Configurational Stereoisomers of Alkenes
Nomenclature of Alkene Stereoisomers
Configurational stereoisomers of the kind shown above need an additional nomenclature prefix added to the IUPAC name, in order to specify the spatial orientations of the groups attached to the double bond. Thus far, the prefixes cis- and trans- have served to distinguish stereoisomers; however, it is not always clear which isomer should be called cis and which trans. For example, consider the two compounds on the right. Both compound A (1-bromo-1-chloropropene) and compound B ( 1-cyclobutyl-2-ethyl-3-methyl-1-butene) can exist as a pair of configurational stereoisomers (one is shown). How are we to name these stereoisomers so that the configuration of each is unambiguously specified? Assignment of a cis or trans prefix to any of these isomers can only be done in an arbitrary manner, so a more rigorous method is needed. A competely unambiguous system, based on a set of group priority rules, assigns a
Z (German, zusammen for together) or
E (German, entgegen for opposite) to designate the stereoisomers. In the isomers illustrated above, for which cis-trans notation was adequate,
Z is *****alent to cis and
E is *****alent to trans.