Acidic Aqueous Decomposition of Thiocyanogen

Abstract

The aqueous reaction of acidic Cl2 with excess SCN- rapidly generates a UV-absorbing intermediate identified as an equilibrium mixture of thiocyanogen, (SCN)2, and trithiocyanate, (SCN)3(-). The decomposition of this mixture can be described as 3(SCN)2 + 4H2O --> 5HSCN + H2SO4 + HCN. Under our conditions the decomposition is sufficiently slow that its kinetics can be studied using standard stopped-flow methodology. Over the pH range 0-2 the decomposition rate law is -d[(SCN)2]/dt = (3/2)[k(disp)K(hyd)2[(SCN)2]2/([SCN-]2[H+]2 + K(SCN)3-[SCN-]3[H+]2 + K(hyd)[SCN-][H+])] with K(SCN)3(-) = 0.43 +/- 0.29 M(-1), K(hyd) = (5.66 +/- 0.77) x 10(-4) M2, and k(disp) = (6.86 +/- 0.95) x 10(4) M(-1) s(-1) at 25 degrees C and micro = 1 M. The K(SCN)3(-) and K(hyd) terms are significant enhancements relative to one of the rate laws conventionally cited. In the proposed mechanism, K(SCN)3(-) refers to the formation of (SCN)3(-) by association of SCN- with (SCN)2, K(hyd) refers to the hydrolysis of (SCN)2 to form HOSCN, and k(disp) is the rate constant for the bimolecular irreversible disproportionation of HOSCN, which leads ultimately to SO4(2-) and HCN. Ab initio calculations support the values of K(SCN)3(-) and K(hyd) reported herein. The high value for k(disp) indicates that HOSCN is a short-lived transient, while the magnitude of K(hyd) provides information on its thermodynamic stability. These results bear on the physiological role of enzymes that catalyze the oxidation of SCN- such as salivary peroxidase and myeloperoxidase.

Simultaneous oxidation of cyanide and thiocyanate at high pressure and temperature

“…(5)). Various authors have proposed the formation of (SCN) 2 and OSCN − as reaction intermediates in different works of oxidation of thiocyanate [40,41]. Because of the severe operating conditions used, the hydrolysis of OSCN − and (SCN) 2 is very fast, obtaining thiocyanate, cyanide and sulfate (Eq.…”

Dynamical Complexity in Electrochemical Oxidations of Thiocyanate

“…10 Trace amount of thiocyanate could also be used as an anticancer reagent through eliminating radicals inside human body. [11][12][13][14] However, the presence of a large quantity of thiocyanate in industrial effluent would impose a great threat on our environment. Therefore, finding a way to manipulate the presence of thiocyanate or convert it to less pollutant sulfur compounds becomes increasingly desired.…”

Reactions of cysteine sulfenyl thiocyanate with thiols to give unsymmetrical disulfides

“…It was necessary to employ approximately 10% excess (SCN) 2 to ensure complete conversion of CSH to CSSCN. The excess is probably required because once thiocyanogen is extracted into the aqueous phase, it undergoes competitive reactions that result in its decomposition [21,22]. Thiocyanogen can be stabilized in 1 M HCl by adding thiocyanate [5], but that was unnecessary in the present case because the rate of reaction of thiocyanogen with CSH is apparently competitive with respect to its rate of decomposition.…”