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Periplanar or Coplanar? Saul Kane and William H. Hersh* Department of Chemistry and Biochemistry, Queens College of the City University of New York, Flushing, NY 11367-1597; *
[email protected] Over the years, organic chemists have paid close attention to our use of language to describe our science, particularly in stereochemistry, where entire articles have been devoted to the pursuit of precision (1–4). However, one term in particular, “periplanar”, has not been subject to such scrutiny. The purpose of this commentary is to alert organic chemists to the fact that current usage of this term is linguistically incorrect. Many textbooks describe the E2 elimination reaction as proceeding via an antiperiplanar transition state and typically go to some length to describe the planarity of the system. Why then the use of the peri prefix? “Peri” is derived from the Greek for “near”, and hence its use is clear in terms such as pericyclic or perigee (the closest approach to the earth of an object in orbit). In fact, the term periplanar was coined by Klyne and Prelog (5) to mean “approximately planar”, with a deviation of ±30°, in this way allowing the terms synperiplanar, synclinal, anticlinal, and antiperiplanar to cover the full range of conformations (Fig. 1). The “clinal” term does not require the “peri” prefix because it does not specify any particular non90° angle. The definition of periplanar is included in the IUPAC “Rules for Nomenclature of Organic Chemistry” (6 ) and on page 21 of the authoritative book Stereochemistry of Organic Compounds (7). However, its original meaning does seem to have been forgotten; but the only acknowledgment of this that we have found is a footnote noting that syn and anti “are often used for ω ≈ 0°” and 180°, respectively (7). Our perusal of recent textbooks shows that about half do not use the term periplanar; instead they use coplanar (8– 10 ) or the terms syn and anti without reference to planarity (11–13). However, the other half use the term periplanar and clearly equate it with coplanar. An inspection of two excellent textbooks from the 1970s, barely 10 years distant from the introduction of periplanar into the chemical language, found a similar split: Allinger et al. used the term coplanar (14 ), whereas Hendrickson, Cram, and Hammond equated trans antiparallel with trans periplanar (15 ). The original need and purpose for the term periplanar lay in descriptions of conformations, where some flexibility with respect to rotation about a single bond was required. Without taking on the question of just how planar is the E2 transition state, and therefore whether or not “periplanar” is chemically correct in that case, it is clear that the term is being used where authors mean “coplanar” or simply “planar”. Despite this common usage, the use of the peri prefix when planarity is indicated should be discouraged because it specifically means approximately planar. We welcome any information that readers may have on when and how the common usage of periplanarity was effectively changed following its adoption by the organic community. Future authors should consider modest revisions in textbooks and papers. We would suggest that “syn-coplanar” and “anti-coplanar” be used.
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0° -60°
sc
-ac -120°
60°
sp -sc
ac ap
120°
180° Figure 1. s = syn, a = anti, p = periplanar, c = clinal.
Acknowledgments We would like to thank G. P. Moss (Queen Mary & Westfield College, University of London), contacted via the IUPAC Nomenclature Home Page at www.chem.qmw.ac.uk/ iupac, and George Axelrad, William Berkowitz, and Norman Goldman (Queens College) for their help. Literature Cited 1. Eliel, E. L. J. Chem. Educ. 1971, 48, 163–167. 2. Mislow, K.; Siegel, J. J. Am. Chem. Soc. 1984, 106, 3319– 3328. 3. Adams, D. L. J. Chem. Educ. 1992, 69, 451–452. 4. Eliel, E. L. Chirality 1997, 9, 428–430. 5. Klyne, W.; Prelog, V. Experientia 1960, 16, 521–523. 6. Commission on the Nomenclature of Organic Chemistry of the IUPAC. J. Org. Chem. 1970, 35, 2849–2867. 7. Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley-Interscience: New York, 1994. 8. Brown, W. H.; Foote, C. S. Organic Chemistry, 2nd ed.; Saunders: Fort Worth, TX, 1998. 9. Hornback, J. M. Organic Chemistry; Brooks/Cole: Pacific Grove, CA, 1998. 10. Wade, L. G. Jr. Organic Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 1999. 11. Jones, M. Jr. Organic Chemistry; Norton: New York, 1997. 12. Fessenden, R. J.; Fessenden, J. S.; Logue, M. W. Organic Chemistry, 6th ed.; Brooks/Cole: Pacific Grove, CA, 1998. 13. Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function, 3rd ed.; Freeman: New York, 1999. 14. Allinger, N. L.; Cava, M. P.; DeJongh, D. C.; Johnson, C. R.; Lebel, N. A.; Stevens, C. L. Organic Chemistry; Worth: New York, 1971. 15. Hendrickson, J. B.; Cram, D. J.; Hammond, G. S. Organic Chemistry, 3rd ed.; McGraw-Hill: New York, 1970.
Journal of Chemical Education • Vol. 77 No. 10 October 2000 • JChemEd.chem.wisc.edu
