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Kinetics and mechanism of oxidation of apple pectin by Cr VI in aqueous acid medium
Selective oxidation of galacturonic acid residues of apple pectin by Cr VI affords CO 2 /HCO 2 H, oxidized pectin, and Cr III as final redox products. The reaction shows first‐order kinetics in [pectin], [Cr VI ], and [H + ], at fixed ionic strength and temperature. Kinetic studies show that the red...
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Published in: | Journal of physical organic chemistry 2008-12, Vol.21 (12), p.1059-1067 |
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Main Authors: | , , , , |
Format: | Article |
Language: | English |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | Selective oxidation of galacturonic acid residues of apple pectin by Cr
VI
affords CO
2
/HCO
2
H, oxidized pectin, and Cr
III
as final redox products. The reaction shows first‐order kinetics in [pectin], [Cr
VI
], and [H
+
], at fixed ionic strength and temperature. Kinetic studies show that the redox reaction proceeds through a mechanism combining Cr
VI
→ Cr
IV
→ Cr
II
and Cr
VI
→ Cr
IV
→ Cr
III
pathways. The mechanism is supported by the observation of free radicals, CrO
(the formation of which implies involvement of Cr
II
and Cr
IV
) and Cr
V
(formed in monoelectronic redox processes) as reaction intermediates. The reduction of Cr
IV
and Cr
V
by pectin was independently studied and found to occur more than 10
3
times faster than pectin + Cr
VI
reaction, in acid medium. At pH 3–5, apple pectin + Cr
VI
redox reaction is slow, oxo‐chromate(V)–pectin species stabilize and remain in solution during several hours. The present results show that these abundant and ubiquitous components of the cell walls of all land plants are able to reduce Cr
IV–VI
or stabilize high‐valent chromium depending on pH. Copyright © 2008 John Wiley & Sons, Ltd. |
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ISSN: | 0894-3230 1099-1395 |
DOI: | 10.1002/poc.1406 |