On the Reaction in the Cobalt(l1)-Hydrogen PeroxideTartaric Acid System To the Editor: I read with interest the article by Paul T. Ruda [J. CHEM. EDUC., 55 615 19781 in which the author suggests the reaction hetween tartaric acid and hvdroeen catalvzed hv .. peroxide . c11ha1tchloride, for the demnnstration of homogeneous catalvsis. The reartion is reoroducihle. but the i n t e r ~ r ~ t a t i o n . given by the author, is incorrect. The color change, seen at the end of the reaction. is not the consequence of the complete oxidation of tartaric acid, hut of totaihydrogen peroxide decomposition. This should be obvious, as, according to the composition of the reaction mixture, given by the-author: 4 . 0 8 8 mol KNaC4H406.4Hz0and -0.18 mol HzOz, there is not enough hydrogen peroxide for the oxidation of all of the tartaric acid to Con. The resulting pink solution, therefore, contains a cohalt(I1)-tartrate complex; this is supported by its absorption spectrum, and the observation, that any further reaction-can he induced only by addition of hydrogen peroxide. Mass-spectrometric analysis of gaseous samples withdrawn in different phases of the reaction indicates simultaneous evolution of 01and C01. As the reaction proceeds. the 07 to coz ratio decreases, and a t the end of thbeaction; when the color of the solution changes from green to pink, only a small amount of COz can be detected, without any Oz. If a sufficient auantitv of hvdronen ~eroxideis provided all of the tartaric aiid hecimesbxid&ed, hut this & not manifested in any color change. The solution remains green provided hydrogen peroxide is present. With the oxidation of the tartrate salt the pH of the solution increases, and in case of larger quantities this can lead to the hydrolysis of Co(III), when all the tartrate becomes destroyed. Based on these experimental findings the following reactions can he s w ~ e s t e dtaking place in the Co(I1)-HzOz-tartaric acid system: ~
~
a
On t h e action of hydrogen peroxide rhe eohnltlll)-tartrate nrrnplex hemme* widired to a green, probohly hinurlenr, CwIl11-tartrate comnound. Thiscol,alt~IIl,-tartrarris reduced bothin tartariracid and hydrogen peroxide to Co(I1)-tartratewith a eonc&itant evolution of COz and 0 2 , respectively. Since the colour of the solution is green throughout the reaction, that is most of the cobalt is present as Ca(III),the first step (oxidation) should be essentially faster, as the reduction of Co(II1)-complex. Zoltin T6th Institute of Physical Chemistry Kassuth Lajos University H-40lO.Debreeen, Hungary Bent Thermometer To the Editor: Robert T. Morrison has kindly pointed out to me two relevant references that I omitted in my discussion of "The Reducing Action of Sodium Borohydride" [J.CHEM. EDUC., 56,540 (1979)l. These are Chaikin and Brown, J.Amer. Chem. Soc., 71,122 (1949) and Johnson and Rickborn, J. Org. Chem., 35,1041 (1970). A consultation of these two papers will perhaps redress the balance a hit. When slaying dragons one is often tempted to overdo tpngs a trifle! David Todd Worcester Polytechnic Institute Worcester, MA 01609 464 1 Journal of ChemicalEducation
Preparation of the Sweetening Agent P4000: A Variation
To the Editor: We have been using for several years the P4000 experiment described in this Journal, hut with a variation which might interest vour readers. In the oreparation orieinallv " described l-(l-propoxy)-2,4-dinitrohenzeneis reduced by aqueous sodium disulfide to a mixture of the two isomeric amino,nitrol-(l-propoxy)henzenes, and these are then separated on the basis of the differine solubilities of their hvdrochloride salts. Our variation is to use column chromatnzra~hv .tn effect the separation. The P4000 is absorbed on alum&ium oxide less strongly than its isomer and is eluted first. In practice, the hands from the two compounds usually overlap, so the eluate is collected in small portions and these are then examined hy thin-layer chromatography, to check their purity. This procedure is a good illustration of how TLC has better resolving power than column chromatography, and shows how by collecting the eluate in portions one can get round the worst of this problem. Understandably, students find this an enjoyable and rewarding experiment. A chromatoeraohv column (20 mm X 600 mm) is aonroximately half-filGd with toluene A d then aluminium oxkfe (125 ml; activity 11) is trickled in with gentle tapping to remove air bubbles. The liquid is then run out of the column until the level is about 80 mm ahove the top of the solid. The mixture of amino,nitro(l-propoxy)henzenes (6 g) is dissolved in acetone (15 ml) and aluminium oxide (25 ml: activitv 11) is added. The solvent is then removed from this mixture ;sing a rotary evaporator to aive a fine free-flowina powder. This powder is trickled in on ;up of the solid in the column. and finilly a l~ttle sand is added. Tht. level of the liquid in the column i i ~diusted so it is just ahove the sand-on-no account should it at any time drop lower-and then the column is carefully filled with toluene. The two compounds are then eluted by running toluene through the column. renlenishine it at the too as necessarv. The initial colorless k G t e is discarded, and the yellow haid is collected in aliquots of about 25 ml. After two such fractions have heen collected, diethyl ether is gradually added to the toluene at the top of the column withgentle stirring to ensure the ether mixes well. Finally pure ether is used to complete the elution of the vellow hand. The contents of each aliquot are examined using a TLC plate (solvent: toluenelethyl acetate = 411). and those havinn the same one component are combined. These solutions are transfered to weighed roundbottomed flasks, and the solvents are removed on a rotary evaporator. T h e ~ 4 0 0 0solidifies at room temperature, and is already pretty pure. The isomer, 4-amino-2-nitro-l-(lpropoxy)benzene, solidifies on cooling to On. The infrared spectra of the two products are recorded, as well as their melting points.
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' de Koning, A. J., J. CHEM. EDUC., 53,521, (1976). J. McK. Woollard University College of Botswana Private Bag 0022 Gaborone, Botswana
