Representing Isomeric Structures Edwin mall The University of Akron-Wayne College, Orrville, OH 44667
For undergraduates taking their first organic chemistry course, learning to draw all the structures of a given formula is often frustrating. T o assist students with identifying all isomers without the inclusion of repeats, I have developed a technique called Representing Isomeric Structures (RIS). The method focuses on carbon skeletons and permits single structures t o represent more than one isomer. The method uses arrows pointing t o unique sites on the carbon skeleton to represent functional groups. For example, the constitutional isomers of CsH13CI possess five unique carbon skeletons. By drawing these five skeletons and declaring the arrow as a chloro group, 1-5 show all the isomers of C6H13C1. Structure 1 represents 1-chloropentane, 2-chloropentane, and 3-chloropentane. C
I
C-C-C-C-C-C
t t t
t t t t t 2
1 C-C-C-C-C
5:
C
C1
I C-C-C-C
t t t
C1
I
I C-C-C-C
t h t
C-C-C-C
t t t t
4
3
T o represent stereoisomers with pairs of enantiomers and meso compounds, arrows are accompanied with the following notations: En= n pairs of enantiomers Mn = n meso compounds The notation EM2specifies one pair of enantiomers plus two meso compounds. Including stereoisomers, all isomers of C4HsCI2are shown in 14-16. The four isomers represented by 15 are 2,2-dichlorohutane (achiral) and 2,3-dichlorohutane (one pair of enantiomers and one meso compound). For 14-16, the cumulative EIM (CEM) is represented by E3M.
C-C-c-C-C 1
Geometric isomers are easily displayed with arrows--the cis and trans isomers of 1-chloropropene are distinguishable by arrows pointing up and down, respectively (13).
t t
F'
C-LC
t t t CEM = E3M
5 RIS is applicable when more than one halogen is Present. For CaHsClz, hold one chlorine fixed and point to the sites occupied by the second chlorine (6-8). For three halogens (C3H6C12Br),hold both chlorine8 fixed and declare the arrow as a bromo group (9-12). C1 C1 C1 C
I C-C-C-C
I C-C-C-C
I I C-C-C
t t t t
t t
t t t
6
7
8
CI
C1
I
I
9
10
C-C-C-C
I t
C-C-C-C
t Gt
f
= 0 for CH30CHzCH2CH3and
CH3CHzOCHzCH3
I = =for
H2C=CHCH2CH3 and CHsCH=CHCH3
C1 CI
I
I
11 CI
C1
I
C-C-b
t t 12 190
t =CI
The method can he extended to represent constitutional isomers of compounds containing functional groups that are in the chain as shown below:
Journal of Chemical Education
t
= Br
The symbol G, used to indicate the existence of geometric isomers, represents both cis and trans 2-hutene. What are the advantages of RIS? Aside from saving time and minimizing the tediousness of generating many structures, students are able to assimilate a number of structures witha single glance. As aresult, students become more adept in recognizing skeleton forms and repeat structures. The benefits to the organic chemistry instructor are twofold. First, grading problems of this type is simplified. For example, all nine dichlorocyclohexanes, both constitutional isomers and stereoisomers, can he represented by the single structure (17).
Secondlv. the instructor h a s m o r e flexibilitv in nroblem selection. while it is unreasonable to ask stud&s'to draw the 39 isomers of C~HICI, it is well within reason t o have them represent all of these isomers using the nine skeleton forms (18-26).
t t t t
t t t t t r I
C-C-C-C-C
t t t t t t t L t t t
7'
C-C-C-C-c 24
C-C-C-C-c-C
I
F
t t t
I
C-C-C-C-c-c-C
C-C-C-C-C-(:
C
1
=
CI
I I
C
C
C-C-C-C-c
t t t
C
I I
C-C-C-C
t d t t
25
26
T h e development of RIS was not intended t o replace traditional structural formulas, b u t rather to enable students t o identify readily all isomers without the inclusion of repeats. In the classroom, I clearly emphasize the appropriateness of each method and students are required to distinguish "drawing" versus "representing" isomers. What are student reactions? They catch on quickly and if given the choice of drawing or representing isomers, they usually o p t for the latter.
Plan Now for 12th Biennial Conference on Chemical Education Circle August 2-6,1992, on yourealendm and plan to join other chemical educators at the 12thBiennial Conference on Chemical Education. The conference will he held on the campus of the University of California, Davis, which is near Sacramento, California. "Changing the Image of Chemistry" is the theme. Program Chair, Arlene Russell, is already planning far keynote speakers, special programs, workshops and other educational programs. If you have suggestions, or are interested in sponsoring a panel, colloquium or workshop, please contact Arlene A. Russell, Department of Chemistry &Biochemistry, University of California, Los Angeles, Los Augeles, California; (213) 825-7570. The Chair of the Conferenceis Fred E. Wood. He is arranging for housing and meeting sites on the University campus. If you have questions about the facilities contact Fred E. Wood, Department of Chemistry, University of California, Davis; (916) 752-6265. The Sacramento Valley and surrounding regions provide numerous opportunities for cultural and recreational activities. Nearby are the Sacramento and American Rivers, with their water-based activities; Old Town in Sacramento with an extensive railroad museum; an outdoor theatre in the round in Sacramento; and other historical sites. Within reasonable driving distance are the Gold Country, Wine Country, Lake Tahoe and Reno, San Francisco, and the Pacific Coast. You may also want to visit Yosemite on your own. If you want to reserve a campsite or hotel room within the park, you must call one year and one day in advance, no earlier. Plan to start calling at 7:00 a.m. Pacific Time to insure a reservation: (209) 252-4848. Be patient and persistent in calling. Tours and trips to various sites will be offered through the conference facilities. If you have general questions about the 12th Biennial Conference on Chemical Education or the general region, please contact the publicity chair: Jim Hill, Department of Chemistry, California State University, Sacramento, 6000 J Street, Sacramento, CA 95819-6057; (916) 218-6684.
Volume 66
Number 3 March 1991
191
