Question

The IUPAC name of:

(A) $\left( E \right),\left( E \right) - 2,4 - hexadiene$
(B) $\left( Z \right),\left( Z \right) - 2,4 - hexadiene$
(C) $\left( E \right),\left( Z \right) - 3,5 - hexadiene$
(D) $\left( Z \right),\left( E \right) - 2,4 - hexadiene$

Answer 1

Stereoisomers are those isomers that possess the same relationship between the substituent’s atoms but having different arrangements of atoms in the space. So, we can say that the relationship between two molecules above is that of diastereomers. So, we can say that diastereomers are the stereoisomers which are not mirror images of each other.
These two molecules have different physical properties – different boiling points, melting points, reactivity, spectral characteristics and so on.

Complete answer: In organic chemistry nomenclature, “cis” is used to the isomer which has two identical groups (e.g. the two chlorines in 1,2-dichlorocyclopentane) are present in the similar direction from the plane of the ring, and trans to the isomer which has groups in opposite directions. We can use the priority system developed by Cahn, Ingold, and Prelog which is explained as;
1. Each carbon in the pi bond is attached to two substituents. For each carbon, these two groups are given priority (1 or 2) according to the atomic numbers of the atom directly attached to the carbon. (e.g. Cl > F)
2. If both substituents with priority 1 are on the same side of the pi bond, the bond is given the naming Z (short for German Zusammen, means “together”).
3. If both substituents priority 1 are on the two-opposite side of the pi bond, the bond is given the naming E (short for German Entgegen, means “opposite”).
So, Z is same as “cis” and E is same as “trans”.

So, in our question methyl is given high priority over hydrogen atom.
So, one double bond will be in cis i.e. Z isomer and another double bond will be in trans i.e. E isomer. There are six carbons in the molecule and there two double bonds at second and fourth position.
So correct naming of this compound will be $\left( Z \right),\left( E \right) - 2,4 - hexadiene$

Hence, option (D), is the correct option.

Note: For any molecules in alkene showing cis-trans isomerism each carbon is must be bonded to two different groups, and that the two carbons must have one substituent common.