Abstract
The reaction between bromine and thiocyanate has been studied between pH 0.3 and 3. The stoichiometry at pH 1 is 4Br2 + SCN" + 4H20 —*· S042" + 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", kn = (3.0 ± 0.7) X 109 M"‘ s'1, and Br2 + HCN(aq) -* BrCN + H+ + Br', k¡6 = 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" + H20 -* HOSCN + 2Br" + H+, fc2 > (6.05 ± 1.1) X 105 M"1 s'1, and Brf(aq) + SCN' + H20 — HOSCN + 3Br* + H+, k3 > (7.1 ± 0.5) X 104 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 the overall mechanism, with previously measured equilibrium and rate constants from this study, and one adjustable parameter.