate esters in SN1 reactions. © 1976, American Chemical Society. All rights reserved.