# 19.7: E,Z Notation

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The configuration about double bonds is undoubtedly best specified by the cis-trans notation when there is no ambiguity involved. Unfortunately, many compounds cannot be described adequately by the cis-trans system. Consider, for example, configurational isomers of 1-fluoro-1-chloro-2-bromo-2-iodo-ethene, $$9$$ and $$10$$. There is no obvious way in which the cis-trans system can be used: A system that is easy to use and which is based on the sequence rules already described for the $$R$$,$$S$$ system works as follows:

1. An order of precedence is established for the two atoms or groups attached to each end of the double bond according to the sequence rules of Section 19-6. When these rules are applied to 1-fluoro-1-chloro-2-bromo-2-iodoethene, the priority sequence is:
• at carbon atom 1, $$\ce{Cl} > \ce{F}$$
• at carbon atom 2, $$\ce{I} > \ce{Br}$$
1. Examination of the two configurations shows that the two priority groups - one on each end - are either on the same side of the double bond or on opposite sides: The $$Z$$ isomer is designated as the isomer in which the top priority groups are on the same side ($$Z$$ is taken from the German word zusammen - together). The $$E$$ isomer has these groups on opposite sides ($$E$$, German for entgegen - across).$$^2$$ Two further examples show how the nomenclature is used: This system is especially useful for oximes, which have the structural feature $$\ce{-C=N-OH}$$. The two possible configurations at the double bond in the oxime of ethanal are $$11$$ and $$12$$: The cis-trans notation does not work well here, and structure $$11$$ has the $$Z$$ configuration and $$12$$ the $$E$$ configuration. In the older chemical literature, these stereoisomers were designated as syn and anti forms, but these names are really no better than cis and trans.

$$^2$$It would have been simpler to remember if $$E$$ stood for same side and $$Z$$ for opposite side, but it is too late now.