Kinetics and mechanism of the reaction between chromium(III) and 3,4-dihydroxyphenylpropionic (dihydrocaffeic) acid in weak acidic aqueous solutions.
The reaction of 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid, hydcafH3) with chromium(III) in weak acidic aqueous solutions has been shown to take place through various oxygen-bonded intermediates. The formation of the oxygen-bonded complexes upon substitution of water molecules of the chromium(III) coordination sphere takes place in at least three stages, the first of which has an observed rate constant k1(obs)=k1K0'[hydcafH3]/[H+] where K0' corresponds to the Cr(H2O)6(3+) complex dissociation equilibrium. The second and third stages are ligand concentration independent and are thus attributed to isomerisation and chelation processes. The corresponding activation parameters are DeltaH2(not equal)=78+/-3 kJmol(-1), DeltaS2(not equal)=-49+/-9 JK(-1)mol(-1), DeltaH3(not equal)=60+/-9 kJmol(-1) and DeltaS3(not equal)=-112+/-39 JK(-1)mol(-1). The kinetic results support associative mechanisms and the nature of the electronic spectra a catecholic-type of coordination at the pH and concentration range studied and reported in this paper. The associatively activated substitution processes are accompanied by proton release causing a pH decrease. At lower acid concentration oxidation of the ligand takes place with concomitant high increase in the UV and VIS absorbance.
Petrou
Athinoula L
AL
Laboratory of Inorganic Chemistry, University of Athens, Panepistimiopolis, Athens 15771, Greece. apetrou@chem.uoa.gr
Paraskevopoulou
Patrina
P
Chrysikopoulou
Maria
M
D002109:Caffeic Acids / Q000737:chemistry*; D002614:Chelating Agents / Q000737:chemistry; D002857:Chromium / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D008024:Ligands; D012996:Solutions; D013053:Spectrophotometry; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
D002109:Caffeic Acids; D002614:Chelating Agents; D008024:Ligands; D012996:Solutions; D014867:Water; D002857:Chromium; C000995:3,4-dihydroxyphenylpropionic acid