Comparative Kinetic and Mechanistic study of oxidation of B-Cyclodextrin by Potassium Dichromate

R. Naz, R. Azmat, N. Qamar, H. Jaffery, S. Nisar
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引用次数: 2

Abstract

*1 R. Naz, 1 R. Azmat, 2 N. Qamar, 1 H. Jaffery and 1 S. Nisar *1 Department of Chemistry, University of Karachi, 75270, Karachi, Pakistan. 2 Department of Chemistry, Jinnah University for Women, 74600 Karachi, Pakistan E-mail: * drr.nazz@gmail.com ABSTRACT A comparative kinetic and mechanistic study of oxidation of β-Cyclodextrin (β-CD) by Potassium Dichromate (K2Cr2O7) in presence of aqueous H2SO4 medium was monitored at λmax 350 nm spectrophotometrically. The oxidation reaction follows first order kinetics with respect to [β-CD] and [Cr2O7 ] and markedly increased by increasing [H]. The slow reaction was accelerated with Fe (III) as a catalyst that enhances the rate significantly. The results of varying temperature were used to compute different activation parameters. The values of activation energy Ea were 37.70KJ mol -1 for catalysed and 50.183 KJ molfor uncatalysed reactions. These clearly indicate that Fe (III) greatly reduced the activation barrier thereby increases the rate of reaction. Oxidation product results due to the oxidation of -CH2OH group present on each glucose monomer of β-CD.A mechanism consistent with kinetic and thermodynamic data is also proposed that involves two-electron reduction of Cr (VI).
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重铬酸钾氧化b -环糊精的动力学和机理比较研究
*1 R. Naz, 1 R. Azmat, 2 N. Qamar, 1 H. Jaffery, 1 S. Nisar *1卡拉奇大学化学系,75270,巴基斯坦卡拉奇。2真纳女子大学化学系,74600,巴基斯坦卡拉奇E-mail: * drr.nazz@gmail.com摘要采用λmax 350 nm分光光度法对重铬酸钾(K2Cr2O7)在H2SO4水溶液中氧化β-环糊精(β-CD)的动力学和机理进行了比较研究。对于[β-CD]和[Cr2O7],氧化反应遵循一级动力学,并随着[H]的增加而显著增加。在Fe (III)的催化作用下,反应速度明显加快。利用温度变化的结果计算不同的活化参数。催化反应的活化能Ea为37.70KJ mol -1,未催化反应的活化能Ea为50.183 KJ mol -1。这清楚地表明,Fe (III)大大降低了活化势垒,从而提高了反应速率。氧化产物是由于β-CD的每个葡萄糖单体上的-CH2OH基团氧化而产生的。还提出了一种与动力学和热力学数据一致的机制,涉及Cr (VI)的双电子还原。
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