The Effects of Crown Ethers on the Rate of Oxidation of Oxalic Acid by the Permanganate Ion An Integrated Senior Chemistry Experiment Bruce Miles and Samuel K. Nyarku' Brandon University, Brandon, MB. Canada R7A 6A9 Crown ethers have been used as complexing agents (Fig. 1) and phase transfer reagents to facilitate a large number of reactions (1-3). They function by selectively capturing metal ions in their cavities by electrostatic forces. By this interaction anions can be soluhilized in organic solvents. Potassium permanganate, for example, is insoluble in all kinds of organic solvents, but in the presence of crown ethers it becomes soluble in a nonpolar solvent such as benzene to produce what has been called "purple benzene" (4). K+MnOl-
+L
-
Kt[L]MnO4-
(1)
I n the organic phase the resulting product exists as an ion pair and has been effective in various oxidation processes (5).An investigation into some of these oxidative processes have been carried out (6), and an interesting experiment is the oxidation of oxalic acid by the permanganate ion in the presence of crown ethers of various cavity sizes. The mechanism and kinetics of this reaction have been very well studied and are used as the basis of an analytical procedure for the standardization of permanganate.
The intense color of the permanganate ion and its strong absorption peak a t 526 nm offer a suitable method to follow the reaction visually and spectrophotometrically. The oxidation of the uxalate ion by perrnanganate in acid medium is known to he first order with respect to theoxalate ion and first order with respect to the permanganate ion (8). T h e reaction is catalyzed by Mn2+ ions and serves as an example of autocatalysis. It is inhibited by the fluoride ion, F-,probably through complexation. The proposed mechanism involves the initial steps:
T h e Mn(II1) species is subsequently destroyed in a reaction involving the oxalate species. As seen in the above steps, (eqs 3-5) Mn(I1) species plays an important role in the oxidation process, and these species may be involved in
' Author to whom correspondence should be addressed.
Figwe 1. S6uctwes of me crown ethers end a crown ether complex.
complexation with the crown ethers. Thus the reaction rate is affected depending on the cavity size of' the crown ether used,and thegreatest effect isohserved with thecrown ether whose cavity size matches that of the Mn2+ ion. Experimental All required reagents are of analar grade and doubly distilled of solutions fur . ohvsiwater is recommended for use in .oreoaration . . cal rneasuremenls. The crown ethers in our experiments were purchaded from Aldrich Ch~rniealCompany and iraed without further purification. Stock solutions for kinetic runs are thermostated at the required temperature for about 20 min. In a tmical kinetic run, 4.00 X M KMnOl solution in 4.0 M H2S04,4.00 X M oralic aeid solution and 4.00 X M crown ether solutions are first thermostated at 25 OC. Then aliquots of the oxalic acid, crown ether and water are added to a 1.0-cm thermostated cuvette. The perrnanganate sohtian is added last to give a total volume of 3.00 mL of solution. The volumes of reagents are measured with a micropipet. To study the effect of the permanganate ion, the volumes of oxalic acid solution and crown ether are kept constant at 1.000 mL and 0.500 mL, respectively, and these of permanganate and water are varied to give a total volume of 3.00 mL. Similarly, to study the effect of oxalie aeid, the volumes of the permanganate ion and crown ether are kept constant at 1.000 mL and 0.500 mL, respectively, while those of oxalic acid and water are varied to give a total volume of 3.00 mL. After mixine. -.the absorbanceof the solutions at 526 nm are recorded at various intervals. These measurements and scans were recorded in our experiments using a Pye Unican SP 1800 recording spectrophotometer with a cell compartment maintained at a specific tamperature using a Colora thermostaticbath, which circulated water at the desired temperature through the cell holder of the spectrophotometer. However, very good results can be obtained visually by noting the time taken far the permanganate color to be discharged at the various temperatures. The absorbance at infinite time, A,, is
Volume 67
Number 3 March 1990
269
En& ol Various Crown Eihen on the Reacllon Rate' no crown
Crown ether cavily diameter (A) mL of oxalic acid mL of crown ether mL of water solution m ~ oKMnO, f Rate canstant (s-') (22.0* C )
-
0.500
-
1.500 1.000 6.00 X 10-4
18-crown-6 15-crown-5 12-crown-4 2.6-3.2 0.500 1.000
1.7-2.2 0.500 1.000 0.500 0.500 1.000 1.000 6.25 X lo-' 3.95 X
1.2-1.5 0.500 1.000 0.500 1.000 6.30 X lo-'
nConcenlrations of solutbna: 4.00 X 10-a M KMnO, in 4.0 M H2S04; 0.10 M Emwn oomwund and 4.00 X l o r 2 M H2Cz0,.
TIME (s) Figure 2. Typlcai p5eud~fIrnuderplot for the oxidation 01 oxailc acid by potassium pemnganate in lhe presence of crown ethers (4.00 X M KMn04 in4.00 MHtS04.4.00 X Moxaiicacid. and 0.10M 18-crown-6at 25 OC).
obtained from solution mixtures where the permanganate color has heen discharged, normally after 30 min. Results and Dlscusslon Plots of in (A - A,) against time are observed to he linear (see Fie. 2). thus confirming that the rate law is first order in permiganate. The pseudo-first-order rate constants obtained from the slopes of these plots are found to be proportional to the concentration of the oxalic acid, indicating that the reaction is also first order in oxalic acid concentration. A notable observation is that, although the pseudo-firstorder rate constant is altered in the presence of the crown ether. it is inde~endentof the crown ether concentration. The table shows the values of the pseudo-first-order rate constants obtained with different crown ethers. These values can be related to the cavity sizes of the different crown ethers used in the experiments. The very high value of the rate constant in the presence of 15-C-5 whose cavity size corresponds to that of Mn2+ (1.60 A) suggests the strongest effect of interaction through possible complexation between crown ethers and Mn2+ ion in particular and also other .-~.--~ ~~ intermediates in the ~rooosedmechanism. I t has been suggested (5) that the crown ethers tend to activate the permanganate ion, but the results of these experiments suggest that the formation of the "naked" ion mav not be the determining factor in these oxidation reactions. It seems more likely that the crown ether interacts
withone or moreofthe reaction intermediatesas outlined in the mechanism, and this leads to the different reaction rates. Some other interesting and simple oxidation reactions have been reported (4-7).The extent and success of these reactions are enhanced in the presence of crown ethers. Among them are the permanganate oxidations of cyclohexene, ortho- and para-xylenes. Our preliminary studies show that the above studies can be extended to these compounds. summary The oxidation of oxalic acid by the permanganate ion in the presence of an acid is a very simple and interesting experiment that lends itself to both visual and spectrophotometric studies. The addition of crown ethers serves to introduce the students to macrocylic ligands, complexing agents, and the role of ligands in altering the rates of chemical reactions. By performing the reaction at various temperatures, extensions can be introduced into this experiment to determine the thermodynamic activation parameters E, for the reactions with various crown ethers and relate them to their cavity sizes and their structures. Acknowledgment Financial assistance from Brandon University is gratefully acknowledged. Literature Clted 1. Gobe1.G. W.; Weber, W.P.J.Chrm.Educ. 1978,55,350,428., ' 2. schrnidt.H.J.: schsfer.J. H. &mu. cham. Inr. ~ d~ .n d1979.16.68. . 3. Ohimoto. T.; Swern, D. J. J. Am. Chem Sac. 1977.54.862A. 4. Hiraoka, Michio. Studios in Orgonie Chemistry, I f Croivn Compounds-Their Charocl~rialicsand ApplVoiions; Elsevier: Amnterdam, 1982. 5. Sam, D. J.: Simmons, H.E. J.Am. Chem. Soc. 1872,94,4024. 6. Herriott, A. W.: Picker, T. TefrohedronLetf.L974.1511. 7. Lee, 0. G.;Chang,V. S. J. 0rg.Chem. 1979.43.2726. 8 Caoke, D. 0. Inorganic Reortion Mechonlama: The Chemical Soeicfv Monograph%for Teachers. Number 33.1978.
Second Edition of Modern Experiments for Introductory College Chemistry Available A new edition of Modern Experiments for Introductory Chemistry, containing experiments from the Journal of Chemical Education that were selected because they reflect exciting innovationsin introductory laboratory work, has just been published. Compiled by H. Anthony Neidig and Wilmer J. Stratton,the experimentsare written for professors to use, not as prestyled, specific directions, but as inspiration and guide for adoption and adaptation in their individual courses. The 144-page paperback is $16.50 ($11.50 foreign)and can be purchased by sending a prepaid order to: Subscription and Book Order Department, Journal of Chemical Education, 20th and Northampton Streets, Easton, PA 18042. All orders must be prepaid and the price includes shipping.
270
Journal of Chemical Education
