Question

Which of the following compounds have a stable trans isomer than its cis form?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A), (D)

In acyclic double-bonded systems, the terminal groups/atoms experience steric hindrance between them.
Steric hindrance between the terminal groups is greater in cis-isomer because bulkier terminal groups are close to each other in cis-isomer.
$\therefore$
Generally, cis-isomer of acyclic alkene system is less stable
than the trans-isomer.
Hence, trans form of compound (a) is stable.

For cycloalkene rings, when the number of carbon is less than 8 its trans form does not exist. Hence, no geometrical isomerism across the pi-bond is observed in 3-7 membered cycloalkenes.

Cycloalkenes with 8 or more carbons in a ring exhibit geometrical isomerism.
From cyclooctene, the ring is large enough to allow the existence of trans-isomers. But cis is the more stable form.

For cycloalkenes having ring size of 12 or more carbon atoms, trans-isomer is more stable than cis-isomer.
Hence, compound (d) has stable trans isomer than cis.