Mechanisms of Rapid Reactions Involving Metal-Ion Complexes

Lappin, Alexander Graham (1975) Mechanisms of Rapid Reactions Involving Metal-Ion Complexes. PhD thesis, University of Glasgow.

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Abstract

Reactions of iron(III) and copper(II) with organic substrates have been studied using stopped-flow spectrophotometry, Two forms of stopped-flow apparatus have been constructed and are described. The transient blue colour formed when aqueous acidic solutions of iron(III) and mercaptocarboxylie acids are mixed has been shown to be due to the rapid formation and subsequent slower disappearance of a sulphur-oxygen bonded monochelate. Visible absorption spectra, extinction coefficients and equilibrium constants have been obtained in the case of mercaptoacetic acid, 2-mercaptopropionic acid and Z-mercaptoisobutyric acid. The effect of ligand structure on these parameters is interpreted in terms of solvation of the free substrate. Complex formation reactions have been shown to proceed by a mechanism involving reaction of the first hydrolysis product of hexaaquo-iron(III), Fe(H2O)5OH2+, with the unprotonated form of the acid. Data are consistent with an Id, mechanism in which the effects of solvation of the mercaptocarboxylic acids are again important. Comparable reactions with phenolic ligands are also consistent with this hypothesis. Other mechanisms are discussed. Medium effects are attributed to the variation of activity coefficients on changing the reaction conditions. The rates of disappearance of the blue complexes have been shown to follow a rate law which is second order in complex suggesting a dimerisation mechanism. Reaction products are iron(II) and the corresponding disulphide and it is argued that such a process reflects the high energy of formation of a mercaptide radical ion species, A bimolecular reaction affords the possibility of simultaneous two electron transfer leading to direct formation of the disulphide product. The implications of such a mechanism are outlined. Methyl substituents have been shown to retard the overall rate of disulphide formation, A detailed analysis of the rate parameters reveals that this may be ascribed more to a reduction in the thermodynamic stability of the monocomplex than to an overall reduction in the rate of dimerisation. Consideration of the mechanism of the reaction with iron(III) has afforded a means of interpreting the kinetic-ally more complex reaction of 2-mercaptosuccinic acid with copper(II), Similarities in the two systems are outlined. The reactions are directly comparable and, under suitable conditions, both monomeric and dimeric copper(II)-mercaptosuccinate complexes are observed. The first acid dissociation constants of the mercapto-carboxylic acids used, have been determined to enable elucidation of the mechanism and evaluation of the kinetic parameters. Thermodynamic quantities associated with the ionisation constants have been shown to reflect solvation effects similar to those observed in the complex formation reactions. It is apparent that the substantial effects occurring in the systems described in this thesis may be ascribed to the solvating nature of the reaction medium. Such an Interpretation may have wider applications in other reactions Involving ionic and neutral reactants in solution.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Inorganic chemistry, Physical chemistry
Date of Award: 1975
Depositing User: Enlighten Team
Unique ID: glathesis:1975-78695
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:01
Last Modified: 30 Jan 2020 15:01
URI: http://theses.gla.ac.uk/id/eprint/78695

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