Kinetics and mechanism of the comproportionation of hypothiocyanous acid and thiocyanate to give thiocyanogen in acidic aqueous solution

Article abstract of DOI:10.1021/ic061470i

The kinetics of comproportionation of hypothiocyanous acid (HOSCN) and thiocyanate (SCN^-) to give thiocyanogen ((SCN)₂) in acidic aqueous solutions have been determined by double-mixing stopped-flow UV spectroscopy. Hypothiocyanite (OSCN^-) was generated at pH 13 by oxidation of excess SCN^- with hypobromite (OBr), followed by a pH jump to acidic conditions ([H⁺] = 0.20-0.46 M). The observed pseudo-first-order rate constants exhibit first-order dependencies on [H ⁺] and [SCN^-] with overall third-order kinetics. The corresponding kinetics of hydrolysis of (SCN)₂ have also been examined. Under conditions of high (and constant) [H⁺] and [SCN ^-], the kinetics exhibit second-order behavior with respect to [(SCN)₂] and complex inverse dependences on [H⁺] and [SCN^-]. Under conditions of low [H⁺] and [SCN ^-], the kinetics exhibit first-order behavior with respect to [(SCN)²] and independence with respect to [H⁺] and [SCN^-]. We attribute this behavior to a shift in the rate-limiting step from disproportionation of HOSCN (second-order dependency on [(SCN) ₂]) to rate-limiting hydrolysis (first-order dependency on [(SCN)₂]). Thus, we have determined the following equilibrium constant by the kinetic method: (SCN)₂ + H₂O ? HOSCN + SCN^- + H⁺; K_hyd = [HOSCN][SCN ^-][H⁺]/[(SCN)₂] = K_hyd/k _comp = 19.8(±0.7) s^-1/ 5.14(±0.07) × 10³ M^-2 s^-1 = 3.9 × 10^-3 M².