Web Release Date: December 13,
Kinetics and Mechanism of the Comproportionation of Hypothiocyanous Acid and Thiocyanate to Give Thiocyanogen in Acidic Aqueous Solution
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019
Received August 4, 2006
Abstract:
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+; Khyd = [HOSCN][SCN-][H+]/[(SCN)2] = khyd/kcomp = 19.8(±0.7) s-1/ 5.14(±0.07) × 103 M-2 s-1 = 3.9
× 10-3 M2.
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