Bonding in Phosphoryl (-PO32-) and Sulfuryl (-SO3-) Group Transfer between Nitrogen Nucleophiles as Determined from Rate Constants for Identity Reactions

Article abstract of DOI:10.1021/ja00308a029

Rate constants have been measured for the bimolecular reactions of pyridines with pyridinium-N-phosphonates (k_y^xp) and pyridinium-N-sulfonates (k_y^xs) for aqueous solution at 25 deg C and 0.1 M ionic strength.The Broensted correlations with the pK of the entering or leaving pyridine can be used to calculate the rate constants for reactions where entering and leaving pyridines are identical.The Broensted selectivities of the "identity" rate constants (β_i=-0.53 for k_x^xp and β_i=-55 for k_x^xs) predict tightness parameters (τ) of 0.35 and 0.56 and bond orders (η) of 0.17 and 0.28 for the N-P and N-S bonds in the transition states for the transfer processes.The data are consistent with first-order "Marcus" theory governing the energy surfaces of the reactions and in particular obey the relationship β_i=β_nuc+β_lg predicted by Lewis and Hu.