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Basic IUPAC Organic Nomenclature

E- and Z- nomenclature of alkenes

On the previous pages, we looked at naming alkenes as cis- and trans-.
It is important to note that the two methods are different (i.e. they are based on different rules) and they are NOT interchangeable, see below for an example.
The cis- / trans- style is based on the longest chain whereas the E/Z style is based on a set of priority rules. 
You need to know both styles if you started studying the syllabus in 2008.

2-butene
 2-chloro-2-butene
cis-but-2-ene
or
(Z)-but-2-ene
cis-2-chlorobut-2-ene
or
(E)-2-chlorobut-2-ene

The E- and Z- style is more reliable and particularly suited to highly substituted alkenes, especially when the substituents are not alkyl groups.

The Cahn-Ingold-Prelog priority rules are used for naming geometric isomers (e.g. E- or Z-alkenes) and other stereoisomers (see later).

These rules are based on atomic number, and the first point of difference.
  • Imagine each alkene as two pieces, each piece containing one of the sp2 C
  • Assign the priority (high = 1, low = 2) to each group on each sp2 C based on atomic number 
  • Determine the relative position of the two higher priority groups
  • If they are on the same side then it is a (Z)-alkene (German; zusammen = together) 
  • If they are on opposite sides then it is an (E)-alkene (German; entgegen = opposite)
  • If there is more than one C=C that can be E/Z, then the location needs to be included with the locant, e.g. (2E,4Z)-
Example: but-2-ene
  • Isomer 1 : 
split the alkene
 priorities
 relative positions
Step 1 : split the alkene
 Step 2 : assign the relative priorities
 Step 3 : look at the relative positions of the higher priority groups : same side = Z, hence (Z)-but-2-ene.
  • Isomer 2 :
E/Z-example
  

The two substituents are -CH3 and H, so since the atomic numbers C > H then the -CH3 group is higher priority. Therefore the two high priority groups are on the opposite side, then this is (E)-but-2-ene.

 

Subrules:

  • Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H
  • Same atom attached ? By moving out one unit at a time, locate the first point of difference and apply rules there.
   

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