T o the Editor: The paper of Burrell on "Balancing Organic Redox Equations" in the February issue called to mind the earlier, quite extensive discussions of this problem in THIS JOURNAL. A summary and bibliography of the early comments and the most general method for balancing any equation, redox or otherwise, are given by Bennett, THIS JOURNAL, 31, 324 (1954). AS Bennett points out, the "Oxidation Number" and "Ion-Electron" methods of balancing equations are useful because they are more convenient and quicker than the general method. While the method proposed by Burrell may have some validity from a mechanistic viewpoint in a number of cases (WALLING,C., "Free Radicals in Solution," John Wiley & Sons, Inc., New York, 1957, chapter l l ) , in view of the somewhat involved rules required and the necessity of finding the change in oxidation number of the oxidant., I think that the claim that this method allows balancing by inspection is overoptimistic. Unless a better understanding of the true mechanism of the reaction heing studied is obtained, the use of this new method will only serve to complicate the problem. I n particular, the application of this method to the oxidation of K4Fe(CN)aby Mn04- ion in acid solution seems to me to be a step backward. If the products quoted in this example are correct, which I doubt, since ferrocyanide can be titrated to ferricyanide in acid solution with permanganate, the substances taking part in the reaction (hydrated forms being disregarded) would be Fe(CN)8-4, H+, Mn04-, Fef3, NOs-, Cop, Mn++, and H20. To ask the student to think of the presence of KOH, Fe(OH)2, Fe(OHh, and unionized HNOs is to undo the work of the general chemistry course. A comparison of the reduction steps used by Burrell written in ionic form with the commonly used half reactions shows that what has been done is to simply leave out the electrons. This has the effect of changing OH-ions, if the reaction takes place in alkaline solution. or HtO molecules in acid solution, to OH radicals when charge equality between reactants and products is preserved. For example:
Incidentally, although Burrell states that a perfect material balance has been obtained without the use of ionic forms in his table of oxidizing agents, a mole of water is missing from the reactant side in the reaction of KMn04 in acid solution, and a mole of water is missing from the product side in the reaction of KzCrzO; in acid solution. Perhaps these omissions were oversights. With reference to the oxidation steps proposed, the same comments apply to the reciprocal relationship of electrons and OH radicals. Actually the balancing of organic redox reactions can he carried out quite readily by the ion-electron method taught in many general chemistry courses as shown by the following examples. 474
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Journal o f Chemicol Education
(A) The oxidation of propylene in alkaline solution for which the over-all molecular equation is
-
+ KMn04CH,COOK + K&08 + MnOl + KOH + HpO
CIHP
The reduction step is the usual half reaction of MnO4ion in alkaline solution, 34-
+ Mn04- + 2H.O
-
-
MnOl
+ 40H-
The oxidation step is C3H6
+ 130H-
CHICOO-
+ COar2+ 8H?O + 10e-
Certainly in alkaline solution, the predominant species are acetate and carbonate ions, not the acids. After the usual cross multiplication, addition, and cancellation of excess substances, the final result is
If the molecular equation is desired, 10 K + ions can be added to each side giving
-
+ 10KMn0, 3CH.COOK + 3K3COa+ lOMn01 + KOH + 4Hr0
3C3Ha
(B) The oxidation of 1-menthol to 1-Menthone by dichromate in dilute sulfuric acid:
+
+
C10H200 KPCI~O, Hi301
-
+
C ~ H ~ OCn(S033
+ H?O
The reduction step in this case is 66-
+ Cr.OiP + 14Ht
-
+ 2Cr+=
+ iH20
The oxidation step is
+ 2H+ + 2e-
CloH?~O ClaHlsO
no organic ions being involved. Again multiplication, addition, and cancellation result in the final ionic equation,
+
3CisH?o0 Cr20i?
+ 8Ht
-
+
3CIOHIP0 2Cr+*
+ 7H:O
Again, if t,he molecular equation is preferred, 2 K+ ions and 4S04-- ions must be added to each side giving.
To the Editor: The merit of Professor Gaddis' "Titration Without Burets," THIS JOURNAL 36, 290 (1959), will be considerably enhanced if the solutions are weighed on a rough balance to the nearest tenth of a gram, neglecting bouyancy corrections. One anticipates a precision of fO.5yO being obtained in this way. Weighing to within 0.010 g with bouyancy corrections should easily surpass the accuracy of an ordinary buret. EDWARD T. R ~ D L E Y MEMPHISSTATEUNIVERSITY MEMPHIS, TENNESSE
