A. G. PINKUS Baylor University, Waco, Texas
t,he increasing availability of isotopes, many compounds are being prepared which contain one or more isotopic atoms. This note will serve to call attention to the isomerism encountered in these compounds and will suggest correlations. It is proposed that isomerism which is the result of the position of an isotopic atom or atoms in a linear or cyclic compound be called isotopic isomerism when the isotopic atom or atoms form a part of the skeleton of the molecule. Isotopic isomerism does not include molecules containing isotopic substitnents (e.g., deuterated benzene) since the isomerism problems encountered here can be treated in the same way as is done for position isomerism. Isotopic isomerism should be clearly differentiated from the term nuclear isomerism1which is used by the physicist for isotopes of an element which differ in energy content, stability, and spin. For e ~ a m p l e , ~ B P having a half life of 18 hours has a metastable state with a 4.4 hour half life which is designated as Be0". It should also be noted here that the term nucleus ismner has been used as another name for chain i ~ o m e r . ~ In view of the close similarity with the physicist's term, it wonld seem preferable to discontinue the use of the former in favor of the term chain isomer. The follo~vingoutline of isotopic isomerism lists examples as a guide in defining the various categories. All can be elaborated further. WITH
Linear Molecules. I n the homologous series starting with methane, isotopic isomerism is first encountered in propane with one isotopic carbon atom (I). The asterisk denotes the isotopic carbon atom I3C or "C; the remaining carbon atoms are I2C. The isomers for doubly laheled propane are (11). c-C-*C
Polymers. The polymerization of isotopic ethylene will serve to illustrate a simple case.
c=c*
-c-c-c-c*-
C-C'C"
I
c*-C-C*
I1
%-Butane and the higher members of this homologous series can be treated similarly, the number of isomers increasing aith the chain length. Monocyclic Molecules. For one isotopic atom no isomerism is possible in benzene or in cyclopropane or in the cycloalkane analogs as long as there are no substituents on the ring. For monosubstituted benzene, there are four possible isomers (111). 'See for example, WIEDENBECK, M. L., P h p . Rev., 88, 67 (1945).
HEARTH, E. M. Phys.Rev., 88,170 (1952). SHRINER, R. L., R. ADAMS,AND C. S. MARVEL, in Gilman's "Organic Chemistry," John Wiley & Sons, New Yark, 1938, Vol. I, p. 154.
VOLUME 34. NO. 6, JUNE, 1957
c=c*
+ -(-c-c*-c-c*-), head to tail polyethylene polymer
--c-c*-c
