Kinetic Studies of the Electron-Transfer Reaction in Iron(II) and Iron(III) Systems. V. The Reaction in Mixed Solvents of Methanol and Water
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概要
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The electron-transfer reaction between Fe(II) and Fe(III) species in mixed solvents of methanol and water was investigated by the use of Fe-59 as the radioactive tracer. The hydrolysis constant of Fe<SUP>3+</SUP>, <I>K</I><SUB>H</SUB>, in the mixed solvent decreases gradually as the mole faction of methanol, <I>x</I><SUB>M</SUB>, increases. At very high acidities, ionic association between Fe<SUP>3+</SUP> and ClO<SUB>4</SUB><SUP>−</SUP> ions occurs, thus forming an ion-pair in the mixed solvent. The apparent rate constant, <I>k</I><SUB>app</SUB>, of the electron-transfer reaction is a function with respect to <I>x</I><SUB>M</SUB>; <I>k</I><SUB>app</SUB> does not vary much in the concentration range <I>x</I><SUB>M</SUB>≤0.3, but it decreases as <I>x</I><SUB>M</SUB> grows higher than 0.3. <I>k</I><SUB>app</SUB> is also dependent upon [H<SUP>+</SUP>] and in a linear relationship with [H<SUP>+</SUP>]<SUP>−1</SUP>. Thus, the two reaction paths, Fe<SUP>2+</SUP>-Fe<SUP>3+</SUP> (<I>k</I><SUB>0</SUB>) and Fe<SUP>2+</SUP>-FeOH<SUP>2+</SUP> (<I>k</I><SUB>H</SUB>), are deduced to be prevailing. Both rate constants, <I>k</I><SUB>0</SUB> and <I>k</I><SUB>H</SUB>, decrease with an increase in <I>x</I><SUB>M</SUB>. The constancy of <I>k</I><SUB>app</SUB> at 0≤<I>x</I><SUB>M</SUB>≤0.3 is a result of compensation between the increasing <I>K</I><SUB>H</SUB> value and the decreasing <I>k</I><SUB>0</SUB> and <I>k</I><SUB>H</SUB> values. By taking into account the values of <I>ΔH</I><SUB>H</SUB><SUP>\neweq</SUP>=7.6 kcal/mol and <I>ΔS</I><SUB>H</SUB><SUP>\neweq</SUP>=−19.1 e.u. at <I>x</I><SUB>M</SUB>=0.2, both of which do not differ greatly from those in pure water, the reaction is concluded to proceed through the hydrogen-atom-transfer mechanism in the mixed solvent, the inhibiting effect of methanol being due to the reduction of the hydrogen bondings between water molecules.
- 公益社団法人 日本化学会の論文
著者
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Wada Goro
Department of Chemistry, Faculty of Science, Nara Women s University
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Endo Akiko
Department of Chemistry, Nara Women's University
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