Which of the following molecules can exist as Z or E isomers? Draw the (E) and (Z) forms.
If one of the carbons on the C=C is attached to two same groups (such as two hydrogens), it can not have E/Z configuration. For example, if we look at the double bond in the top molecule, carbon is attached to two methyl groups. Therefore, this alkene can not have E/Z configuration.
The bottom molecule has two double bonds. The one on the left is not capable of E/Z because a carbon on the double bond is attached to two methyl groups. However, the double bond on the right is capable of E/Z.
LINKS
Organic Chemistry Course
Online Organic Chemistry Tutoring
Transformation Tutoring