Representing Isomeric Structures Five Applications Edwin Thall The University of Akron-Wayne College, Orwille, OH 44667
In a recent publication in this J o u m l , ' I introduced a new method called Representing Isomeric Structures (RIS). In this approach I use arrows to point to unique sites on the carbon skeleton to represent functional groups. This helps students to write all isomers without duplicates. For example, the eight constitutional isomers of CsHllCl and one hydroxyl group on the leR side are shown in 1-3. Structure 1reoresents three structures: - 1-chloropentane, 2-chloropentak, and 3-chloropentane. The method also accommodates stereoisomers. as demon----.-strated in 4 for the dichlorocyclohexanes. Pairs of enantiomers and meso compounds are specified with the following notations. En= n pairs of enantiomers of horizontal lines. If rotated 90' clockwise, hydroxyl groups in 5 align with arrows in 6. Because FP arrows desM" = n meso compounds ignate functional groups directly above, they need not be The cumulative E/M (CEM) acknowledges two pain of endefmed. and two meso comoounds (M2,. antiomen (E2) In addition to saving time and &inimiz& the tedium of eeneratine manv structures. RIS allows students to assimaate struc&es in a glance. 1n this paper, five applications are presented: Fisher projections, reaction products, functional group isomers, cyclic hydrocarbons, and 'H-NMR signals. ~
~
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Representing Fisher Projections Structures for chiral molecules are conveniently represented with two-dimensional formulas called Fisher projections. These illustrations are especially useful for compounds that contain several chiral carbons because they save space, furnish important structural detuil, and are easv to draw. The RIS a~oroachsimolifies the taskof writing-many Fisher proje&ns by assigning arrows with asterisks to distinguish groups on the right and left sides of the horizontal line. Although Fisher projection (FP) arrows accommodate a variety of structures, they are ideally suited for symmetrical molecules. The Fisher projection for one of the 2,3,4,5,6-pentahydmxyheptanes is shown by 5. For the same configuration, represented by 6, FP arrows specify four hydroxyl groups on the right side
' Thall, E. J. Chem. Educ. 1991, 68,190-191.
The mirror image for 6 is obtained by inverting the entire row of arrows (Fig. 1).To establish enantiomers, rotate the mirror image 180' and slide it over the original row of arrows. Since the two rows do not superimpose, enantiomers exist. If 7 is tested in the same manner, as seen in the second example of Figure 1, the rows of arrows superimpose, thereby confirming a meso structure. All 2,3,4,5,6-pentahydroxyheptanes-six pairs of enantiomers and four meso compounds--are illustrated in Figure 2. To save space, enantiomers are noted, but represented with a single entity.
OH HOH H H I ? I
PP
C-C-C-C-C-C-C
tffff
mirmt image
minor image m m d 180'
f l l 1l I f f f l
enantiomerr
Figure 1. Testing superimposability for stereoisomers of 2,3.4,5,6pentahydroxyheptane. Volume 69 Number 6 June 1992
447
--
l!! l ! 1I I
enantiomers
i P i
OH HOHOH
c-c-C-c-c-C-C
i
I!!!
tllr
enantiorners
[tit
enantiomers
l r !!
1 1111
enantiomers enantiomers
f
[
enantiom
!t!rl !r I i !
--
meso
Figure 2. Representing 2,3,4,5,6-pentahydroxyheptaneswith FP arrows (CEM = E"'). The following two examples demonstrate FP arrows for nonsymmetrical molecules.
enantiomers
t!rr !r!r frr! frrr
enantiomers enantiomers enantiomers enantiomers enantiomers
Figure 3. Representing 2,3,4,5,-tetrahydroxyheptanes with FP arrows (CEM = E'). amow: Chlom groups are designated by conventional arrows, while loops identify hmmo groups. Representing Reaction Products The RIS approach can be extended to isomeric products, including stereoisomers, derived fmm reactions. Assuming primary, secondary, and tertiary reactivities of 1.0,3.8,5.0, eq 1 predicts the isomers and percentages that are expected from the monochlorination of 2-methylbutane.
The 16 stereoisomers of 2,3,4,5-tetrahydroxyheptaneare reoresented hv, eieht rows of arrows in Fieure 3. Due to the lack nfsymmetry, meso cumpounds arc ahsrnt, and you should not attempt to mvert or mtate arrows h a remmdcr, 1 lnhrl the first armw wlth a Imp to hmder ~uperunpnrabhty The four enantiomeric pairs of 2-hromo-3,4-dichloropentane are illustrated in Figure 4. In this case, the unsymmetrical positioning of chloro or bromo groups require that we label the
-
Applying RIS guidelines to eq 1,the same information, in more concise form, is represented as
Br CI CI I I I
c-c-c-c-C
11 f 11 I 11 f 11 f
enantiomers enantiomers
enantiomers enantiomers
Figure 4. Representing 2-bromo-3,4-dichioropentanes with FP arrows (CEM = E'). 448
Journal of Chemical Education
Chlorination of racemic 2-chloropentane produces five pairs of enantiomers, one meso compound, and an achiral structure. With the use of E M notations to designate pairs of enantiomers and meso compounds, the reaction products are represented by
Chlorination Summary of (R)-2-Chloropentane
Products
2.2-dichloro
ff t4t
WS Specification Number of Number of fractions optically active fractions achiral
1
I
(R)
GI2
hv
2*
I'i t t t ta 1t * L 1°,O,a, I
C-C-c-C
C-C-C
2
I =Nu
0
131, but tertiary structures must be drawn in the conventional manner (14-16).
While the CEM offers insight into the types of stereoisomers associated with a chemical formula, knowing the number of fractions is often useful for isomers formed via reaction. The cumulative FIA (CFA), which provides information regarding fractions, tabulates seven fractions (F7) for eq 3, none of which are optically active (A'). The results expected from the chlorination of (R)-2chloropentane are summarized in the table. When the RIS approach is used, this gives the same pertinent information in equation fonn.
c-c-c-c-c-
22'2'
c-c-c-c-c
Representing Cyclic Hydrocarbons
The cornerstone of RIS is based on incorporating arrows within carbon skeletons to represent functional groups. If methyl groups are treated as ordinary functional groups, demonstrated in 17 for one of the C6H14 skeleton forms,
c4-c-c-c
t t t t t
[s] [2R3Rl[2R4RlIRl [2R3S]2R4S
CFA = ~
7
(4)
RIS specifications, when applicable, are included below arrows. Brackets signify optically active fractions. Equation 4 clearly indicates that five of the seven fractions are optically active; diastereomers ([2R3RI/[2R3Sl and [2R4Rl/2R4S) and the meso compound (2R4S) are readily discernible. Representing Functional Group Isomers
Until now, the RIS method has focused on positional isomers, that is, structures that differ in the location of the functional group. The three isomers indicated by 1, for example, differ in the position of the chloro group. Ethyl alcohol and dimethyl ether portray another isomeric relationship: They differ in the type of functional group present. In this section, labeled arrows are used to represent distinct functional group isomers. The formula CSHIZOallows for 14 constitutional isomers-eight alcohols and six ethers. To avoid duplication of skeletal forms, alcohol and embedded ether functional groups are assigned conventional and looped arrows, respectively. This approach enables all C&IlzO isomers to be represented within three skeleton forms (%lo). To further emphasize functional groups, the eight alcohol arrows are pointed up, and the six ether arrows point down.
Primary, secondsly, and tertiary amines are treated as separate functional groups in the representation of the constitutional isomers of C5H13N.Primary and secondary amines are readily designated with labeled arrows (11-
~
5
confused students might erroneously refer to the structure designated by the leftmost arrow as l-methylpentane. To avoid oversights of this kind, I recommend writing out carbon skeletons completely. However, there is one exception: alkyl groups attached to cyclic skeletons. The dimethylcyclohexanes, represented by 18, parallel the dichlorocyclohexanes(4): Both possess common structures and nomenclatures. Due to the "closed" nature of cyclic structures, methyl groups, which are specified by arrows within the cyclohexane skeleton, are not misleading when determining nomenclature. Next, consider the cyclic structures of C6Hlz:The 20 isomers are represented within five skeletons (19-23). Besides drawing isomers in condensed form, RIS neatly organizes structures by 3-, 4 , 5 - , and &membered rings.
The aromatic ring makes benzene derivatives ideally suited for RIS. Derivatives of C&Ilz are represented in 2426. For 24 and 25, arrows with the methyl group symbol (Me) represent the mesitylenes and the ethyltoluenes, respectively. The selection of m-xylene, structure 24, permits a single skeleton to represent all the mesitylenes. NumVolume 69 Number 6 June 1992
449
the
the
hers within brackets show the number of isomers associated with each skeleton form. If an arrow points to a hoxed alkvl mouD. (261.~henvlmourn are considered attached to each unique siLaof the ilk$ group. Structure 26 represents two isomers: n-propylbenzene and iaopropylbenzene. Other common alkyl groups include butyl (four sites), pentyl (eight sites), hexyl(l7 sites), and heptyl(39 sites). Groups higher than heptyl lead to numbers of sites that are considerably larger. Structure 27 represents the following four butylbenzenes. n-butylbenzene sec-butylbenzene isobutylbenzene tert-butylbenzene
With the use of arrows that point both to and from the aromatic ring, the single skeleton shown in 28 represents: ortho-, meta-, andparan-butyltoluenes ortho-, meta-, andparosec-butyltoluenes ortho-, mela-, andparoisobutyltoluenes ortho-, meta-, andparatert-butyltoluenes To review some principles regarding aromatic isomers, are represented in the 22 benzene derivatives of C1~Hl4 2 9 5 4 . For arrows pointing toward the benzene ring, the symbols "Me" and "Et" specify methyl and ethyl groups, respectively Arrows pointing away from the aromatic system signify phenyl gmups attached to each unique site of the boxed alkyl group. For example, 30 comprises six structures. oraho-, meta-, andpamn-propyltoluenes ortho-, meta-, andpamisopropyltoluenes Representlng 'H-NMR Signals Although the final RIS application does not directly involve isomeric structures, a connection is evident: Arrows that designate unique sites within the carbon skeleton also
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Journal of Chemical Education
point to sets of equivalent hydrogens. For example, the arrows shown in structure 1point to the sites that cause the three 'H-NMR signals anticipated from n-pentane. I often use NMR arrows for a quick projection of a structure's spectrum. The NMR signals associated with 2,2,3-tribromobutane are shown in 35. Splitting of signals is noted by the numbers in the brackets. For the tribromobutane cited, three sets of equivalent hydrogens give rise to a singlet (no brackets), a doublet, and a quartet. For signals that are split into multiplets, the hydrogens are designated by an "m" in brackets. The doublet and multiplet that are generated by isobutane, are represented accordingly in 36.
Since diastereotopic hydrogens do not normally exhibit identical chemical shifts, they lead to different 'H-NMR signals. For l-bromo-1,2-dichloroethane,three distinct chemical shifts are predicted (37). By directing two arrows to the same carbon, diastereotopic protons are acknowledged. Concluding Remarks The RIS approach has been extended to accommodate Fisher projections, reaction products, functional group isomers, cyclic hydrocarbons, and 'H-NMR signals. These applications are intended not to replace traditional methods, but to help students recognize isomeric relationships. When used properly, RIS saves time and space, and it can serve as a master plan for writing out conventional structures.
