Kinetic Study of the Dissociation of Sodium Cryptate(2,2,1)

The rate of dissociation of sodium cryptate(2,2,1) (NaCry⁺) has been measured at various temperatures and pressures in dimethyl sulfoxide (Me₂SO), N,N-dimethylformamide (DMF), and acetonitrile (MeCN) by using a high-pressure stopped-flow apparatus with conductometric detection. There are two reaction paths: an acid-independent path (k_d1), CryNa⁺ → Cry + Na⁺, and an acid-dependent path (K_d2), CryNa⁺ + Hdca → CryH⁺ + Na⁺ + dca^-, where Hdca is dichloroacetic acid. Activation parameters are as follows: in Me₂SO, k_d1/S^-1 = (7.65 ± 0.05) × 10^-1 (25 °C), ΔV‡_dl/cm³ mol^-1 = 2.1 ± 0.7, ΔH‡_dl/kJ mol^-1 = 70.4 ± 0.8, and ΔS‡_d1/j mol^-1 K^-1 = -11 ± 3; in DMF, k_d1/s^-1 = (3.37 ± 0.03) × 10^-1 (25 °C), k_d2/mol^-1 kg s^-1 (25 °C), ΔV‡_dl/cm³ mol^-1 = 2.0 ± 0.2, ΔV‡_d2/cm³ mol^-1 = -8.8 ± 1.0, ΔH‡_d2/kJ mol^-1 = 73.0 ± 0.9, ΔH‡_d2/kJ mol^-1 = 30 ± 5, ΔS‡_d1/J mol^-1 K^-1 = -9 ± 3, and ΔS‡_d2/J mol^-1 K^-1 = -140 ± 20; in MeCN, k_d2/mol^-1 kg s^-1 = 73.5 ± 0.5, ΔV‡_d2/cm³ mol^-1 = -16.0 ± 0.8, ΔH‡_d2/kJ mol^-1 =41.1 ± 1.3, and ΔS‡_d2/J mol^-1 K^-1 = -71 ± 4. The reaction volume for the formation of NaCry⁺ in Me₂SO has also been measured dilatometrically: ΔV° = 3.3 ± 0.4 cm³ mol^-1. The reaction mechanism is discussed on the basis of these parameters.