Oxidation and Reduction Reactions in Organic ... - ACS Publications

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Oxidation and Reduction Reactions in Organic Chemistry Ivan A. Shibley Jr., Katie E. Amaral,* David J. Aurentz, and Ronald J. McCaully Division of Science, Penn State Berks, Reading, Pennsylvania 19610, United States *[email protected]

Students taking their first organic chemistry course often do not recognize oxidation-reduction reactions. When students learn about oxidation-reduction in general chemistry, they are introduced to it in terms of the loss or gain of electrons. But the notion of gaining or losing electrons becomes less pertinent to organic chemistry. When students are given an organic synthesis, they rarely think about reagents that will act as either oxidizing or reducing agents and they tend to lump these reagents with the many other reagents that they memorize. If students are to apply oxidation states when analyzing the products of organic reactions, the concept needs to be taught in a relevant form. Oxidation and reduction has been written about extensively in this Journal. Many articles explain oxidation numbers as they are taught in general chemistry courses (1-3) and one article explored oxidation numbers in biochemistry (4). Different approaches have been examined in organic chemistry. One approach proposed using electron density as a way of helping students gain more theoretical understanding of oxidation and reduction (5). Another article explained the topic with reference to an “average oxidation number” for carbon (6). The variety of approaches proposed for introducing oxidation and reduction in the literature is paralleled by the variety of approaches used in organic textbooks. Many textbooks treat oxidation-reduction reactions in a cursory manner and often do so with vague or fairly complicated heuristics. The approach described in this article is conceptually straightforward so that students can quickly assess an organic reaction in terms of oxidation and reduction. Because of the simplicity of this approach, instructors could use oxidation and reduction as an organizing framework for teaching organic chemistry. The concept could be introduced with the very first reactions that are presented such as the halogenation of an alkane or an alkene. The approach described in this article provides students with a simpler approach to oxidation and reduction that can be utilized for many organic reactions. Textbook Treatment of Oxidation and Reduction Textbooks take a variety approaches to discuss oxidation and reduction (7-20). A summary of 11 textbooks is provided in the supporting information. Although most textbooks still mention oxidation in terms of a loss of electrons (perhaps as a tie into general chemistry), they often describe oxidation as a decrease in hydrogen atoms or an increase in oxygen, nitrogen, or halogen atoms. The more complicated approaches involve a calculation of the oxidation state of each carbon atom involved in the reaction. Such an approach requires determination of formal charge or a multistep procedure with several subsets to determine the oxidation state of carbon. Similar oxidation levels may have

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different oxidation states depending on the substituents on carbon. Most organic textbooks concede that oxidation and reduction (as defined in terms of electron gain or loss) has little meaning in organic chemistry, yet the authors use several pages describing algorithms to determine whether a given reagent is oxidized or reduced. A Simpler Approach to Oxidation and Reduction A relatively simple approach to teach students how to view oxidation and reduction is to add the total number of heteroatoms, π-bonds (triple bonds count as two), and rings in each organic reactant and then do the same for each organic product. A higher number for the organic product compared to a given organic reactant indicates oxidation, whereas a lower number signifies reduction. Two examples of organic reactions that demonstrate the approach are shown in Figures 1 and 2. A slightly more complicated transformation is exemplified in the synthesis of hirsutene shown in Figure 3. From the decrease in oxidation number, it is clear that the transformation requires a reducing agent, which in this case was tributyltinhydride. (Organometallic hydrides are considered to be variants of metal hydride reducing agents. Although they are oxidized in the process of reducing organic reactants, the focus in this article is on the organic reactants or products.) The approach also has application to a group of compounds related to folic acid (Figure 4). Consideration of oxidation levels is also useful in cases where neither oxidation nor reduction occurs because the alteration of the molecule does not involve an oxidizing or reducing agent. A simple example of this conversion is shown by the pinacol rearrangement (Figure 5) in which the product is at the same oxidation state as the starting material. The approach is also applicable to a more complex situation involving the tautomers of pterin (Figure 6) in which all tautomers have the same oxidation state. This approach to understanding oxidation-reduction reactions also follows the rule that the sum of the oxidation levels of the reactants must equal the sum of the oxidation levels of the products. For this to be true, one must add an additional consideration: the total oxidation numbers on both sides of the equation involves the difference in the number of molecules reacting and the number of molecules formed in the balanced equation. For example, in the case of the formation of benzopinacol from benzophenone (Figure 7), three reactants are involved (two benzophenone molecules and one isopropanol molecule) and two products are obtained (one benzopinacol molecule and one acetone molecule). The difference between the number of reactants and the number of products is one, and thus, 1 needs to be added to the total oxidation number on the side of

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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 12 December 2010 10.1021/ed100457z Published on Web 10/08/2010

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Journal of Chemical Education

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Figure 5. Application to a rearrangement that is neither an oxidation nor a reduction: the pinacol rearrangement. Figure 1. Oxidation of toluene demonstrating the oxidation-reduction approach.

Figure 6. Examples of compounds with the same oxidation state: tautomers of pterin.

Figure 2. Oxidation of 1-phenyl-1-propene.

Figure 3. Reductive synthesis of hirsutene.

Figure 4. Relative oxidation levels of compounds related to folic acid.

Figure 7. Oxidation numbers for reactants and products for a reaction in which a bond is formed: formation of benzopinacol from benzophenone.

the equation having the smaller number of molecules, in this case, the products. An example involving bond breakage is shown by the decarboxylation reaction in Figure 8. The number of products formed is two (cyclohexane carboxlic acid and carbon dioxide) and the number of reactants is one (1,1-cyclohexane dicarboxylic acid) so the difference of one needs to be added to

the total oxidation number on the side of the equation that has the smaller number of molecules, in this the reactants. Although there does not seem to be any simple corollary of the oxidation-reduction approach to inorganic oxidizing or reducing agents in general chemistry, the formalization of utilizing electron half-reactions can still be applied. Every increase in

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r 2010 American Chemical Society and Division of Chemical Education, Inc.

In the Classroom

Figure 8. Oxidation numbers for reactants and products for a reaction in which a bond is broken: 1,1-cyclohexane dicarboxylic acid to cyclohexane carboxlic acid. Figure 10. Conversion of styrene to phenylacetylene.

Figure 9. Half reaction for the oxidation of toluene.

oxidation level of one unit is equivalent to the loss of two electrons. The general chemistry method of writing half reactions for inorganic reagents can then be applied to balancing reactions. An example of the oxidation of toluene to benzoic acid is shown in Figure 9. Oxidation-Reduction Approach Applied to Synthesis Early in the study of organic chemistry students are faced with the sequence of reactions involving the conversion of an alkene to an alkyne. There seems to be reluctance by students to proceed from a double bond to a single-bond intermediate when they are trying to generate a triple bond. If the process is viewed from the point of view of oxidation and reduction, the process becomes clearer. For example, the student can reason that the synthesis involves an oxidation, that is, going from one π-bond to two π-bonds. Reaction of the alkene with a halogen (bromine) brings about the oxidation. Even though the original π-bond is lost, the single bonded dihalogen intermediate is at the proper oxidation state for the desired product. The final elimination step remains and is neither an oxidation nor a reduction. The approach is shown in Figure 10 for the synthesis of phenylacetylene from styrene. The oxidation-reduction approach could be employed in the more challenging synthetic problem involving the analysis of the Johnson progesterone synthesis (21). An examination of the oxidation levels for the first step indicates that the reaction is neither an oxidation nor reduction but is basically a complex intramolecular rearrangement catalyzed by acid (Figure 11). An examination of the oxidation levels for the second step indicates an oxidation and ozone, a reagent that students encounter early in organic courses, is employed. The last step is neither an oxidation nor a reduction as indicated by the oxidation levels and is an intramolecular aldol cyclization. It is not uncommon for problems such as this to leave out the reagents and to ask the


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Figure 11. Synthesis of progesterone.

student to supply reasonable reagents. A consideration of the oxidation-reduction levels would be a significant help in solving the problem. Once the student has determined the nature of the transformation (oxidation, reduction, or neither), he or she is ready to choose the reagent needed to bring about the transformation. The number of oxidizing and reducing reagent that students encounter in beginning organic chemistry is rather limited and fall into distinct categories. As shown in the table of oxidizing agents (Table 1 in the supporting information), the categories of reagents are halogens, halogens attached to a heteroatom, transition-metal oxides, and group 6A oxides. The categories of reducing agents (Table 2 in the supporting information) consist of catalytic hydrogenations, group 3A metal hydrides, organometallics, solvated electrons, and metals in acid. Clearly there are differences in specificity and reactivity of the reagents in the various categories that the student will need to


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consider, but familiarization with the general nature of oxidizing and reducing reagents should eliminate a considerable amount of memorization. Benefits to Students Students often think of organic chemistry as a collection of reactions that they need to commit to memory. The underlying similarities among reactions are sometimes lost. Each textbook author takes a slightly different approach to create unifying themes. However, none of the books rely on oxidation-reduction reactions as extensively as they could. A teacher wishing to provide additional coherence to organic chemistry could utilize the approach suggested above to lead learners through reactions by continually referring to oxidation levels. Because the procedure is relatively simple, an instructor no longer needs to introduce a reaction with a title such as “reduction of alkenes” but can write a reaction and then ask students whether the reaction is reduction or oxidation, which can lead them to start classifying reagents as either oxidizing or reducing agents. The use of oxidation levels to help understand organic chemistry could be used as a unifying theme in the course. Even if an instructor chooses not to focus on oxidation or reduction, the simplified protocol for determining whether an organic molecule has been oxidized or reduced should clarify a topic that sometimes seems mysterious to students. As Gregg argued in 1945, “The student might be able to appreciate many organic reactions more fully if he [or she] recognized these reactions as oxidationreduction reactions.” (22) Literature Cited 1. 2. 3. 4. 5. 6.

Kauffman, J. M. J. Chem. Educ. 1986, 63, 474–475. Calzaferri, G. J. Chem. Educ. 1999, 76, 362–363. Woolf, A. A. J. Chem. Educ. 1988, 65, 45–46. Halkides, C. J. J. Chem. Educ. 2000, 77, 1428–1432. Anselme, J.-P. J. Chem. Educ. 1997, 74, 69–72. Menzek, A. J. Chem. Educ. 2002, 79, 700–702.

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7. Jones, J., Maitland Organic Chemistry, 3rd ed.; W. W. Norton: New York, 2005. 8. Bruice, P. Y. Organic Chemistry, 6th ed.; Prentice Hall: Boston, MA, 2011. 9. Ege, S. N. Organic Chemistry-Structure and Reactivity, 5th ed.; Houghton Mifflin: Boston, MA, 2004. 10. Fox, M. A.; Whitesell, J. K. Organic Chemistry, 3rd ed.; Jones and Bartlett: Sudbury, MA, 2004. 11. Hornback, J. M. Organic Chemistry, 2nd ed.; Brooks/Cole-Cengage: Florence, KY, 2006. 12. Loudon, G. M. Organic Chemistry, 4th ed.; Oxford University Press: New York, 2002. 13. McMurry, J. Organic Chemistry, 7th ed.; Thomson, Brooks/Cole: Belmont, CA, 2008. 14. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice Hall: Upper Saddle River, NJ, 1992. 15. Solomons, T. W. G.; Fryhle, C. B. Organic Chemistry, 9th ed.; John Wiley: Hoboken, NJ, 2008. 16. Sorrell, T. N. Organic Chemistry, 1st ed.; University Science Books: Sausalito, CA, 1999. 17. Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function, 5th ed.; Freeman: New York, 2006. 18. Wade, L. G. Organic Chemistry, 6th ed.; Prentice Hall: Upper Saddle River, NJ, 2006. 19. Brown, W. H.; Foote, C. S.; Iverson, B. L.; Ansyln, E. V. Organic Chemistry, 5th ed.; Brooks/Cole-Cengage: Belmont, CA, 2009. 20. Fox, M. A.; Whitesell, J. K. Organic Chemistry, 3rd ed.; Jones and Bartlett: Sudbury, MA, 2004. 21. Gravestock, M. B.; Johnson, W. S.; McCarry, B. E.; Parry, R. J.; Ratcliffe, B. E. J. Am. Chem. Soc. 1978, 100 (13), 4274–4282. 22. Gregg, D. C. J. Chem. Educ. 1945, 22, 548–553.

Supporting Information Available Table of oxidizing agents; table of reducing agents; table of organic textbooks and a summary of the oxidation-reduction treatment. This material is available via the Internet at http://pubs.acs.org.

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Oxidation and Reduction Reactions in Organic ... - ACS Publications

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