2012 #5 CIS/TRANS in CYCLOALKENES???!!!

[Wendy_Liu_3A]

1. Could anyone explain why compound B is cis?
2. Do we always include cis/trans on cycloalkenes? Why is cis/trans necessary when we can only draw cycloalkenes in one way?

THANK YOU

[Chem_Mod]

Without a picture, nobody knows what you are referring to.

[Wendy_Liu_3A]

In my subject I meant Final 2012 Question #5. Here is a photo of the problem:
The answer is Z-cyclooct-4-enone. Thank you!

[Chem_Mod]

Cis refers to the fact that the parent chain (the cyclooctene ring) is on the same side of the double bond.

Typically, E/Z is used for double bonds in ring systems.

When the ring is 7-membered or smaller, it is not possible to form anything except a Z double bond. However, when we increase ring size to 8-membered or above, there is enough flexibility in the skeleton to allow an E double bond. Basically, for 8-membered rings or above you need to specify double bond geometry. For 7-membered rings and smaller, there is no need.