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Kinetics and Mechanism of Oxidation of Methyl Glycol and Ethyl Glycol by N-Bromosuccinimide in Alkaline Medium Catalysed by Os(VIII)

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Kinetics and Mechanism of Oxidation of Methyl Glycol and Ethyl Glycol by N-Bromosuccinimide in Alkaline Medium Catalysed by Os(VIII) Oxidation Communications 36, No 3, 565–572 (2013) Oxidation in the presence of halogen-containing compounds KINETICS AND MECHANISM OF OXIDATION OF METHYL GLYCOL AND ETHYL GLYCOL BY N-BROMOSUCCINIMIDE IN ALKALINE MEDIUM CATALYSED BY Os(VIII) R. A. SINGHa*, KAMINI SINGHa, ABHISHEK KUMARa, S. K. SINGHb aChemical Kinetics Research Laboratory, Department of Chemistry, T. D. P. G. College, 222 002 Jaunpur (U.P.), India E-mail: [email protected]; [email protected] bDepartment of Chemistry, Sri J.N.P.G. College, Lucknow (U.P.), India ABSTRACT The kinetics of Os(VIII)-catalysed oxidation of methyl and ethyl glycol by N-bro mo- succinimide has been investigated in alkaline medium. The reaction shows first order kinetic with respect to methyl and ethyl glycols at their low concentration tended to zero order at their higher concentration. N-bromosuccinimide and osmium tetroxide also show first order kinetics. The dielectric constant of rate constant has positive effect. Negligible effect of variation of ionic strength of the medium on rate constant was observed. Increase in temperature markedly increase the rate constant of oxidation of methyl and ethyl glycols by N-bromosuccinimide under experimental condition. A plausible rate law can be proposed as – d[NBS] kdK1K2[NBS] [S] [Os(VIII)T][OH ] – = – – , dt 1 + K1[OH ] + K1K2 [OH ] [S] where S = glycol, viz. methyl and ethyl glycol. Keywords: oxidation, methyl glycol (MG), ethyl glycol (EG), osmium tetroxide, N-bromosuccinimide [NBS], kinetic and mechanistic approach. AIMS AND BACKGROUND Kinetic investigations involving aliphatic, cyclic, aromatic ketones and various oxidis- ing agents have been reported by many workers1,2. N-bromosuccinimide is a potent oxidising agent and has been used in the determination of several organic compounds3. Most of the investigations on NBS oxidation of organic substrates have assumed that the molecular NBS acts only through its positive polar end producing Br+, which is * For correspondence. 565 + subsequently solvated. Thus H2O Br has been considered as the effective oxidising species of NBS in acidic medium. In the presence of mercuric acetate protonated form of NBS, i.e. N+BSH, has been also considered4 as reactive species of NBS in acidic medium. However, there are only few reports on the oxidation reaction of NBS in alkaline medium5. Some investigations involving NBS oxidation of ester6, alcohol7,8 and ketones9,10 are reported. Very little data are reported on Os(VIII) as an independent oxidant11–13 although it has been widely used as a catalyst14–16. Recently some attempts have been made on kinetics studies of Os(VIII) oxidation of some substrate, but literature on kinetic investigation of oxidation of reducing substrate by Os(VIII) is scanty. This paper reports the kinetics and mechanism of oxidation of methyl and ethyl glycols by osmium tetroxide in alkaline medium. EXPERIMENTAL All the reagents used were of highest purity available A. R. grade. The reagents employed were methyl and ethyl glycols (E. Merck), osmium tetroxide (Johnson & Matthey), N-bromosuccinimide (NBS) (E. Merck). All the solutions were prepared in doubly distilled water. Fresh solution of NBS was always prepared and its strength was checked iodometrically. The solution of methyl and ethyl glycols was prepared by weighing desired amount and dissolving the same in appropriate volume of dou- bly distilled water. Solutions of mercuric acetate (E. Merck) and sodium perchlorate were prepared in doubly distilled water. The solution of catalyst osmium tetroxide was prepared by dissolving the sample in NaOH of known strength. Table 1. Effect of variation of concentration of reactants in the oxidation of methyl glycol at 35oC 6 4 2 4 [Os(VIII)] × 10 [NBS] × 10 [Methyl glycol] × [NaOH] × 10 k1 × 10 (mol dm–3) (mol dm–3) 102 (mol dm–3) (mol dm–3) (s–1) 1 2 3 4 5 5.64 6.67 2.00 2.00 5.03 5.64 10.00 2.00 2.00 5.01 5.64 12.50 2.00 2.00 5.03 5.64 16.67 2.00 2.00 4.96 5.64 20.00 2.00 2.00 4.95 5.64 25.00 2.00 2.00 4.97 5.64 10.00 0.50 2.00 1.22 5.64 10.00 1.00 2.00 2.48 5.64 10.00 1.50 2.00 3.62 5.64 10.00 2.50 2.00 5.74 5.64 10.00 3.00 2.00 6.50 5.64 10.00 4.50 2.00 8.04 to be continued 566 Continuation of Table 1 1 2 3 4 5 1.41 12.50 2.00 2.00 1.30 2.82 12.50 2.00 2.00 2.57 4.23 12.50 2.00 2.00 3.78 7.05 12.50 2.00 2.00 6.40 8.46 12.50 2.00 2.00 7.67 5.64 10.00 2.00 2.00 1.89 5.64 10.00 2.00 2.00 3.01 5.64 10.00 2.00 2.00 3.73 5.64 10.00 2.00 2.00 5.55 5.64 10.00 2.00 2.00 6.26 Table 2. Effect of variation of concentration of reactant in the oxidation of ethyl glycol at 35oC 6 4 2 4 [Os(VIII)] × 10 [NBS] × 10 [Ethyl glycol] × [NaOH] × 10 k1 × 10 (mol dm–3) (mol dm–3) 102 (mol dm–3) (mol dm–3) (s–1) 5.64 6.67 2.00 3.00 2.47 5.64 10.00 2.00 3.00 2.42 5.64 12.50 2.00 3.00 2.40 5.64 16.67 2.00 3.00 2.40 5.64 20.00 2.00 3.00 2.41 5.64 25.00 2.00 3.00 2.39 5.64 12.50 0.50 3.00 0.58 5.64 12.50 1.00 3.00 1.18 5.64 12.50 1.50 3.00 1.82 5.64 12.50 2.50 3.00 2.64 5.64 12.50 3.00 3.00 2.88 5.64 12.50 4.50 3.00 3.92 1.41 10.00 2.00 3.00 0.58 2.82 10.00 2.00 3.00 1.14 4.23 10.00 2.00 3.00 1.70 7.05 10.00 2.00 3.00 2.80 8.46 10.00 2.00 3.00 3.52 5.64 10.00 2.00 2.00 1.06 5.64 10.00 2.00 2.00 1.46 5.64 10.00 2.00 2.00 1.74 5.64 10.00 2.00 2.00 2.98 5.64 10.00 2.00 2.00 2.68 The rate increases with an increase in [glycol]0 (Tables 1 and 2, Fig. 1). However, from the linear plots of 1/k1 versus 1/[glycols] with intercepts, it appears that the order in substrate falls from unity to zero at higher [glycol]. To study the effect of alkali on the reaction rate, kinetic studies were made on the sample in NaOH of known strength. The desired volumes of solution of NBS, osmium tetroxide, mercuric acetate, sodium 567 acetate and sodium hydroxide were taken in a black coated reaction vessel and ther- mostated water bath was used to maintain the desired temperature to within ±0.1oC. Requisite volumes of all reagents except NBS were introduced into a reaction vessel and equilibrated at 35oC, a measured volume of NBS solution equilibrated separately at the same temperature was rapidly poured into the examining aliquots of the reaction mixture and NBS was detrmined iodometrically using starch indicator. 12 MG 9 1 6 k g EG l + 5 3 0. 0 3 6 9 12 7 + lg [Os(VIII)] Fig. 1. Under the conditions of Tables 1 and 2 STOICHIOMETRY AND PRODUCT IDENTIFICATION The reaction mixture containing a known excess of [N-bromosuccinimide] over [methyl glycol] and [ethyl glycol] were kept in the presence of alkaline medium and osmium tetroxide at 35oC. O C H2C R—CHOH – Os(VIII) 3 + 2 NBr + 7 OH + 2H2 O H2C CH OH C 2 O O C H2C | | | – NH 2 RCOOH + 2R COOH + Br + 2HBr + 5H2O + H2C C O 568 The end products were the corresponding acids which were identified by spot | || test. Here R stands for CH3 and C2H5 group. R stands for H and R stands for CH3 and C2H5 group in methyl and ethyl glycol, respectively. RESULTS AND DISCUSSION The kinetics of oxidation of methyl and ethyl glycols by NBS was investigated at several initial concentrations of reactants. The linear plot of log [NBS] versus time indicates a first order dependence of rate on [NBS]. Effect of ionic strength was found to be negligible, which is maintained by sodium perchlorate both in methyl and ethyl glycols. On increasing the concentration of sodium hydroxide the value of first order rate constant increases non-linearly in oxidation both of methyl glycol and ethyl glycol by N-bromosuccinimide, showing positive effect of sodium hydroxide on the rate constant of reactions. The rate increases proportionally with increasing concentration of catalyst as confirmed by a plot of rate versus [Os(VIII)] that yields a straight line passing through origin. Thus, the oxidation of the both glycols, viz. methyl and ethyl glycols, respectively, exhibits the same results, which confirm first order dependence on [Os(VIII)]. All the results and data are collected in Tables 1 and 2. The kinetic investigations of oxidation by N-bromosuccinimide have been car- ried out extensively in alkaline medium. In alkaline medium, N-bromosuccinimide gives hypobromite anion as: – – – RNBr + 2OH RN + OBr + H2O The reactive species of the oxidant could be either hypobromite ion or NBS itself.
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