IUPAC Nomenclature of Organic Isomers

 

E/Z Nomenclature - Alkenes

R/S Nomenclature - Optical isomers

 

Priority Rules for E/Z Nomenclature

Priority Rules

· Priorities are based on the atomic numbers of the atoms directly attached to

the carbon atoms of the double bond or the chiral (stereogenic) centre. Thus H always has lowest priority.

· If two groups have the same atom attached to the double bond or chiral centre, compare the atomic numbers of the second atom in each group. If necessary, go to the third atom …, etc.

e.g. -CH2CH3 has priority over -CH3

· Multiply bonded atoms are considered as an equivalent number of singly bonded atoms.

e.g.

 

E/Z Nomenclature for Geometric Isomers

Assign priorities (high, low; 1,2) to the groups on each carbon of the double bond.

(Z) –Isomer: Has the groups of highest priority on the same

(German; ‘zusammen’,) remember it as ‘Z is for Zame Zide’) side of

the double bond.


(E) –Isomer: Has the groups of highest priority on opposite

(German; ‘entagegen’) sides of the double bond.

 

 

Priority Rules for R/S Nomenclature

 

· Priorities are based on the atomic numbers of the atoms directly attached to

the carbon atoms of the double bond or the chiral (stereogenic) centre. Thus H always has lowest priority.

· If two groups have the same atom attached to the double bond or chiral centre, compare the atomic numbers of the second atom in each group. If necessary, go to the third atom …, etc.

e.g. -CH2CH3 has priority over -CH3

· Multiply bonded atoms are considered as an equivalent number of singly bonded atoms.

e.g.


R/S Nomenclature for Optical Isomers


Assign priorities (highest-lowest; a,b,c,d) to the four different groups attached to the chiral carbon atom.


View the molecule along the axis from the chiral centre to the lowest priority group (H if it is present).

(R) –Isomer Has the priority sequence a ® b® c in a clockwise direction.

(S) – Isomber Has the priority sequence a ® b® c in an anti clockwise direction.