Toward Consistent Terminology for Cyclohexane Conformers in

Wikipedia and IUPAC also split on the half-chair conformation structures, with .... Fessenden , R. J. ; Fessenden , J. S. ; Logue , M. W. Organic Chem...
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In the Classroom

Toward Consistent Terminology for Cyclohexane Conformers in Introductory Organic Chemistry Donna J. Nelson* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States and Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019 *[email protected] or [email protected] Christopher N. Brammer Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States [email protected]

Great variation in cyclohexane conformer terminology is found across many current excellent introductory organic chemistry textbooks (1-17). Most texts disagree not only on conformer selection, but also on their names. For example, many such texts (1-13) use the cyclohexane conformer names, chair, half-chair, twist-boat, and boat, as shown in Table 1. Some use this nomenclature in combination with additional or alternate names (9-13). Five texts use alternate names for the twistboat conformer, such as twist (7, 8), skew boat (10), or skew-boat (9, 11). Two textbooks use additional names for the boat conformation, which are true boat (12) and full-boat (13). Others (14-17) discuss only two or three of these conformers. Table 1 shows the wide variation not only in conformers presented across the texts, but also in the names of those conformers. Conformer names used interchangeably within a text are designated in Table 1 by asterisks (*) and by carets (∧). Table 1 also provides textbook publication descriptors, such as year of publication and current edition of each text compared. Although two texts (1, 5) are no longer in print, they are included for the sake of completeness, because they are still readily available, often used as supplementary material in courses, and therefore merit evaluation. Because students often consult online resources to supplement information in their texts, two Internet resources were also examined and included (Table 1): Wikipedia (18) and IUPAC (19). Wikipedia, which is often a helpful source, uses all five conformer names most often found in Table 1, chair, half-chair, twist-boat, twist, and boat, with the notation that twistboat and twist refer to the same conformer. However, Wikipedia also gave envelope as an alternate name for the half-chair conformer. The name envelope is not used as a cyclohexane conformer name in any textbook known to the authors. However, envelope-like has been used to describe the form of the half-chair conformer, which has five coplanar carbons and is shown in Figure 1 (20). Nevertheless, the use of the term envelope could confuse introductory organic chemistry students, who consult Wikipedia for clarification of the cyclohexane conformer nomenclature in their textbooks. The IUPAC Web site (19) presents the chair, boat, and four coplanar carbon half-chair conformations. This Web site does not mention the twist-boat, but seems to attempt inclusivicity in other twist conformer alternate names: skew boat, skew form, 292

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skew, and stretched form. Therefore, this Web site might not be very helpful to students. Another possible point of confusion is the structure of the half-chair conformation. This conformation is usually included in introductory organic chemistry texts (1-13), being omitted from only four (14-17) of the texts in this study. However, the structure of the half-chair conformer is inconsistent among the 13 textbooks that address it (Table 1). The two structures proposed for this conformer are shown in Figure 1: one with five coplanar carbons (1-11) and one with four coplanar carbons (12, 13). Wikipedia and IUPAC also split on the half-chair conformation structures, with the former presenting (18) the five coplanar carbon structure, and the latter endorsing (19) the four coplanar carbon structure. The importance of being consistent and reducing confusion in teaching introductory organic chemistry terminology has been convincingly discussed previously (21). Similar to that report (21), we propose herein adopting a set of structures and names that will bring consistency and clarity to cyclohexane conformations and nomenclature in textbooks. Proposals, Rationale, and Explanation Students in introductory organic chemistry often consult textbooks, other than the course-adopted textbook, to clarify difficult concepts or supplement weak points in a text; a convenient solution is to read multiple explanations by different authors. Although multiple explanations are useful, these explanations should agree on at least basic facts and preferably use the same terminology, to avoid further confusion and making it harder than is necessary (21). Cyclohexane Conformers and Their Names The cyclohexane conformer names proposed herein for use in introductory organic chemistry classes are chair, half-chair, twist-boat, and boat. Most textbooks examined in this study use these names (Table 1), and it will be argued below that these are less likely to be confused than some alternatives. Figure 2 shows a typical energy diagram for the cyclohexane conformers, chair, half-chair, twist-boat, and boat, which is based on reported (22-24) relative conformer energies. The half-chair (22) and twist-boat (23) forms are 10.8 and

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In the Classroom Table 1. Cyclohexane Conformers Discussed in Introductory Organic Chemistry Textbooks and Internet Resourcesa

a

*Interchangeable names. ∧Interchangeable names.

Figure 1. Two structures for the half-chair conformer of cyclohexane.

5.5 kcal/mol above the chair conformation, respectively. The boat conformer is ∼1-2 kcal/mol above the twist-boat form (24). Some texts use twist instead of twist-boat. However, this is less desirable, because students sometimes confuse the name twist as pertaining to the half-chair conformer. Indeed, one popular graduate-level text substitutes the term twist for half-chair (25). Therefore, we propose not using the term twist at all. The term twist-boat is a more descriptive term for this structure, because the structure is more similar to the boat than to the chair form, as seen in Figure 2. The term half-chair conveys simultaneously (i) that the half-chair structure resembles more the chair conformer than the boat conformer, and (ii) that the half-chair structure is closer to the chair than the boat on the energy diagram pathway for conformational interconversion of cyclohexane (Figure 2). Similarly, the term twist-boat will reinforce that the corresponding structure and energy diagram position are closer to the boat structure than to the chair; the term twist would not similarly convey this. In addition, we recommend discontinuing the terms skew, skew form, skew boat, and skew-boat. Structure of Half-Chair Conformer Two structures for the half-chair conformer are often used: one with four coplanar carbons and one with five coplanar carbons (Figure 1). There are multiple reasons for adopting the former for introductory organic chemistry, as shown in the conformational pathway in Figure 2. First, the structure with

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Figure 2. Conformational changes during the inversion of cyclohexane.

four coplanar carbons is calculated to be more stable than the one with five coplanar carbons. Decreasing the number of coplanar carbons in the structure decreases the number of eclipsing methylene units and the resulting strain. The four coplanar carbon structure has fewer eclipsing methylene units than the five coplanar carbon structure (26), so it incurs less strain as a result (26). Second, the structure with four coplanar carbons is predicted to lie on the lowest-energy pathway for the interconversion of cyclohexane conformers (20, 26). Third, the fivecarbon coplanar structure is predicted to lie on a pathway that by-passes the twist-boat structure (20) and goes directly from the

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half-chair to the boat form (20); this is not the conformational pathway typically taught in introductory organic chemistry. The four-carbon coplanar structure is predicted to lie on the pathway leading from the chair to the twist-boat form (20), which is shown in Figure 2 and which is typically taught in introductory organic chemistry. Summarizing, the four-carbon coplanar structure is the preferred half-chair structure, because it is predicted (i) to be more stable than the five-carbon coplanar structure and (ii) to lie on the cyclohexane conformer interconversion energy surface, whereas the five-carbon coplanar structure does not. However, the less-stable structure with five coplanar carbons was given as the half-chair conformer structure by 11 of the 13 texts (1-11) examined herein; only two texts (12, 13) presented the more stable half-chair structure which has four coplanar carbons. Nevertheless, the half-chair conformation structure with four coplanar carbons is the more stable conformer (20, 26), and it is identified in the literature as most likely (20, 26). Therefore, it is recommended to use the four-carbon coplanar structure for the half-chair in introductory organic chemistry texts. Summary The cyclohexane conformer names recommended herein for use in introductory organic chemistry are chair, half-chair, twist-boat, and boat. Most of the textbooks examined in this study use these names, although many use them in combination with other names. There is little likelihood of confusing these conformer names, as there is with some alternate names currently in use. The half-chair conformation structure with four coplanar carbons is recommended for use in introductory organic chemistry. As defined, the recommended names and structures are consistent, descriptive, pedagogically useful, and in agreement with research literature. Literature Cited 1. Fessenden, R. J.; Fessenden, J. S.; Logue, M. W. Organic Chemistry, 6th ed.; Thomson Brooks/Cole: Pacific Grove, CA, 1998; pp 135-137. 2. Fox, M. A.; Whitesell, J. K. Organic Chemistry, 3rd ed.; Jones and Bartlett Publishers: Sudbury, MA, 2004; pp 232-236. 3. Hornback, J. M. Organic Chemistry, 2nd ed.; Thomson Learning, Inc., Brooks/Cole: Belmont, CA, 2006; pp 196-199. 4. Loudon, M. Organic Chemistry, 5th ed.; Oxford University Press: New York, 2009; pp 269-276. 5. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice-Hall, Inc.: Upper Saddle River, NJ, 1992; pp 455-459.

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6. Sorrell, T. N. Organic Chemistry, 2nd ed.; University Science Books: Enfield, NH, 2006; pp 85-87. 7. Solomons, T. W. Graham; Fryhle, C. B. Organic Chemistry, 10th ed.; John Wiley & Sons, Inc.: New York, 2011; pp 163-165. 8. Wade, L. G. Organic Chemistry, 7th ed.; Pearson Education, Inc.: Upper Saddle River, NJ, 2010; pp 109-113. 9. Bruice, P. A. Organic Chemistry, 6th ed.; Prentice Hall: Upper Saddle River, NJ, 2011; pp 103-105. 10. Carey, F. A; Giuliano, R. M. Organic Chemistry, 8th ed.; McGraw-Hill: New York, 2011; pp 111-115. 11. Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry, 6th ed.; W. H. Freeman and Co.: New York, 2011; pp 141-142. 12. Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic Chemistry; Oxford University Press: New York, 2001; pp 460-453. 13. Jones, M. J.; Fleming, S. A. Organic Chemistry, 4th ed.; W. W. Norton & Co. Inc. (Np): New York, 2010; pp 197-199. 14. Ege, S. Organic Chemistry: Structure and Reactivity, 5th ed.; Houghton Mifflin Co.: Boston, MA, 2004; pp 180-182. 15. Brown, W. H.; Foote, C. S.; Iverson, B. L.; Ansyln, E. Organic Chemistry, 5th ed.; Brooks Cole Publishing: Pacific Grove, CA, 2009; pp 80-86. 16. Smith, J. G. Organic Chemistry, 3rd ed.; John Wiley & Sons Inc.: New York, 2011; pp 140-141. 17. McMurry, J. Organic Chemistry, 7th ed.; Brooks/Cole: Upper Saddle River, NJ, 2008; pp 117-122. 18. Wikipedia, The Free Encyclopedia. Wikimedia Foundation: San Francisco, CA, 2009. http://en.wikipedia.org/wiki/Cyclohexane_ conformation (accessed Nov 2010). 19. IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by McNaught, A. D., Wilkinson, A.; Blackwell Scientific Publications: Oxford, 1997. XML online corrected version: http://goldbook.iupac.org created by Nic, M. ; Jirat, J.; Kosata, B.; updates compiled by Jenkins, A. ISBN 0-9678550-9-8. DOI: 10.1351/goldbook. (accessed Nov 2010). 20. Nasipuri, D. Stereochemistry of Organic Compunds; New Age International (P) Ltd., Publishers: New Delhi, 1990, Chapter 10, pp 244-247. 21. Adams, D. L. J. Chem. Educ. 1992, 69, 451. 22. Anet, F. A.; Bourn, A. J. R. J. Am. Chem. Soc. 1967, 89, 760. 23. Squillacote, M.; Sheridan, R. S.; Chapman, O. L.; Anet, F. A. L. J. Am. Chem. Soc. 1975, 97, 3244. 24. (a) Leventis, N.; Hanna, S. B.; Sotiriou-Leventis, C. J. Chem. Educ. 1997, 74, 813. (b) Sauers, R. R. J. Chem. Educ. 2000, 77, 332. 25. Carey, F. A.; Sundberg, R. J. Organic Chemistry, Part A: Structure and Mechanisms, 7th ed.; McGraw-Hill: New York City, 2008; p 152. 26. (a) Hendrickson, J. B. J. Am. Chem. Soc. 1961, 83, 4537. (b) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127.

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