Question

Which of the following is Z isomer?
A.

B.

C.

D.

Answer 1

Hint: As we know that in alkenes there are two different substituents present at the each end of the carbon-carbon double bond that can occur as a pair of stereoisomers. The designators E and Z are very helpful to unambiguously designate the stereochemistry of alkene than the trans and cis.

Complete step by step solution:
In order to use the naming system, first we have to identify the higher priority group which is located at the each carbon of the double bond by using the similar priority rules we use in the case of R/S systems. When there is the presence of a higher priority group on the same side of the double bond then we designate it as Z-alkene. When there is the presence of the higher priority group on the opposite side of the double bond then we designate it as E-alkene.
The priority of the compounds is given based on their atomic number. In the left hand side, $C{H_3}$ has the highest priority when compared to hydrogen as the atomic number of carbon is greater when compared to hydrogen. On the right hand, the bromine has the highest priority when compared to chlorine as the atomic number of chlorine is less when compared to Bromine.
So, Option (A) is correct.

Note:We have to know that there is no possibility of designating all the alkenes as E or Z. When one of the double bonded carbon contains the same substituents, the alkene isn't considered as stereogenic and in such cases there is no possibility to designate E or Z configuration.