Although many compounds can be named uniquely using the cis/trans approach, there are
compounds which cannot be so distinguished. Take for example the two isomers of
1-bromo-1-chloroprop-1-ene. Which of the halogens, bromine or chlorine is to be trans to the
methyl, and which one cis?
To overcome this difficulty, the IUPAC system introduces a different approach to alkene
nomenclature, an approach which prioritizes the two groups on each end of the double bond and names
according to the orientation of the two highest priority groups. The system involves a set of
arbitrary rules as follows:
Rules for Assigning E or Z.
- The two ends of the double bond are considered separately.
- The two groups at each end are prioritized (as noted above, the two groups must be
different if stereoisomers are to exist.)
- The designation E (entgegen) is given to the stereoisomer in which the two highest
priority groups are across the double bond from each other.
- The designation Z (zusammen) is given to the stereoisomer in which the two highest
priority groups are on the same side of the double bond.
The prioritization of the groups is also done by following a set of arbitrary rules, in this case
known as the Cahn-Ingold-Prelog rules:
The Cahn-Ingold-Prelog Rules for Assigning Group Priority in Stereochemistry.
- Groups to be prioritized are compared atom by atom starting with the
atom connected to
the site of the stereoisomerism, in this case the carbons of the double bond.
- The atoms being compared are prioritized according to their atomic number: the higher
the atomic number, the higher the priority.
Decreasing priority: I, Br, Cl, F, O, N, C, H.
- If the atoms being compared have the same atomic number, but differ in mass number
(that is they are isotopes) then the prioritization is by increasing mass number.
Decreasing priority: T, D, H. (i.e. 3H, 2H, 1H).
- If the two atoms are identical, then move along the chain of atoms away from the
stereochemical centre comparing atom by atom until a difference is found.
Decreasing priority: BrCH2-, FCH2-, HOCH2-,
- For multiple bonds, consider each bond of the multiple bond as an individual
connection to the next atom:
- that the two systems of naming the alkene stereoisomers are quite different and cannot be
intermixed in a name. For the rest of this tutorial set, the E/Z method will be used.
- that the cis/trans system is used exclusively for the stereoisomerism due to a ring system.
Examples of the naming of alkene stereoisomers:
| ||Alkene Structure||IUPAC Name||Model|
- Examples 1 and 2 show the use of E/Z for routine alkene structures for which cis/trans
could have been used. Reminder: do not mix the systems.
- Examples 5 and 6 show how the rules require you to move down the chain away from the
stereocentre (the double bond) until a difference is found.
- Example 6 illustrates that the functional group named as a suffix takes precedence in
the numbering system and is given the lowest possible number.
- Example 7 shows the isotope precedence rule applied.
- Example 8 shows the C=C taking precedence over the C-C.
- Example 8 also illustrates that di, tri, etc, can be used when more than one double
bond is present, and that in these cases the location of the stereochemistry will have to be given
(here by the (3Z) prefix showing that the Z stereochemistry applies to the double bond at carbon 3).
(Strictly, with only one stereocentre there is no real need to put the locant in. However, if
more than one stereocentre must be labelled the locants must be added.)