KINETICS AND MECHANISM OF THE REACTION OF BROMINE WITH THIOCYANATE

The reaction between bromine and thiocyanate has been studied between pH 0.3 and 3. The stoichiometry at pH 1 is 4Br2 + SCN- + 4H2O --> SO42- + BrCN + 7Br- + 8H+. At lower pH values, between 3.5 and 4 mol of bromine are consumed, because some CN- escapes as HCN(g). There is a fast initial stage in which 1 mol of bromine is consumed for each mole of thiocyanate. At low acid concentrations the remaining 3 mol of bromine are consumed in the slower second stage of the reaction. In high acid concentrations a very slow third stage (Br2 + CN- --> BrCN + Br-) is observed in which 1 mol of bromine is consumed by cyanide. Acid retardation of the final stage is brought about by conversion of CN- to HCN, shown by the disparity between the rate constants for the following two reactions: Br2(aq) + CN- --> BrCN + Br-, k13 = (3.0 +/- 0.7) X 10(9) M-1 s-1, and Br2 + HCN(aq) --> BrCN + H+ + Br-, k16 = 52 +/- 5 M-1 s-1. For the first step of the initial stage, electrophilic attack by bromine or tribromide on thiocyanate followed by hydrolysis to produce HOSCN, the reactions and measured rate constants are Br2(aq) + SCN- + H2O --> HOSCN + 2Br- + H+, k2 greater-than-or-equal-to (6.05 +/- 1.1) X 10(5) M-1 s-1, and Br3-(aq) + SCN- + H2O --> HOSCN + 3Br- + H+, k3 greater-than-or-equal-to (7.1 +/- 0.5) X 10(4) M-1 s-1. Further oxidation of HOSCN proceeds to a sulfonic acid species before cleavage of the sulfur-carbon bond to release CN-, which then reacts with the stoichiometrically remaining mole of bromine to constitute the final stage of the reaction. Excellent agreement with experiment is obtained through integration of a simplified version of thc overall mechanism, with previously measured equilibrium and rate constants from this study, and one adjustable parameter.