Acidic Aqueous Decomposition of Thiocyanogen

摘要

The aqueous reaction of acidic CI_2 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+4H_2O->5HSCN+H_2S0_4+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_(di)sPK_(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)X10~(-4)M~2,and K_(disp)=(6.86(+-)0.95)X10~4 M~(-1)s~(-1)at 25 deg C and mu=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,Khyd refers to the hydrolysis of(SCN)_2 to form HOSCN,and /cdjsp is the rate constant for the bimolecular irreversible disproportionate of HOSCN,which leads ultimately to SO_4~(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.