In the Laboratory
The Microscale Laboratory
W
A Microscale Oxidation Puzzle
Michael W. Pelter,* Rebecca M. Macudzinski, and Mary Ellen Passarelli Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323; *
[email protected] It has been reported that sodium hypochlorite in acetic acid solution selectively oxidizes secondary alcohols to ketones in the presence of primary alcohols (1). A discovery-oriented experiment based on selectivity was recently published in this Journal (2). This experiment was based upon the selective oxidation of 2-heptanol in equal mixture with 1-heptanol. Herein we report our adaptation of this reaction to a “puzzle” approach by using a diol as the substrate for oxidation. The diol under investigation has both a primary and a secondary hydroxyl group (see Scheme I for an example using diol 1). The students are told that there are three possible outcomes to the reaction: (i) only the primary alcohol is oxidized to the aldehyde (2); (ii) only the secondary alcohol is oxidized to the ketone (3); or (iii) both alcohols are oxidized (4 ). Their assignment is to perform the reaction and determine the structure of the product through interpretation of the IR spectrum. As a prelab question, the students are asked “How could compounds 2, 3, and 4 be distinguished by IR spectroscopy?” 1°
O H
OH 2
HO
NaOCl
2° HO
OH
Discussion The oxidation of diol 1 by sodium hypochlorite solution proceeds quantitatively (based on GC analysis) and is selective for the secondary hydroxyl group, yielding the hydroxyketone 3 as the only product, as confirmed by NMR and GC/MS analysis. The IR spectrum shows the presence of both an OH stretch and a C=O stretch. Therefore, students must remember how to distinguish between an aldehyde and a ketone. Although the carbonyl stretches are identical, the ketone lacks the aldehydic C–H stretch. Any diol containing both a primary and a secondary diol can be utilized. We use 2-ethyl-1,3-hexanediol (1) and 2,2,4trimethyl-1,3-pentanediol.W Both these diols are commercially available at an inexpensive price. Conclusion We use this experiment in place of the traditional oxidation of cyclohexanol to cyclohexanone (3), to introduce our students to the operation of the FTIR and to the procedure for obtaining an IR spectrum of a liquid sample. The experiment can be used at any time during the semester, provided that IR interpretation has been covered in lecture. It is well received by the students because of its relative ease and because they get to use the FTIR. The important lessons of this experiment are the use of IR spectral analysis in product identification, and the selectivity of chemical reactions.
O 1
Acknowledgment
3
We would like to thank National Science Foundation (DUE-9650826) for the funds to purchase an FTIR.
both O H
O 4
Scheme I
WSupplemental
Material
Only the oxidation of diol 1 to compound 3 is presented in this lab summary. Experimental procedures for the oxidation of both compounds are in this issue of JCE Online.
Hazards Standard laboratory safety practices should be observed at all times while performing this experiment. Glacial acetic acid is a dehydrating agent, an irritant, and causes burns. Dispense it in a fume hood and avoid contact with skin, eyes, and clothing. Sodium hypochlorite solution emits chlorine gas, which is a respiratory and eye irritant. Dispense it in a fume hood.
Literature Cited 1. Stevens, R. V.; Chapman, K. T.; Stubbs, C. A.; Tam, W. W.; Albizati, K. F. Tetrahedron Lett. 1982, 23, 4647. Stevens, R. V.; Chapman, K. T.; Weller, H. N. J. Org. Chem. 1980, 45, 2030. 2. Shadwick, S. R.; Mohan, R. S. J. Chem. Educ. 1999, 76, 1121. 3. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory, 3rd ed.; Wiley: New York, 1994.
JChemEd.chem.wisc.edu • Vol. 77 No. 11 November 2000 • Journal of Chemical Education
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