57668-47-0Relevant articles and documents
Lifetimes of imidinium ions in aqueous solution
Dalby, Kevin N.,Jencks, William P.
, p. 7271 - 7280 (2007/10/03)
Imidinium ions, ArN=CNR2+, were generated in aqueous solution from the solvolysis of fluoro and chloro formamidines at pH 9.3 and 25°C. Rate constants for the hydration of five imidinium ions were determined from a kinetic analysis of their trapping by thiolacetate anion, CH3COS-, in the presence of a pool of competing fluoride anion, and a rate constant of k(AcS)- = 5 x 109 M-1 S-1 for diffusion-controlled trapping of the carbocations with thiolacetate anion. The rate constants, k(s), for hydration of the imidinium ions, XArN=CNC4H8O+ are 3.6 x 105, 5.8 x 105, 1.5 x 106, 1.6 x 106, and 1.8 x 106 s-1 for X = H, 4-Cl, 3-CN, 4-CN, and 3-NO2, respectively. In a similar experiment a rate constant of k(s) = 3.3 x 107 s-1 was obtained for hydration of the imidinium ion 4-NO2-ArN=CNCH3(OCH3)+, by using azide anion to trap the cation and a rate constant of k(az) = 5 x 109 M-1 s-1 for diffusion-controlled trapping of the imidinium ion with azide ion. The partitioning rate constant ratio k(AcS)-/k(s) for 4-ClArN=CNC4H8O+ decreases by approximately 6-fold in 45% v/v glycerol/water in contrast to k(az)/k(s), which remains almost constant, showing that thiolacetate, but not azide anion, combines with the imidinium ion at a diffusion-limited rate. The reactivity of the imidinium ions deviate from behavior predicted by the N+ scale in a manner that may be explained by a difference in the selectivity of the cation when compared to the more stable N+ carbocations.
