In small, cyclic alkenes, the two ring carbons must be on the same side of the alkene (cis) due to ring strain. If you try and make trans-cyclopentene for example you will quickly have a very bent molecular model kit.

The first cyclic alkene with a stable trans- geometric isomer is trans cyclooctene. Once the ring size increases past eight, then geometrical isomerism is possible. So in larger rings we should be specific about whether the double bond is cis or trans (or E/Z).

That was in a section on cis trans in *rings*.

In *rings* cis/trans is redundant if there are two chiral centers and you determine R/S on them both.

It is (E) and also cis. I wonder if this is a glitch with the nomenclature software at pubchem.

No human being would name this “trans”

E-Z is preferred and is non-ambiguous, but in simple cases cis-trans- will do.

It’s not energetically possible for 5- or 6- membered rings. Seven membered rings can be trans but have a short lifetime. Only at 8-membered rings do they start to be reasonably stable.

Glad you find it helpful, Tom. Best wishes on your graduate program. James