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Acidic aqueous decomposition of thiocyanogen
Jon J Barnett1, Michael L McKee, David M Stanbury
1Department of Chemistry, Auburn University, Auburn, AL 36849, USA.
Inorganic Chemistry
|August 3, 2004
Summary
The reaction of chlorine with thiocyanate ions forms thiocyanogen and trithiocyanate intermediates. Their decomposition kinetics were studied, revealing a new rate law important for understanding enzyme catalysis.
Area of Science:
- Chemical Kinetics
- Reaction Mechanisms
Background:
- The reaction between acidic chlorine (Cl2) and excess thiocyanate ions (SCN-) generates UV-absorbing intermediates.
- These intermediates, thiocyanogen ((SCN)2) and trithiocyanate ((SCN)3-), decompose via a complex reaction pathway.
Purpose of the Study:
- To investigate the kinetics and mechanism of the decomposition of thiocyanogen and trithiocyanate mixtures.
- To determine the rate law and identify key intermediates and rate constants involved in the decomposition process.
Main Methods:
- Stopped-flow methodology was employed to study the slow decomposition kinetics over a pH range of 0-2.
- Kinetic data were analyzed to derive a rate law and determine equilibrium and rate constants.
Main Results:
- A novel rate law was established for the decomposition, incorporating terms for trithiocyanate formation (K(SCN)3-) and thiocyanogen hydrolysis (K(hyd)).
- Specific values for K(SCN)3- (0.43 M-1), K(hyd) (5.66 x 10(-4) M2), and k(disp) (6.86 x 10(4) M-1 s-1) were determined at 25°C.
- Ab initio calculations supported the determined values for K(SCN)3- and K(hyd).
Conclusions:
- The results indicate that hypothiocyanous acid (HOSCN) is a short-lived intermediate, with its thermodynamic stability informed by K(hyd).
- The findings provide insights into the reaction mechanism relevant to enzymes like salivary and myeloperoxidase that catalyze thiocyanate oxidation.