Consider bicyclo[4.4.0]decane: the two rings in this molecule can have cis or tr
ID: 900861 • Letter: C
Question
Consider bicyclo[4.4.0]decane: the two rings in this molecule can have cis or trans ring fusion, as shown below. Build models of each one!
Then decide how many of the molecules below can also be both cis and trans ring fused by building models:
A. None of these can be both cis and trans ring fused
B. One of these can be both cis and trans ring fused
C. Two of these can be both cis and trans ring fused
D. Three of these can be both cis and trans ring fused
E. All of these can be both cis and trans ring fused
Explanation / Answer
A. None of these can be both cis and trans ring fused
The rings with an unsaturation at the fusion cannot rotate due to the rigid pie bond (no free rotation) ,that needs considerable amount of energy to flip the p-orbital (perpendicular to the ring ) by 90 degree, much needed for rotation around the pie bond.
Also cyclobutane has considerable ring strain ,so its unabe to flip to undergo conformational strain.
(last structure) The cycloocta(b)furan ring with single bond can have both cis and trans conformations as both ring have less ring strain and can rotate around the fusion bond, but has steric hindrance due to oxygen in the furan ring with lone pair of electrons that would interact with the bridged carbon atoms at axial position in trans conformation so only cis conformation with ,one equatorial and one axial carbon is possible.