Practice Problem 8

Determine whether the following compound should be described as the Z or E isomer of 3-methyl-2-pentene.

Solution

There is no doubt about the priority of the CH₃ and H substituents on the second carbon atom in this molecule.

There is no way to assign priority to the atoms bound to the third carbon in this molecule -- they are both carbon atoms. We therefore proceed down the substituent. The next atoms we encounter in the CH₃ group are all hydrogen atoms. But the next atoms in the CH₂CH₃ substituent are two hydrogen atoms and a carbon atom. Thus, the CH₂CH₃ group has the higher priority.

We therefore conclude that the two substituents with the highest priority are on the same side of the C=C double bond.

This compound is therefore the Z isomer and it would be named as follows: (Z)-3-methyl-2-pentene.