Chemical Education Today
Letters The Clemmensen Reduction The article by J.-P. Anselme on oxidation-reduction in organic chemistry (J. Chem. Educ. 1997, 74, 69) gives an introductory overview, well suited for undergraduate students, on two of the most important synthetic transformations. The author clearly states that the purpose of the article is a description of the overall redox processes without mechanistic implications. However, regarding the Clemmensen reduction of carbonyls, the inclusion of sequential reactions in which alcohols are ultimately deoxygenated to hydrocarbons is strange and, to some people, perhaps, may even be quite confusing. It is well established that the Clemmensen reduction does not involve the formation of alcohols and, in fact these substances fail to give the reaction (e. g. March, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992; pp 1209– 1211). Even though pinacols (vic-diols) are often side products of the Clemmensen reduction, the mechanism is still unclear and numerous intermediates including zinc carbenoids have been proposed. There is a certain evidence of a complex pathway involving single electron transfers (SETs) to give a cascade of radical anions followed by nucleophilic displacements (Di Vona, M. L.; Rosnati, V. J. Org. Chem. 1991, 56, 4269). It should also be noted that alcohols are not intermediates in the complementary Wolff–Kishner
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reduction either, which is otherwise correctly discussed in the paper. These remarks are not intended to detract from the interesting work by J.-P. Anselme, but to avoid a misinterpretation of the Clemmensen reduction which could be considered as a combination of hydrogenation and deoxygenation of the carbonyl group. Juan C. Palacios and Pedro Cintas Department of Organic Chemistry-UEX E-06071 Badajoz, Spain
. Balancing Organic Reactions Anselme’s wish to “balance the books” in organic redox reactions (J. Chem. Educ. 1997, 74, 69) without involving often complex mechanisms has been dealt with previously by simply assigning oxidation numbers (ON’s) to atoms which change in the redox process (J. Chem. Educ. 1988, 65, 45). The student is required to write suitable Lewis structures and allocate bond electrons to the more electronegative atom in a bond. Balancing redox couples is then common to inorganic and organic reactions.
Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
Chemical Education Today
Letters Letters continued from page 938
Thus the alkene hydrogenation is written as C
C
C (0)
(0)
(0)
C
C
C
C
+
(-I)
H2
(-I)
C
C
H
H
(+I)
(+I)
C
C
C
or more concisely as 2 C(0) + H2(0) → 2 C(-I) + 2 H(I) For ethene, the carbon couple would be C(-II)/C(-III). The Clemenson reduction is C(II) + 2 Zn(0) → C(-II) + 2 Zn(II). Alcohol oxidation with nitric acid becomes O
H C
+
N
OH
C
OH
OH +
(II)
(V)
(0)
O
N (III)
OH
+
H2O
O
Oxidation with chromates vary with the reaction medium. In aqueous media, as in the old “breathalyser” test for ethanol, green Cr(III) is the stable reduced state. In organic media, using reagents such as pyridinium halochromates, a black solid with Cr(IV) is formed. The stoichiometry changes from 2 Cr(VI) + 3 C(0) → 2 Cr(III) + 3 C(II) to Cr(VI) + C(0) → Cr(IV) + C(II). The unquantifiable discussion on changes in electron density is not particularly helpful. Charges on atoms are nonobservables which are evaluated by ab initio calculations. They vary greatly with basis set and method of apportioning electron density. Correlations with ESCA C(Is) shifts at least put carbons in order of oxidation state. The ON method avoids these difficulties by “ionizing” covalent molecules. This is usually a meaningful exaggeration unless more distant neighbors greatly alter charge distributions. In any event the conclusions drawn from Anselme’s analysis is no different from those by the shorter ON balancing in all the reactions considered. A. A. Woolf University of the West of England Bristol BS16 1QY, UK
. Precision and Accuracy Your moratorium on papers concerning balancing equations could be profitably extended to papers on error propagation and significant figures. Formulas for error propagation in all the basic functions were rigorously derived and published almost 30 years ago by H. H. Ku (1969), “Notes on the Use of Error Formulas”, pp 331–341 in “Precision Measurement and Calibration”, NBS Special Publication No. 300, Vol. 300, H. H. Ku, Ed., U.S. Government Printing Office, Washington. Every university library has a least one copy of this basic document. Standard procedures for determining and maintaining significant figures in calculations are given in ASTM Desig970
nation E-380-91, American Society for Testing and Materials, Philadelphia. This standard has been adopted by many if not all U. S. Government departments and administrations, and it is contractually binding in many if not all federal contracts and grants. University researchers should read the boiler plate in their grant proposals; they probably have promised to adhere to the ASTM standard. Correct methods for dealing with the arithmetic of research are obviously important to your readers, otherwise you would not be inundated with papers on these already solved and standardized problems. The obvious need could be met by including as a monthly feature in JCE a box listing the standard references in the problem areas. Robert M. Sykes Department of Civil & Environmental Engineering & Geodetic Science The Ohio State University Columbus, Ohio
W. Robert Midden’s article, “Rounding Numbers: Why the ‘New System’ Doesn’t Work” (J. Chem. Educ. 1997, 74, 405) is good and should get more attention, especially in Japan. The “New System” is a common practice when rounding numbers in Japan. It has been around for many years, and school children are still learning this system. In Japanese, it is called “shi-sha, go-nyu,” which translates to “drop 4, enter 5”. This is exactly the “New System” of always rounding five up. This Japanese practice of rounding numbers is being used by everyone as far as I know. But in banking and finance, numbers are never rounded up. The last digits are always dropped. It is called “kiri-sute”, which literally means “cut and throw it away”. So consumers never get the benefit of the last digit when receiving an interest on their deposits. As pointed out in the article, the “New System” and prevailing Japanese practice of “shi-sha, go-nyu” do accumulate error. I think that at least scientists and engineers should use the Round 5 Even rule. How about the people in finance? Ken Muranaka K’s Garden Nishioji, Suite 401 Kasuga Hachijo Sagaru Minami-ku, Kyoto 601-8312 Japan
Two recent articles published in this Journal (1,2) involve round-off rules. The first article proposes to round up when dropping a 5 whereas the second supports the “old” rule of rounding to an even number when dropping a 5. Both of these articles miss the meaning of significant figures, and strict adherence to either method could cause confusion for students. According to the rule of significant figures, one rounds off a number so as to contain all of the certain digits (figures) plus one uncertain digit (figure). This uncertain digit (figure) is uncertain by at least one unit. Thus, it does not
Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
Chemical Education Today
matter which of the above methods is used. This conclusion is implied in the first article (1) but is not pursued. For example, rounding 0.8665 to three significant figures gives 0.867 by the former method and 0.866 by the latter method. However, the two results are the same within the minimum experimental uncertainty, 0.867± 0.001 and 0.866±0.001. What is important for students to understand is that there is uncertainty in measured values, and the final value must convey to other persons this uncertainty. Thus, we have the rule of significant figures and the round-off procedures. Until students actually determine the numerical value of the uncertainty in the last digit (figure), it is important for them to realize that the last digit is uncertain by at least one unit, and strict adherence to either of the above methods contradicts this. Literature Cited 1. Guare, C. J. J. Chem. Educ. 1991, 68, 818. 2. Midden, W. R. J. Chem. Educ. 1997, 74, 405. Douglas Rustad
Department of Chemistry Sonoma State University Rohnert Park, CA 94928 The authors reply: It is always surprising to find that so few people understand significant digits and their implications for rounding off. It is even more surprising to find that a rule that has been in place on a global scale for centuries, a rule that Midden and Rustad have accepted throughout their lifetimes but without realizing it, is not workable. Rounding rules are not necessary whenever it is possible to calculate uncertainties in numerical results. But unfortunately, all text books seem to carry them. The rule I suggested, “raise the last retained digit in a calculated number by one if the first dropped digit is 5 or higher, otherwise do not raise”, is extremely simple and should cause no confusion among students. It also happens to be the standard used in accounting, banking, business, making out your income tax to the nearest dollar, and by the spreadsheet on my computer when I specify the number of decimal places to be retained. Wouldn’t it be nice if science faculties related science to the rest of the world? To be more specific, taking the example used by both Midden and Rustad, 35.7/41.2, it is easy to establish what the correct minimum uncertainty in the quotient is if you do not, as Midden and Rustad did, ignore the rules while doing the division. By convention, each number is taken to be uncertain by at least one in the tenths position. Since each number may have three values, there are nine possible outcomes for the division. Carrying four digits, the range of quotients then is seen to be: 35.8/41.1 = .8710 35.8/41.2 = .8689 35.8/41.3 = .8668
35.7/41.1 = .8686 35.7/41.2 = .8665 35.7/41.3 = .8644
35.6/41.1 = .8661 35.6/41.2 = .8640 35.6/41.3 = .8619
The second digit is essentially unchanged, showing a value of six in eight of the nine quotients. The third digit is quite variable, showing values of one through eight, and would be the first uncertain digit to be retained. The fourth digit then is so shaky that it should be discarded and not used as a criterion for rounding or anything else. The diagonal, top left to bottom right, .871 – .866 – .861, represents the maximum, best estimate and minimum quotients and is covered by .866 ± .005, not by ± .001 which is not the minimum experimental uncertainty as Rustad claims. If you insist upon avoiding the easy calculation of the maximum, best estimate and minimum to get the range and correct uncertainty, and just round off Midden’s 10 digit number, .866504854, the best bet is .867, the value that agrees with my rule and would be used throughout the world by millions of people who are not confused by any scientific training. Midden’s claim that all ten digits in his quotient are significant is incorrect as shown by my calculations above and the definition of significant digits, which is “all the certain digits plus the first uncertain digit”. If all ten digits in his quotient are significant, his 35.7 and 41.2 must have been exact numbers (no uncertainty), something that does not occur in experimental measurement, and there should be no rounding at all. Charles J. Guare 8 Anita Drive Scotia, NY 12302
Douglas Rustad raises important points: students should be aware of the meaning of significant figures, they should understand the concept of uncertainty in measurements, and they should understand the basis for statistical treatment of errors when they apply any type of rounding rules. But he is in error when he says that it does not matter which of the two rounding methods (1, 2) students use. While this is true for the example Rustad cited, he has failed to take into account other cases where the accumulative error is greater for the rounding up method (2) than it is for the round even method (1), as explained by Midden (1). Because of smaller accumulative error, the round even method (1) introduces less error on average than the round up method (2). While students should never mindlessly apply any rounding rule, when they do appropriately use such a rule, the round even rule will minimize errors on average. Literature Cited 1. Midden, W. R. J. Chem. Educ. 1997, 74, 405. 2. Guare, C. J. J. Chem. Educ. 1991, 68, 818. W. Robert Midden Department of Chemistry Bowling Green State University Bowling Green, OH 43403-0213
[email protected] .
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