E-Z Notation
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what is the problem that comes with using the cis-trans notation
it only works for some compounds
why doesnt the cis-trans notation work with: H Cl H Br | | | | C=C C=C | | | | F Br F Cl
• there are 4 different groups • so the idea of two identical groups being in a cis-trans relationship doesnt work • as there are not two groups that are the same
what system would we use instead rather then cis-trans notation
the E-Z system
what is the first step when using the E-Z system
• work out the part of the name that can be used for both isomers • by using the normal nomenclature rules
what would the common name between these two isomers
1-bromo-1-chloro-2-floroethene
what is the second step in the system
using the priority rules to decide which of the two toms on the left hand side of the double bond has higher priority
how is priority decided in this case
• it is decided by the atom with the higher atomic number • H=1 and F=9 • so F has the higher priority
what is step 3
• do the same thing for the atoms on the right hand side of the double bond • Cl = 17 and Br = 35 • so Br has higher priority
what is the final step of the E-Z system
decide whether the two higher priority atoms are at opposite ends if the molecule (across and up / down) or not
what kind of isomer do you have if the higher priority atoms are at opposite ends of each other
an E-isomer (equivalent of trans)
what kind of isomer do you have if the higher priority atoms are at opposite sides but no opposite ends of each other
a Z-isomer (equivalent of cis)
what would be the isomer of the first and second molecule previously shown
• the first one would be a Z-isomer • the second would be an E-isomer • as in the first the F and Br are only at opposite sides • whereas in the second they are are opposite ends
what is a way of remembering the difference between E and Z isomer
• the E in E-isomer stands for enemy • as the high priority atoms are enemies with each other • ans therefore want to stay as far away from each other as possible • so they would be at opposite ends • Z-isomers are just the other option
where do the letters E and Z come from in the first place
• the letter E used comes from the german word entegen • which means opposite • the letter Z used comes from the german word zusammen • which means together (opposite sides but shoulders touching kinda thing)
what is the definition of structural isomer
• compounds with the same molecular formula - but with different structural formulae
what is the definition of a stereoisomer
• compounds with the same molecular and structural formula - but with a different arrangement of atoms or groups in a 3 dimensional field
what is the definition of a geometric isomer
• compounds containing a C=C bond - with atoms or groups attached at different positions
what is the definition of restricted rotation
when the atoms or groups bonded to a C=C bond have their postions fixed