notes on nomenclature
(a)
(b)
Figure 1. Geometry of a pair of daubiy-bound atoms. A and B. and their nearest neighbors. Chemical Abltractl Service Columbux, Ohio 43210
KURT LOENING ROY M. ADAMS
Gene*. College Beaver Follr, Pennsylvonim 15010
Stereoisomerism about a Double Bond (1-3) Isomers of the type H\C/C'
H\C/C1
I1
II
H/ C \ C I
CI /C\H
have long been designated cis when the like groups were on the same side of the molecule and trans when on opposite sides. However, until recently, it has not been clear in such a case as
The rules for specifying these configurational differences are: I. For each double bond to he described configurationally, determine which of the two groups attached to each of the doubly bound atoms has the higher priority according to the sequence rules of Cahn, Ingold, and Prelog (4). 11. That configuration in which the two groups of higher priority are on the same side of the reference plane (Fig. 1) is assigned the stereochemical descriptor Z (from the German zusammen); that configuration in which these groups are on opposite sides is assigned the descriptor E (from the German entgegen). The application of these rules is illustrated in Figure 2. Those wishing to apply these rules to compounds with several double bonds should consult the original references, Sec. E. of the Rules for the Nomenclature of Organic Chemistry IUPAC ( 3 ) or the Index Guide to Chemical Abstracts 15, 61. Literature Cited (1) Rlackmd. J. E., Gladys. C. L.. henin& K. J. Amar Chem Sor. 911. 509i19Wi. (2) ~ l ~ ~ k ~J.r E., n dmadys. . C.
which would be designated as cis and which trans. Similarly, the isomeric oximes of unsymmetrical ketones have been designated syn and anti. although once again there were no unequivocal rules for unambiguously applying the descriptors to specific isomers
In general terms the problem reduces to the following: there is need for a general method of specifying the configurational differences represented by a pair of doubly hound atoms, A and B, and their nearest neighbors, 1, 2, and :I, 4, respectively (Fig. 1). Since the molecular configuration of this system is such that all of the atoms lie in the same plane, P , the necessary and sufficient condition for stereoisomerism is that the atoms or groups attached to A (1 and 2 ) and B (3 and 4). respectively, he unequal. Thus the two possible configurations shown in Figure 1 can be differentiated by indicating that groups 1 and 3 are on the same side of the reference plane, Q (Fig. l a ) , or on opposite sides of the reference plane (Fig. lh). 60
/
Journal of Chemical Education
L.. Petrarca. A. E.. and Rush, J. E..
L.. Patrarca. A. E.. Poueil, W. H..and Rush. J. E..
J. Cham. Doc. 8.3911368l. (:I1 IUPAC Tentative Ruler for the Nomenclature of Orranic Chemistry. See. E. Fundamental Sfereochemirtry. J. 018. Chem 15. 285161 11970). (41 Femelius. W . C . , h n i n g , K . L.,andAdams. R.M., J. Chem. Educ. ( 5 ) Combined Intmduelionrlndsreato VolumeM, Chem Abs 66. 96ll967l. (6) introduction ta the CA Index Guide. Chem A b r . 76. 20811972-16).
.
Figure
2. Examples illustrating application of the rules for assignment of
E and Z.
