Question
Question: Which of the following compound(s) has/have ‘Z’ configuration? i. has/have ‘Z’ configuration?
i.
ii.
iii.
A.(i) only
B.(ii) only
C.(iii) only
D.(i) and (iii)
Solution
When the groups with higher atomic number (higher priority) are present on the same side of the double bond, then it is Z configuration but when present on the opposite side of the double bond, the configuration will be E.
Complete step by step answer:
There are two types of isomers- geometrical and optical isomers. Isomers are those which have the same molecular formula but different arrangement in space. Geometrical isomers can be cis-trans, E-Z, syn-anti types.
E-Z type of geometrical isomers are used when no same groups are present on both sides of the double bond. Groups are assigned priority on both sides as per Cahn-Ingold-Prelog priority rules. Z type isomers are those which have the same priority groups on the same side of the double bond whereas E type isomers are those in which groups of same priority lie on different sides of the double bond.
Priority is assigned as per their atomic number. The atom having higher atomic number is given high priority and atom having lower atomic number is given lower priority.
In (i), the higher priority atoms are chlorine and bromine on either side of the double bond and since both of them are located on the same side that is pointing upwards and facing each other, it is Z isomer.
In (ii), the higher priority groups are again chlorine and bromine on either side of the double bond but here, both are located opposite to each other, so it is E isomer.
In (iii), the higher priority groups are bromine and methane on respective sides, and they are facing each other by being located upward. Thus, it is also a Z isomer.
Hence, the correct option is (D).
Note:
We use cis-trans terminology when there are two same types of groups attached on the either side of the double bond or opposite side. When same groups on adjacent side, it is cis isomer and when same group on different sides, then it is trans isomer. Same is the case of syn-anti type.