Chemical Education Today
Letters Survey of Binary Azeotropes Our laboratory also settled on the water/1-propanol system as an example of a low-boiling azeotrope, for the same reasons recently outlined by Smith, Cooke, and Glinski (J. Chem. Educ. 1999, 76, 227). This replaced the ethanol/ carbon tetrachloride system, which was discontinued on the grounds of toxicity of the latter component. One immediate effect of this change was that the number of breakages of the vessel used to boil the mixture increased dramatically. This is thought to be due to the increased temperature and consequent change in the rates of heating and cooling. Certainly a change to the methanol/ethyl acetate system has resolved the problem, without the need for any changes to the apparatus used. This suggests that caution needs to be exercised if mixtures in the higher range of boiling points are selected; this principally affects those involving water. Paul C. Yates Department of Chemistry Keele University Staffordshire ST5 5BG, UK
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Cheating Probabilities? Probably Not Charles Jonah ( J. Chem. Educ. 1998, 75, 1089) criticized the note by Rizzuto and Walters ( J. Chem. Educ. 1997, 74, 1185) that gave a binomial-theorem-based calculation of “cheating” probabilities (an unfortunate term). Jonah set up a specific case, two students each achieving 80% on a 50question, 5-choice exam, where they had the minimum possible number of answers in common. Since each must have “missed” (i.e., answered incorrectly) 10 in order to achieve 80% scores, Jonah reasons that this minimum possible number is 30 (each student errs on a different set of 10 answers). He takes Rizzuto and Walters’ tables to give this scenario a probability of 5.83 × 10 {10. He concludes that one would be led to suppose “that it is very unlikely that two students would have exam grades around 80% without cheating”. However, there are many, many more ways of any two students achieving 80% than this particular one (i.e., all those where various combinations of their “wrong” answers
are in common). The specific event whose probability is reported is indeed highly unlikely because it is only in their correct answers that this pair of students’ responses are identical; its probability will thus be even lower than stated. “Cheating” involving just correct answers is itself implausible; such an outcome would imply genuine knowledge (“knowledge” probably including prior sight of the exam paper!). Rizzuto and Walters are partly aware of the problem; their original paper correctly states that “the probability…of identical responses becomes larger the better the students”. However, they do not appear to be aware of the magnitude by which the neglect of this factor would distort the probabilities. The most obvious case is for two students achieving 100% scores. Thus, Jonah’s intentionally ironic criticism shows us that we have no intuitive guide to the probability level at which to suspect “cheating”. His remarks make clear that, without more detailed analysis, Rizzuto and Walters’ paper regrettably cannot find practical application. The difficulty of deeper analysis lies in the assumption (necessary for Rizzuto and Walters) that all responses are equally probable. This demands that the “cheater” either has, or applies, zero knowledge in the test. In any realistic scenario, the cheater will surely copy only part of another script, else he or she would have 100% congruence of responses (but not necessarily score 100%). Any attempt to analyze for possible clues to “cheating” is paradoxically best restricted to considering congruence in the incorrect answers. With a colleague, I have recently analyzed the probabilities of Multiple Choice Question exam scores where the extent of the guessed element is defined (that part where the “equally probable” assumption holds) and thus binomial theory can be applied (1). It reveals that, for defined knowledge levels, high probabilities for a very wide scatter of scores are unavoidable, unless guessing is inhibited. For many typical Multiple Choice exams, we should ask “exactly who is cheating whom?” between examiner and examinee. Literature Cited 1. Burton, R. F.; Miller, D. J. The Statistics of Multiple-Choice and True/False Tests; Assess. Eval. Higher Educ. 1999, 24.4, in press. David J. Miller Institute of Biomedical & Life Sciences University of Glasgow, G12 8QQ Scotland, UK
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Chemical Education Today
Letters trans-Cyclohexane-1,2-diamine
Micropreparation of [RuH2(PPh3)4]
I read with interest the article by Walsh, Smith, and Castello on the resolution of trans-cyclohexane-1,2-diamine and the determination of its optical purity by chiral solidphase HPLC (1). The remarkable growth in the manufacture of single-enantiomer drugs does, indeed, mandate that the laboratory techniques discussed by the authors be introduced into the undergraduate curriculum. I am writing to offer one update and one correction to the article. A recent report in C&E News brings us up to date on the dramatic increase in the production of chiral drugs as pure enantiomers (2). Senior editor Stephen Stinson states that worldwide sales of these drugs reached almost $90 billion in 1997. This is more than double the amount that was predicted for 1997 just a few years earlier and that was cited by Walsh et. al. Of the 100 top-selling drugs today, no fewer than 50 are single enantiomers. I must correct the statement by the authors that the first successful resolution of the diamine was reported in 1972. In fact, it was resolved 35 years earlier by Jaeger and Bijkerk (3). These workers used the same diastereomeric tartrate method described by Walsh et. al. My interest in the compound dates to the early 1960s when I was a student in the research group of T. S. Piper at the University of Illinois. I routinely resolved it by Jaeger and Bijkerk’s method and used it as a ligand for preparing optically active coordination compounds (4 ). In the days before chiral solid-phase HPLC the optical purity of the compound was established by showing that its diastereomeric tartrate could be recrystallized to a constant optical rotation.
The micropreparation of [RuH2(PPh3)4] recently reported in this Journal (1) is a good proposal for the advanced inorganic chemistry lab. However, the reaction for the synthesis of the starting material, [RuCl2(PPh 3) 3]:
Literature Cited 1. Walsh, P. J.; Smith, D. K.; Castello, C. J. Chem. Educ. 1998, 75, 1459. 2. Stinson, S. C. Chem. Eng. News 1998, 76(Sep 21), 83. 3. Jaeger, F. M.; Bijkerk, L. Proc. Acad. Sci. Amsterdam 1937, 40, 12. 4. Treptow, R. S. Inorg. Chem. 1966, 5, 1593. Richard S. Treptow Department of Chemistry and Physics Chicago State University Chicago, IL 60628-1598
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RuCl3?3H2O + 3PPh 3 + CH3OH → [RuCl2(PPh 3) 3] + 3H2O + HCHO + HCl poses problems for credibility unless a good supporting reference can be provided. The fact that it is the sole unbalanced equation among those involved in the article would be irrelevant if not were for the formation of HCHO, which implies that the solvent (CH3OH) is the reducing agent. If this were the case, why does the procedure described afterward make use of more than twice the stoichiometric amount of PPh3 when this class of 16-electron species has low tendency to dissociate, a carefully deoxygenated solvent is used, and a very small amount of PPh3—if any—is blocked by the HCl produced?. It should be noted that this synthesis is not unique. A significant number of PPh 3 complexes of the later transition metals—which are useful starting materials for other compounds—are prepared in high yield in a one-pot synthesis from commercial products. Representative examples, other than the one mentioned, are ReOCl3(PPh3) 2 from HReO4 (2), RuCl2(PPh3)4 from RuCl3?3H2O (3), OsCl2(PPh3) 3 from (NH4) 2OsCl6 (4), Wilkinson’s catalyst from RhCl3?3H2O (5) and AuCl(PPh 3) from H[AuCl 4]?4H2O (6 ). I have failed to find in the literature a good explanation for the mechanism of these interesting reactions, which doubtless require a significant number of elementary steps, but in none of the above syntheses is formation of HCHO mentioned. Most of the early reports only mention either the complex or the complex plus “other products”, presumably because the other products are of secondary interest. However, in order to obtain good yields of these valuable complexes, an excess of PPh 3 is regularly used, consistent with the assumption that PPh3 works as the reducing agent. That PPh 3 is the main reducing agent becomes evident in the closely related synthesis reported in ref 3, where RuCl3?3H2O is dissolved in CH3OH, refluxed for 5 minutes, allowed to cool to room temperature, and treated with a 6-fold excess of PPh3. In this moment the solution begins to darken to deep brown. Furthermore, we can read in ref 4 (preparation of the homologous osmium complex) the formation of PPh3Cl2; in ref 5 the formation of Cl2PPh 3, followed by hydrolysis to OPPh3; and in ref 6 the formation of OPPh 3. Thus, it is improbable that CH3OH plays the role indirectly attributed and that HCHO forms. HCHO is a more kinetically active reducing agent than CH3OH and often works as a good source of CO for preparing compounds such as OsH(Cl)(CO)(PPh3)3 from Na2OsCl 6 (7 ) and trans-RhCl(CO)(PPh 3) 2 from RhCl 3?3H2O (8), as well as RuH2(CO)(PPh 3)3 from RuCl 3.3H 2O (9).
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
Chemical Education Today
Letters Literature Cited 1. Linn, D. E., Jr. J. Chem. Educ. 1999, 76, 70. 2. Johnson, N. P.; Lock, C. J. L.; Wilkinson, G. Inorg. Synth. 1967, 9, 145. 3. Hallman, P. S.; Stephenson, T. A.; Wilkinson, G. Inorg. Synth. 1970, 12, 238. 4. Elliott, G. P.; McAuley, N. M.; Roper, W. R. Inorg. Synth. 1989, 26, 184. 5. Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 27. 6. Braunstein, P.; Lehner, H.; Matt, D. Inorg. Synth. 1990, 27, 218. 7. Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg. Synth. 1974, 15, 53. 8. Evans, D.; Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 79. 9. Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg. Synth. 1974, 15, 48. Francisco J. Arnáiz Laboratorio de Química Inorgánica Universidad de Burgos 09001 Burgos, Spain
The author responds: There is agreement with Arnáiz that some ambiguity exists concerning the reducing agent in some of the preparations of transition metal phosphine complexes. The JCE article referred to ( J. Chem. Educ. 1999, 76, 70) does indicate in eq 1 that methanol is the reducing agent in the preparation of [RuCl2{P(C6H5)3}3]. The emphasis here was purposeful and pedagogical; whether good or bad, that is up to the reader to decide. I always make it a point to give a plausible balanced chemical equation for a chemical reaction because I think it serves the students well, to help them to determine how the “reaction goes”. Nevertheless, I think the matter of which atom is oxidized here, C or P, deserves some special comments. I would agree with Arnáiz that P(III) can undergo oxidation in such reactions but these are cases where no oxidizable alcohol is present. The case of P(V) formation (i.e., P(C6H5)3Cl2) in the preparation of [OsCl 2{P(C 6H5) 3} 3], which Arnáiz cites, employs the alcoholic solvent 1,1-dimethylethanol, one without a hydrogen on the α -carbon and which is not readily oxidized (1). One authoritative source makes the statement that the preparations of organophosphorus complexes of ruthenium involve “interaction of RuCl3?3H 2O, K2OsCl6 or other halide species with PR3 in an alcohol or other solvent”, and further, “either hydride or CO may be abstracted from the solvent molecule” (2). An estimate of the energetics of these two processes can be made using compilations of thermochemical data, assuming and ∆∆S° ≈ 0 and ∆Cp = 0. Here the P(III) to P(V) conversion can be approximated by ∆G °[PV/PIII] ≈ ∆H°[PIII/PV ] = {124 kJ mol{1 using a model
for P(III) oxidation (PCl3 + Cl2 = PCl5) (3). Likewise the dehydrogenation of methanol proceeds to the acetal and we can estimate here, ∆H °[CH3OH/CH2(OCH3)2] = {147 kJ mol{1 (4, 5). This simple analysis suggests that the oxidation of methanol to the acetal is the most likely scenario. It would be helpful, however, to have some corroborative spectroscopic data. A color change in a reaction such as the one Arnáiz cites can be misleading unless the species displaying the colors are known with certainty. The need to add excess phosphine could be to force an equilibrium to completion, and a color change occurring subsequent to addition may be the result of forming a more potent oxidant at the ruthenium center upon further phosphine coordination. From a kinetic standpoint, the construction of two 5coordinate centers, [RuCl2{P(C 6H 5)3} 3] and P(C 6H 5)3Cl2, would require a very crowded transition state. From all this it can be taken that there is evidence that the reduction at the ruthenium center in eq 1 involves a hydride transfer reaction. The only source of hydride, albeit not highly active, is the solvent methanol. The product HCHO is a more active reductant than methanol but it is not present in large amounts and reacts in methanol to form the less reactive 1,1-dimethoxymethane. Another analogous reaction is clearly represented by the reduction of RuCl3, which forms ethanal, in refluxing ethanol and 1,5-cyclooctadiene to prepare di-µ-chloro(η4-1,5cyclooctadiene)ruthenium(II) polymer (6 ). It might also be pointed out that ruthenium complexes such as [RuH 2N 2{P(C 6H 5) 3} 3 ] and [RuH 2{P(C 6H 5)3}3 ] decarbonylate methanol to give [RuH2(CO){P(C6H5)3}3] (7, 8). This would agree with the generally accepted hydride transfer mechanism, first to give HCHO and then the decarbonylation of formaldehyde to give H2 and [RuH2(CO){P(C6H5)3}3]. Literature Cited 1. Elliot, G. P.; McAuley, N. M.; Roper, W. R. Inorg. Synth. 1989, 26, 184. 2. Cotton F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1998; p 894. 3. Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; Dekker: New York, 1985; pp 157, 192. 4. Stull, D. The Chemical Thermodynamics of Organic Compounds; Wiley: New York, 1969. 5. Pilcher, G.; Fletcher, R. A. Trans. Faraday Soc. 1969, 65, 2326. 6. Alber, M. O.; Ashworth, T. V.; Oosthuizen, H. E.; Singleton, E. Inorg. Synth. 1989, 26, 69. 7. Cole-Hamilton, D. J.; Wilkinson, G. Nouv. J. Chim. 1977, 1, 141. 8. Linn, D. E., Jr.; Halpern, J. J. Am. Chem. Soc. 1987, 109, 2969. Donald E. Linn Department of Chemistry Indiana University-Purdue University at Fort Wayne Fort Wayne, IN 46805-1499
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