Lifetimes of Imidinium Ions in Aqueous Solution

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 × 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 × 105, 5.8 × 105, 1.5 × 106, 1.6 × 106, and 1.8 × 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 × 107 s-1 was obtained for hydration of the imidinium ion 4-NO2ArNCNCH3(OCH3)+, by using azide anion to trap the cation and a rate constant of k az = 5 × 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.