Τίτλος:
Kinetics and Mechanisms of the Chromium(III) Reactions with 2,4-and
2,5-Dihydroxybenzoic Acids in Weak Acidic Aqueous Solutions
Περίληψη:
The reactions of 2,4- and 2,5-dihydroxybenzoic acids (dihydroxybenzoic
acid, DHBA) with chromium(III) in weak acidic aqueous solutions have
been shown to take place in at least two stages. The first stage of the
reactions has an observed rate constant k(1(obs)) = k(1)[DHBA] + C and
the corresponding activation parameters are Delta H-1(2,4)(not equal) =
49, 5 kJ/mol(-1), Delta S-1(2,4)(not equal) = -103, 7 J mol(-1) K-1,
Delta H-1(2,5)(not equal) = 60, 3 kJ/mol(-1), and Delta S-1(2,5)(not
equal) = -68, 0 J mol(-1) K-1. These are composite activation parameters
and the breaking of the strong intramolecular hydrogen bonding in the
two ligands is suggested to be the first step of the (composite) first
stage of the reactions. The second stage is ligand concentration
independent and is thus attributed to a chelation process. The
corresponding activation parameters are Delta H-2(2,4)(not equal) = 45,
13 kJ/mol(-1), Delta S-2(2,4)(not equal) = -185, 9 J mol(-1) K-1, Delta
H-2(2,5)(not equal) = 54, 55 kJ/mol(-1), and Delta S-2(2,5)(not equal) =
-154, 8 J mol(-1) K-1. The activation parameters support an associative
mechanism for the second stage of the reactions. The various
substitution processes are accompanied by proton release, resulting in
pH decrease.
