2. NOMENCLATURE OF GEOMETRICAL ISOMERISM
• To represent the various geometrical isomers, following nomenclatures are used :
• Cis-trans nomenclature
• E-Z nomenclature
• Syn-anti nomenclature
3. CIS-TRANS NOMENCLATURE
• Cis is a Latin words and it means “ this side of”
• Cis indicates that the identical atoms or groups are on the same side of the
carbon double bond or ring
4. TRANS ISOMER:
• Trans is a Latin word which means “ the other side of”
• Trans indicates that the identical atoms or groups are on opposite side of the
carbon double bond or ring.
5. CIS-TRANS NOMENCLATURE
• Thus, cis-trans isomers are stereoisomers, that is pairs of molecules which have
the same molecular formula but whose identical atoms or groups are in different
orientation in three dimensional space
• Geometrical isomerism is possible because each double bonded carbon is attached
to it.
6. CIS-TRANS NOMENCLATURE
• LIMITATIONS :
• Cis trans nomenclature is not available for molecules which contain four different
connective substituents.
7. E-Z NOMENCLATURE
• This system can be used to specify the configuration about any carbon-carbon
double bond unambiguously by using a set of priority rules.
• This system is devised in 1964 by R.S.Cahn , C.K. Ingold and V.Prelog.
8. E-CONFIGURATION
• E is taken from German word entgegen which means “ opposite”
• If the two groups of highest priority are on the opposite sides of the double bond,
the bond is assigned as “ E Configuration”
9. Z-CONFIGURATION
• Z – Configuration is take from German word zusammen which means “ together”
• If the two groups of highest priority on the same sides of the double bond, the
bond is assigned as “ Z” configuration.
10. E-Z CONFIGURAATION
• The E-Z system is based on a set of “ priority rules” , which allow to rank any
groups.
• The general strategy of the E-Z system is to analyze the two groups at each end
of the double bond. At each end, rank the two groups, using the CIP PRIORITY
RULES
• Then, see whether the higher priority group at one end of the double bond and the
higher priority at the other end of the double bond are on the same side or on
opposite sides of the double bond.
11. CIP PRIORITY RULES
• RULE 1 :
• Each atom is assigned a priority. Priority is based on atomic number; higher the
atomic number, the higher the priority.
12. RULE 2
• For isotopes, the higher the atomic mass the higher the priority.
• For example, deuterium has higher priority than protium.
13. RULE 3
• If priority cannot be assigned on the basis of atomic number or atomic mass
considering the first atom of a group, then look at the next set of atoms and
continue until a priority can be assigned.
• Priority can be assigned at the first point of difference.
• If the atoms directly linked to the double bond are the same, then the second,
third, fourth, etc. atoms(away from the double bond) are ranked until a
difference is found.
14. RULE 4
• In the case of double or triple bonds, atoms participating in the double or triple
bond are considered to be bonded to an equivalent number of similar atoms by
single bonds, that is, atoms of double and triple bonds are duplicated or
triplicated.
15. E-z configuration
• To assign E-Z system, first determine the groups of highest priority on each
carbon. If the two highest priority groups are on the same side of the double bond,
the configuration is Z. If they are on opposite side of the double bond then the
configuration is E.
17. SYN-ANTI NOMENCLATURE
• Geometric isomerism is also possible in compounds containing oximes (C=N) and
AZO compounds (N=N).
• Thus, Syn- Anti nomenclature is used to indicate the geometrical isomerism
containing C=N or N=N bonds such as oximes and azo compounds.
18. SYN-ANTI NOMENCLATURE
• In aldoxime, the syn or anti configuration is determined by the position of the
lone pair of electrons on nitrogen atom with respect to alkyl (R) or aryl (Ar)
group.
• SYN CONFIGURATION:
• In aldoxime, syn configuration is one in which the lone pair on nitrogen and the
alkyl or aryl group on same side or H or OH is present in same side.
19. ANTI CONFIGURATION
• In aldoxime, anti configuration is one in which the lone pair on nitrogen and the
alkyl or aryl group on opposite side or H and OH is present in opposite side.
20. SYN-ANTI NOMENCLATURE
• Azo compounds contain two lone pair of electrons on each nitrogen atom.
• SYN CONFIGURATION:
• If both lone pair are present on same side, then assigned as syn configuration
• ANTI CONFIGURATION:
• If both lone pair are present on opposite side, then assigned as ani configuration.
