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A Mnemonic for Ozonolysis Ben Ruekberg* Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, United States ABSTRACT: Many students find product prediction in ozonolysis reactions of alkenes and alkynes difficult, and they often have greater difficulty discerning the starting compounds when presented with ozonolysis products. The mnemonic device suggested here can help students figure out what the ozonolysis product (or products) will be. Once mastered, this mnemonic can help students identify unsaturated compounds that could have given rise to the ozonolysis products. KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Mnemonics/Rote Learning, Aldehydes/Ketones, Alkenes, Alkynes, Carboxylic Acids, Oxidation/Reduction any students find product prediction in ozonolysis reactions of alkenes and alkynes difficult, and they often have even greater difficulty discerning the starting compounds when presented with ozonolysis products. Although the mnemonic device suggested here does not provide a mechanism, reaction conditions, or reagents, it can help students figure out what the ozonolysis product (or products) will be. This mnemonic applies directly to problems with a reductive workup. It can also serve as a starting point for ozonolysis reactions with other than reductive workups with the instructor’s pointing out the identity of product groups susceptible to further oxidation (and their corresponding oxidation products), which need not be discussed here. Once mastered, this mnemonic can help students identify unsaturated compounds that could have given rise to the ozonolysis products. Thus, the mnemonic can aid students as they begin to master techniques that have appeared in the literature1 3 for solving these problems. Instructors should tell students that when they see the word “ozonolysis”, they should read it as “O’s on olysis,” because it is a process that “breaks each double bond and puts O’s on and breaks each triple bond and puts O’s and Oh’s on.” The ozonolysis of alkenes, as the simplest case, should be discussed first. This may be done by the widely used, nonmechanistic, cut-and-attach method to which the mnemonic may be appended. The first step is to redraw the molecule, in pencil, with any double bonds drawn extra long. (It is a good idea to encourage students to redraw molecules as a step in solving problems.) The second step is erasing the middle of each double bond (This is the “-lysis” part.), and the third step, putting “O’s on” the ends of each gap created in each erstwhile double bond. These steps are illustrated, for 1-methylcyclohexene, in Figure 1. In the case of alkynes, students follow the same steps, “stretching” and breaking all three bonds, but must remember to put “ OH’s on” the ends of the third broken bond (with the instructor emphasizing that “ OH” as can be pronounced as “Oh!” and, presumably, apologizing for the pun), as well as an O on each of the remaining pairs of bond ends, as before, giving carboxylic acids. Although its use as an analytical tool has diminished, ozonolysis remains not only a valid teaching tool1 but a potential
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Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. The steps of “O’s on olysis” of 1-methylcyclohexene.
synthetic method, in which case determining the satisfactory precursors to the desired product is important. Once learned, the mnemonic, applied in reverse, is also helpful for predicting the possible original alkenes or alkynes from the ozonolysis products, often in conjunction with the chemical formula of the starting material, the isoprene rule, and degrees of unsaturation.2 For example, one could tell if malondialdehyde, or a tautomer thereof, were the product in the ozonolysis of cyclopropene or 1,4-cyclohexadiene if the chemical formula of the starting material is known. First, one redraws, in pencil, the product or products pointing the oxygen ends of each pair of carbonyl groups toward one another. (Clearly, one pair should be considered at a time, and, as a general rule with reductive workup, one must pair carbonyls with carbonyls and carboxylic acids, if any, with carboxylic acids.) Next, the O’s (and OH’s, if any) are erased and the ends of each bond are joined together to restore the multiple bond(s) of a molecule that could have given rise to the ozonolysis product or products. Although not intending to disparage the importance of learning mechanisms, this easily remembered, humorous mnemonic could be helpful in predicting the products or starting materials for the ozonolysis of alkenes and alkynes.
Published: March 31, 2011 843
dx.doi.org/10.1021/ed1010882 | J. Chem. Educ. 2011, 88, 843–844
Journal of Chemical Education
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’ AUTHOR INFORMATION Corresponding Author
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’ REFERENCES (1) Gross, R. A., Jr. An Analysis of Ozonolysis for Classroom Instruction. J. Chem. Educ. 1996, 73, 1019–1021. (2) Gross, R. A., Jr. Ozonolysis Problems that Promote Student Reasoning. J. Chem. Educ. 2006, 83, 604–609. (3) (a) Gross, R. A., Jr. Bond Analysis as a Tool in the Solution of Ozonolysis Problems. Chem. Educ. 2010, 15, 361–364. (b) Gross, R. A., Jr. Improving Reasoning Skills in Organic Students Using Degradation Problems. Chem. Educ. 2006, 11, 372–377.
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dx.doi.org/10.1021/ed1010882 |J. Chem. Educ. 2011, 88, 843–844
