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    STUDIA CHEMIA - Issue no. 1-2 / 2001  
         
  Article:   A COMPARATIVE STUDY ON THE OXIDATION OF HYDROXY- AND MERCAPTO- COMPOUNDS BY CR(VI).

Authors:  I. BALDEA, DANA-MARIA SABOU.
 
       
         
  Abstract:  Both, the thiols or mercapto-acids (RSH) and the alcohols or hydroxy-acids (ROH) are easily oxidised by various oxidising agents as Cr(VI), Mn(VII) Fe(CN)63-, Ce(IV), etc. The importance of such a comparison of the two classes of compounds consists of the resemblance of OH and SH functions, as well as the involvement of such oxidation in many biological processes. Our comparative study is based on the literature data as well as our research in the field of oxidation of various thio-compounds as ethane thiol, mercaptoacetic acid, mercaptosuccinic acid, L(+)-cysteine, glutathione, thiophenol benzylmercaptane and alcohols or hydroxy-acids as benzyl alcohol, glycolic acid, lactic acid, malic acid, ascorbic acid by Cr(VI). Kinetic rate laws are compared and mechanisms are discussed. There is some similar behaviour of two classes of compounds. Using Cr(VI) as oxidising agent, the rate laws, at acidity held constant are of the form: and Many differences were noticed. Generally, the oxidation of hydroxy-compound takes place at the carbon atom bearing the function and carbonyl products were obtained. On the contrary, thiols are oxidised at sulphur atom and disulphide products were obtained. However, the rate laws are similar. Generally, in the Cr(VI) oxidations, intermediate condensed compounds esters or thio-esters have been identified either directly by spectrophotometrical means, or indirectly by the inference on the rate law. With hydroxy-compounds the two-electronic charge transfer prevailed. With thio-compounds both mono- and two-electronic charge transfer takes place, depending on the nature of thiol. The involvement of free radicals has been also proved. Both classes of oxidation are strongly influenced by the acidity of the medium. Other medium effects were noticed.  
         
     
         
         
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