E,Z-System of nomenclature-
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In tri & tetra substituted ethylenes, such as CH3 -CH = C(Br)Cl, then E,Z – system of nomenclature is applicable.This is based on priority rules.Groups on each carbon atom of the double bond are assigned priority 1 & 2 according to CIP rule(Cahn, Ingold, Prelog rule).
According to sequence rule –
(a) Atom of higher atomic number gets priority i.e. groups are ranked in decreasing atomic number of atoms by which they are bonded to assymetric carbon atom .
I>Br>Cl>O>N>C>H
decreasing priority
(b) If the first atom in the two groups have same atomic number then relative priority of the group is decided by the atomic number of the next atom in the two groups.
Ex.1—- In CH3 & –CH2–CH3
In both first atom is Carbon but next atom in –– CH 3 is H , while in ––CH2––CH3 , next atom is ‘C’. So ethyl group is given higher priority than methyl group.
Ex.2 — –CHO is given higher priority than –CN because in group 
, C is directly attached to oxygen while in –CN group C is directly attached to N. (at. no. of oxygen is greater than N).
(c) Multiple bonds are treated as separate single bonds.
Ex.1—- –CH=O is given priority over –CH2OH
I>Br>Cl, OH>NO2>NH2>COOH>CHO>CH2OH
According to sequence rule , groups on each carbon atom of double bond are assigned priority 1 & 2.
when groups of highest priority (i.e 1 ) are on the same side of the double bond, then configuration is Z. If the first priority (i.e 1) groups or atoms are on the opposite side of double bond, the configuration is E.
E comes from German word’ Entgegen’ means opposite & Z comes from German word ‘ Zusammen’ means together.
Example:
In -CH3 & -H priority is given to -CH3 because atomic number of carbon is more than hydrogen . So according to sequence rule, number 1 is given to -CH3 and number 2 to – H.
Similarly, in -Cl and -Br , priority is given to -Br because atomic number of Br is more than Cl. So according to sequence rule , number 1 is given to – Br & number 2 to – Cl.
In figure 1 , groups of highest priority are on opposite side of the double bond . So it is E-isomer.
In figure 2, groups of highest priority are on same side of double bond . So it is Z-isomer.
Q — Label the following geometrical isomers as E or Z.
Ans. According to E,Z-System of nomenclature,
Q — Assign E or Z configuration to each of the following compounds:
Ans. According to E,Z-System of nomenclature,
[because highest priority on same side of the double bond -CH2-CH3 group has higher priority than -CH3]
Note: As we know that atomic number of H is less than C.
