Flow Chart Determination of Isomeric Relationships Kersey A. Black Joint Science Center, 1lth St. and Dartmouth Ave., The Claremont Colleges, Claremont, CA 9171 1
One difficult juncture in the first semester of an organic course comes with the introduction of stereochemistry. Texts commonly provide adequate definitions for isomeric relationships, as well as a few examples of each. Nonetheless, difficulty in dealing with three-dimensional objects on a two-dimensional surface, combined with the abstract nature of stereochemical definitions often proves frustrating and ultimately overwhelming. In the end, students often remain unable to determine stereochemical relationships between isomeric structures, a capability that is prerequisite to exploring the chemical and phgsiral implications. To help address this problem, I provide the students with adflow chart"or"decision tree"sty1e presentation ofstereochemical relationships. The structure of the flow chart is based on one of the most common types of stereochemical exercises they encounter, that of determining the proper stereochemical relationship between two isomeric structures. Given two structures, the chart leads them through a
loeical Drocess hv which the correct isomeric rela- - ~ decision ~ ~ tionship can he deduced. within the flow chart are examples that illustrate each tvoe ". of relationshio. A varietv of structure depictions are used for these examples that consist of a mixture of acvclic and alicvclic com~ouuds.On the hack of the handout fprovide an introducti& to using the handout, as well as a fairly standard set of definitions of the stereochemical relationships and some clarification of terms. I have used this handout for a few years and find that this flow chart presentation is particularly effective in helping the student to distinguish between the different types of stereochemical relationships. I t also defines the task of determining stereochemical relationships more like solving a puzzle, with the resultant increase in motivation, while at the same time providing a logical process by which they can come to the correct answer. The graphic for the handout and the accompanying discussion of its use are presented here ~~~
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ISOMERIC RELATIONSHIPS BETWEEN ORGANIC COMPOUNDS Completely Supenmposable Structures?
'Cis-Trans Isomers'"
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Any pair of compounds beyond this point are ISOMFRS
Const tutlonal Isomers
Any pair of isomers beyond this point are STEREOISOMERS Identical after simple rotation
cyclic stnrcture?
and
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H" HO
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NH2
HO aa,+~z CH3
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The flow chert for determining isomeric relationships.
Volume 67 Number 2
February 1990
141
b o t h to illustrate t h e method and in the hope that it will b e of direct use to others.
Determlnlng Isomeric Relatlonshlps The "flow chart" is designed to show you how to determine logically through a series of questions any isomeric relationship that may exist between two molecular structures.
1. Do they have the same molecular formula? First check that the two structures have the same molecular formula. To be isomers they must have the same molecular formula, though as we will see in a moment this is itself not sufficient. If they donot, then they are entirely different compounds, and the hunt has ended before it begins. If they do have the same molecular formula, then start a t the top of the flow chart with the two structures you are comparing, asking sequentially the questions provided. Each questionltest is designed to reveal a particular type of relationship between the structures. If the two structures oass a given teat, then . further questioningltesting is required to determine what is a progressively more subtle isomeric relationship.
2. Are the two structures "identical"? This first test an the flow chart is actually a preliminary check that the two materials are in fact not identical in all respects. "Identical" means that the two structures, with no changes in conformation, are completely superimposable on each other, atom by atom. If they are identical, then they are the same compound and not isomers. Again, the show is over before the fun begins. This test is included as the first step in the flow chart, just to give you some examples of not so obviously identical structures. If the two structures under consideration get by the ahove two sand traps, then you are the proud surveyor of "isomers" and should continue onward in the chart to determine the precise isomeric relationship. (Isomers: Two compounds that have the same molecular formula hut that are not identical in all respects.)
3. Do the isomers differ in the order of atom connectivity? If the two structures differ in how the atoms are connected. then they are related as "ct,nutitutional isomers". (Con~tirurionolisomers Isomers that differ in their order of atom connectrvity.) If they are nor constitutional isomers, then they fall under the general ruhric of "atereuisomers", and we must continue unward to determine exactly which type. (Srereoiaomera: l~umersthat hare thesame hasicstructure (orderofatomeonnectivity),but that differ in the spatial arrangement of their atoms. The atoms in these iso~~~~~
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mers have different orientations relative to each other in threedimensional space. This is a general classification that includes isomers that fall into the more specific classifications of conformational isomers and configurational isomers (enantiomers and diastereamers).)
4. Can the isomers be made superimposable by simple rotation around single bonds? Next check to see if the two structures can be rendered superimposahle through simple rotation about single bonds. If so, this reveals them to be "conformational isomers". lConformotiona1 isomers lronforrners): .~ ~, ~, Stereoisomers that can he rendered comoletelv superimposable by the relatively facile rocation ahout single bonds. Conformational isomers are unique among rterroioumers in that they readily interconvert and cannot he separated at room temperature.) If not confarmationally related, then they fall under the general designation "configurational isomers". (Configurational isomers: isomers that can he made superimposahle only by breaking and reforming bonds, either sigma or pi.) We must then continue on through the chart to determine the exact type of configurational isomers. Changes in conformation are allowed during the remaining structure comparisons. ~
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5. Are the two configurational isomers related as object and (non-superimposable)mirror image? If so, then they are "enantiomers". Note that any amount of conformatianal change is legitimate while checking this possibility. (Enontiomers: Two isomers that are related as obiect and nonsuperimpwable mirror image. For a compound containing a single chiral carbon center. for example, m e iwmer will have the H ahsolute configuration, and the other will have the S nhrolute conficura. tion.) If the two configurational isomers are not enantiomen, then they are related as "diastereomers". (Diastereomers: Two configurational isomers that are not related as object and mirror image and are not superimposahle. For example, if one isomer has two chiral eenters, both R, then a diastereomer of this compound will have one R and one S ehiral center. (Note that the enantiamer would heve bath centers S.)) (Cis-trans isomers: This subgroup of diastereomers includes isomers that differ in configuration about a double bond. Thus, E and Z isomers of the same compound are also diastereomers. I t also includes disubstituted cyclic compounds where the suhstituents differ as to being on the same side (cis relation) on opposite sides (trans relation) of the ring. An old form of nomenclature refers to this group as "geometric isomers".)
l l h BCCE-Program Description Plannow toattend the 11th Biennial Conference on Chemical Education to be held at GeorgiaTech in Atlanta, August 5-9,1990. In addition to poster and general-paper sessions, the program will include:
Plenary Lecturers: Mark Wrighton, the Brasted Lecturer, Marjorie Gardner, Walter McCrone, and Bassam Shakhashiri. Symposia:Education for the New Technology; Forensic Science; Paper Chemistry; ChemicalDemonstrations; Modern IR Spectroscopy; the Environment; Computers: Interfacing, Hypercard and Hypertext, State-of-the-Art Hardware, Graphics, Artificial Intelligence, Spreadsheets, Software, and Videodisc Applications; Undergraduate Faculty Enhancement Programs; Improving the Public Image of Chemistry; Anti-AIDS Drugs; Plastics; Teaching Physical Chemistry; Low Cost Instructional Materials; General Chemistry Lshoratories; Nuclear Chemistry: Nuclear Medicine, Nuclear Power, and Radioactivity in Industry and Research; Microscale; Basic and Advanced Inorganic Chemistry; Polymers; Chemical Microscoov: World of Chemistrv: General Chemistrv Textbook Authors and Editors: Reactivitv Network: AP Chemistrv:.0rzaniEchemistrv: s ~~~~,~~~~ Geoehemistrv: ~ h~e m ~ -b u~~.~~~~~~~~~ r c~,er: a i ~ u a t stud" e ~~, in ~ .. . ~ a h o r a t o r v ~ s s e s s m e~nut i l d Success: Chemical Education; CEPUP; Screnre Education Ilesenrch Chemistry in the Toy Store; Coamic Chemidry; ChrmCom; Chemical Resuurces in Muuuumr;Travelling Chemritry Demonstration Presentation: History of thechemistry Set; PreHigh-School Chemistry; Reform in Science Education; Chemical Competitions; and the FIPSE Follow-llp Symposium.
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Workshops: Microscale; Computer Data Acquisition and Analysis; Project SERAPHIM: Game-port Interfacing, Appleworks, One-Computer Classroom, and KC? Discoverer and the Periodic Table Videodisc;Laboratory Safety; Fires and Explosions; ChemCom; Institute of Paper Science and Technology Site Visits; Applied Chemistry for Inner-City High School Students; Polymer Chemistry Demonstrations and Experiments; Environmental Chemistry; Modern IR Spectroscopy; Fun with Polymers; Demonstrations, Novel and Easy; World of Chemistry; Superconductors, Semiconductors, and Metals; Chemical Microscopy; Chemistry in the Toy Store; Chemistry Can Be Fun (ICE Workshop); Travelling Chemistry Presentation; and the FIPSE Follow-Up Symposium. For more information about the program, registration, and local arrangements, contact Dr. Toby F. Block, General Chair, School of Chemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400.
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Journal of Chemlcal Education
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