In the Classroom edited by
Todd P. Silverstein Willamette University Salem, OR 97301-3922
A Neat Trick Using Oxalic Acid Dihydrate and Potassium Permanganate and Other Experiments with Small Organic Amine or Oxygenated Compounds Malcolm A. Kelland Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway
[email protected] Potassium permanganate (KMnO4) is a powerful oxidizing agent. Its oxidizing power has been documented in a range of demonstrations in this and other journals, as well as a number of books (1, 2). One well-known demonstration is that of adding a few drops of glycerol (propane-1,2,3-triol, Figure 1) to a small (1-2 g) pile of solid KMnO4. After a short induction time, dependent on the size of the KMnO4 crystals, flames and white smoke appear as an exothermic redox reaction ensues (3, 4). The balanced reaction is as follows: 14KMnO4 þ 4C3 H5 ðOHÞ3 f 7K2 CO3 þ 7Mn2 O3 þ 5CO2 þ 16H2 O I have carried out an investigation of the reaction of solid KMnO4 (approximately 1.0 g either as supplied crystals or after powdering with a pestle and mortar) with up to 1.0 g of a number of other organic chemicals both solid and liquid. In so doing, I came across an interesting demonstration that could be used as a simple magic trick. All the reactions described below were carried out in a fume hood while wearing safety glasses and a laboratory coat. Background Alcohols First, I looked at alcohols and polyalcohols besides glycerol. I found that the less viscous liquid ethylene glycol (ethane-1,2-diol, Figure 1) gave a very similar reaction with KMnO4, including a similar short delay time before producing flames. The balanced reaction is as follows: 10KMnO4 þ 4C2 H4 ðOHÞ2 f 5K2 CO3 þ 5Mn2 O3 þ 3CO2 þ 12H2 O However, addition of several drops of 1,3-propanediol and small monohydroxyl liquid alcohols, such as methanol, ethanol, or isopropyl alcohol, did not cause any visible reaction with
Figure 1. Structures of glycerol (propane-1,2,3-triol) and ethylene glycol (ethane-1,2-diol).
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powdered KMnO4 crystals, even after 10 min. It seems at least two hydroxyl groups on neighboring carbon atoms (vicinal hydroxyl groups) are needed for a dramatic reaction with flames. Further evidence for the need for vicinal groups was found from studies with diamines, described in the next paragraph. Solid polyalcohols such as polyvinylalcohol (PVA), lactose, glucose, sucrose (ordinary cane sugar) (Figure 2), starch, and hydroxyethylcellulose (HEC) also did not react rapidly in a dramatic fashion when a powdered sample of each one was mixed with powdered KMnO4, even when a few drops of water were added. However, a delayed frothy reaction does ensue with glucose, sucrose, and lactose when ample water (2-3 mL) is added and the mixture stirred. Amines Next, I considered switching one or more of the hydroxyl groups in the polyhydroxyl compounds with an amine group. For example, I found that mono-, di-, and triethanolamine, which are all liquids at room temperature, reacted in a similar way with KMnO4 as glycerol (Figure 3). In addition, ethylenediamine (1,2-diaminoethane) and tetraethylenepentamine, both liquids, also gave a similar reaction, producing flames within seconds of their addition to KMnO4 crystals (Figure 4). However, 1,3diaminopropane and monoamines, such as isopropylamine ((CH3)2CH2NH2) and n-butylamine (C4H9NH2) gave no reaction with KMnO4. As observed with the hydroxyl compounds, this is further evidence that vicinal groups are needed for a reaction to take place with solid KMnO4. In addition, it was found that compounds in which the protons on the vicinal hydroxyl or primary amino groups are replaced with methyl groups gave no reaction with KMnO4. The examples that were tested were N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA) and 1,2-dimethoxyethane. Carboxylic Acids Primary alcohols can be oxidized to aldehydes or further to carboxylic acids. As pure polyaldehydes are not easily accessible from suppliers, I investigated the reaction of some simple and inexpensive nonpolymeric polycarboxylic acids with KMnO4. Oxalic acid (HOOC-COOH) is the simplest dicarboxylic acid and is very well known for its reaction with KMnO4 in aqueous redox titrations. The reaction is autocatalytic and surprisingly complicated, involving the less-known 3 CO2- radical (5, 6).
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Vol. 88 No. 3 March 2011 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed1004118 Published on Web 12/14/2010
In the Classroom
Figure 2. Structures of glucose (top left), sucrose (top right), and lactose (bottom).
back of the throat to get as moist a breath as possible and not to disturb the pile of powdered crystals. If the reagents are not crushed and powdered, the reaction seldom works. The water vapor in the breath is sufficient to initiate a reaction after a short delay of usually 5-10 s. No flames are produced, just a hissing or “whoosh” noise with smoke, leaving a dark brown residue. The reaction is not violent and does not spit solid material in any direction. However, as a precaution, it is important to immediately withdraw your face from the pile after quickly breathing on it. (It goes without saying that safety goggles must be worn for all chemistry demonstrations.) It is this demonstration that could be used as a sort of magic trick. Hazards
Figure 4. Structures of ethylenediamine (1,2-diaminoethane) (left) and the major component of commercial tetraethylenepentamine (right).
Crystalline KMnO4 is harmful if swallowed or inhaled. It is also harmful if absorbed through the skin. As demonstrated in this article, KMnO4 is a strong oxidizing agent and may react very exothermically with organic materials. The demonstrator should wear safety glasses and keep the solid or solution from contact with the skin. Take care not to allow the solid to come into contact with flammable materials other than when carrying out the demonstration. Oxalic acid (dihydrate or anhydrous) is toxic, corrosive, and can cause severe irritation, especially to the eyes, and possible skin burns. Therefore, the need for safety glasses and avoiding contact with skin also applies to oxalic acid.
The overall reaction in aqueous solution is as follows:
Demonstration
6Hþ þ 5H2 C2 O4 þ 2MnO4 - f 2Mn2þ þ 10CO2 þ 8H2 O
The demonstration can be carried out on an open bench as I have demonstrated many times to my students. It is very important to make up the mixture of powdered oxalic acid dihydrate and KMnO4 crystals immediately before carrying it out. About 1 g of each chemical is enough. The mixture must not be put in a closed bottle before the performance. As I mentioned earlier, this mixture can react by itself after a delay of a few minutes forming a lot of gas, which could cause the container to explode. The mixture of crystals is placed on a flat heat-proof surface. You can then give a little patter, saying that you can make this pile react using the power of your mind. So you lean into the pile, with safety glasses on and stare at it with serious concentration. After a few seconds, and without the audience knowing, you slowly breathe hot breath on the pile for 1-2 s. Immediately after doing this, as a safety precaution, withdraw at least 1 m from the pile. In a few seconds the exothermic reaction begins spreading through the pile with a “whoosh” noise and producing smoke. If done properly, it usually dumbfounds the audience, until you explain the secret. The secret is that moisture in the breath condenses as liquid water on the pile, dissolving a small quantity of the reactants. This triggers a redox reaction, which is exothermic and the heat generated continues to accelerate the reaction. The carbon-containing products are carbonate salts and CO2, which contain carbon in oxidation state þ4, the most stable oxidation state. As mentioned earlier, I would underline again that if the reagents are not crushed and powdered the reaction seldom works. I have done this reaction over 100 times and it has always been safe and consistent giving no flames or explosion. In fact, for those who are familiar with the KMnO4 plus glycerol reaction, the reaction of oxalic acid dihydrate with KMnO4 initiated by moist breath may seem a little tame. But as you need to get fairly close to this chemical mixture to initiate the
Figure 3. Structures of mono-, di-, and triethanolamine (left to right).
Oxalic acid is available as the dihydrate (HOOCCOOH3 2H2O) and the anhydrous compound. When powdered (crushed) crystals of anhydrous oxalic acid were thoroughly mixed with powdered KMnO4 crystals, no reaction took place for at least 15 min. However, if one drop of water is added, an immediate reaction takes place through the pile, with a “whoosh” or hissing noise and smoke but no flames. A possible explanation for the lack of flames is that oxidation oxalic acid does not form any flammable organic intermediates, in contrast to glycerol, which can react with atmospheric oxygen when hot. Besides water vapor and CO2, the final products of the reaction in the residue are a mixture and include black manganese(IV) oxide and manganese and potassium salts, probably as carbonates as observed in the reaction of KMnO4 with glycerol. Commercially available oxalic dihydrate crystals, taken from a new bottle, look a little sticky and less pourable as a powder than the anhydrous crystals. When powdered crystals of the dihydrate were mixed with KMnO4 crystals as supplied, no immediate reaction took place. However, on standing for some time (usually 10-30 min), a reaction took place, with similar observations as described earlier with KMnO4 and anhydrous oxalic acid when a drop of water was added. It seems that powdered oxalic dihydrate crystals are just sufficiently moist to initiate a reaction with KMnO4 without the need for a drop of water. I also found that I could initiate the exothermic reaction of crushed, powdered oxalic acid dihydrate crystals with powdered KMnO4 by breathing on the pile for 1-2 s from a distance of roughly 10-20 cm. It helps to breathe slowly and with a normal open mouth (not puckered for whistling or blowing) from the
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In the Classroom
Figure 5. Structures of ascorbic acid (left) and citric acid (right).
reaction, it is important that the reaction is sufficiently tame so as not to give the slightest possibility of injury. The residue left on the heat-proof surface usually contains unreacted KMnO4. The permanganate ions are a strong oxidizing agent, and proper disposal requires that it should be reduced with a 1 M solution of sodium hyposulfite until it is colorless, and then it can be washed down the drain. Interestingly, moist breath does not initiate a reaction between anhydrous oxalic acid and KMnO4, no matter how finely powdered you make the crystals. But as mentioned earlier, one drop of water will produce the desired effect. Addition of a drop of water is also known to initiate a violent reaction, with flames, between an intimate mixture of powdered ammonium salts and zinc metal powder (7). However, I could not promote a reaction by breathing moist breath for up to 10 s on to this mixture. I investigated if the reaction of other mixtures of organic acid compounds and KMnO4 could be initiated in a controlled fashion by breathing on them, but I did not find any. Formic acid (HCOOH), sold commercially as a 98% aqueous solution, reacts immediately with crystals of KMnO4. As drops of the acid are added to the pile of KMnO4 crystals, steam (or smoke) is produced, but no flames, and the pile glows red-hot. Liquid glacial acetic acid (CH3COOH) gives no reaction at all with KMnO4, even if drops of water are added. Powdered glyoxylic acid dihydrate (HOOCCHO 3 2H2O) looks even more moist and sticky than oxalic acid dihydrate. Not surprisingly, it reacts immediately when mixed with KMnO4 crystals, in a similar fashion to oxalic acid dihydrate. It seems even powdered glyoxylic acid made from a fresh commercially available bottle has sufficient “free” water to trigger an immediate reaction. Powdered glycolic acid (HOOCCH2OH), which also looks moist,
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reacted with KMnO4 crystals immediately as well. Malonic acid (HOOCCH2COOH), in which the carboxylic acid groups are now not vicinal, does not react with KMnO4 on mixing or by breathing on it, but one or two drops of water causes a slow reaction producing a frothy mixture and steam. Ascorbic acid shows the same reactivity as malonic acid but citric acid gave no reaction at all, even when water was added (Figure 5). Conclusion The reaction of small, liquid or solid amine or oxygenated organic compounds with solid potassium permanganate varies considerably. Vicinal hydroxyl or amino groups (but not tertiary amino groups) in liquid organic compounds are required to produce flames from reaction with solid potassium permanganate. Solid organic compounds need water to initiate a reaction, and this can be provided for by the water of crystallization in hydrated compounds, by adding a drop of water, or in the case of oxalic acid dihydrate, by breathing moist, warm breath onto the reaction mixture. Acknowledgment Mark Case (Emmaus High School, Emmaus, PA 18049) is thanked for checking this demonstration. Literature Cited 1. Steffel, M. J. J. Chem. Educ. 1990, 67 (7), 598–599. 2. Ruoff, P. J. Chem. Educ. 1987, 64 (7), 624. 3. Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison, WI, 1983-1992; Vol. 1, Experiment 1.35, p 83. 4. JCE Software: Chemistry Comes Alive. http://www.jce.divched.org/ JCESoft/CCA/samples/cca3glycerin.html (accessed Nov 2010). 5. Kovacs, K.; Vizvari, B.; Riedel, M.; Toth, J. Phys. Chem. Chem. Phys. 2004, 6, 1236. 6. Kovacs, K.; Grof, P.; Burai, L.; Riedel, M. J. Phys. Chem. A 2004, 108, 11026. 7. Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison,WI, 1983-1992; Vol. 1, Experiment 1.20, p 51.
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