Document Detail


Kinetics and mechanism of the comproportionation of hypothiocyanous acid and thiocyanate to give thiocyanogen in acidic aqueous solution.
MedLine Citation:
PMID:  17198438     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
The kinetics of comproportionation of hypothiocyanous acid (HOSCN) and thiocyanate (SCN-) to give thiocyanogen ((SCN)2) in acidic aqueous solutions have been determined by double-mixing stopped-flow UV spectroscopy. Hypothiocyanite (OSCN-) was generated at pH 13 by oxidation of excess SCN- with hypobromite (OBr-), followed by a pH jump to acidic conditions ([H+] = 0.20-0.46 M). The observed pseudo-first-order rate constants exhibit first-order dependencies on [H+] and [SCN-] with overall third-order kinetics. The corresponding kinetics of hydrolysis of (SCN)2 have also been examined. Under conditions of high (and constant) [H+] and [SCN-], the kinetics exhibit second-order behavior with respect to [(SCN)2] and complex inverse dependences on [H+] and [SCN-]. Under conditions of low [H+] and [SCN-], the kinetics exhibit first-order behavior with respect to [(SCN)2] and independence with respect to [H+] and [SCN-]. We attribute this behavior to a shift in the rate-limiting step from disproportionation of HOSCN (second-order dependency on [(SCN)2]) to rate-limiting hydrolysis (first-order dependency on [(SCN)2]). Thus, we have determined the following equilibrium constant by the kinetic method: (SCN)2 + H2O HOSCN + SCN- + H+; Khyd = [HOSCN][SCN-][H+]/[(SCN)2] = khyd/kcomp = 19.8(+/-0.7) s-1/ 5.14(+/-0.07) x 103 M-2 s-1 = 3.9 x 10-3 M2.
Authors:
Péter Nagy; Kelemu Lemma; Michael T Ashby
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't; Research Support, U.S. Gov't, Non-P.H.S.    
Journal Detail:
Title:  Inorganic chemistry     Volume:  46     ISSN:  0020-1669     ISO Abbreviation:  Inorg Chem     Publication Date:  2007 Jan 
Date Detail:
Created Date:  2007-01-02     Completed Date:  2007-03-13     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0366543     Medline TA:  Inorg Chem     Country:  United States    
Other Details:
Languages:  eng     Pagination:  285-92     Citation Subset:  IM    
Affiliation:
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
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MeSH Terms
Descriptor/Qualifier:
Hydrogen-Ion Concentration
Kinetics
Thiocyanates / chemistry*
Water / chemistry*
Chemical
Reg. No./Substance:
0/Thiocyanates; 0/hypothiocyanous acid; 302-04-5/thiocyanate; 7732-18-5/Water

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