See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The ease of rotation about a single C-C bond connecting two rings depends on the steric and torsional strain involved, as well as the hybridization/geometry at the junction carbons and the ring strain. More specifically, the barrier to rotation is influenced by: (1) the size of the rings (larger rings with more substituents create more steric hindrance), and (2) the internal angles of the rings at the junction carbon (which determines how eclipsed the substituents become during rotation). Key principle: The rotational barrier about the central single bond (the bond being rotated, indicated by the arrow) depends on the steric bulk on either side. Larger rings have more atoms and substituents flanking the central bond, increasing steric clash during rotation. Also, rings with more double bonds (more sp2 carbons) tend to be more planar and create more A(1,3) strain or steric interaction during rotation. Analysis of each structure: - Structure A: cyclopentadiene (5-membered, 2 double bonds) on top + cyclopropane/cyclopropene (3-membered, 1 double bond) on bottom. The 5-membered ring is moderate in size; the 3-membered ring is very small and has a fixed small internal angle (~60°), creating significant angle strain but limited steric bulk. The small 3-membered ring has restricted geometry. - Structure B: cyclohexadiene (6-membered, 2 double bonds) on top + cyclopentadiene (5-membered, 2 double bonds) on bottom. Both rings are relatively large with multiple double bonds (more sp2 centers, more planar rings). This combination creates the greatest steric interaction during rotation because both sides of the bond have larger, more planar rings with substituents that clash during rotation. Hence B has the highest rotational barrier. - Structure C: cyclohexadiene (6-membered, 2 double bonds) on top + cyclopropane/cyclopropene (3-membered, 1 double bond) on bottom. One large ring and one very small ring. The 3-membered ring provides less steric bulk than the 5-membered ring, so the rotational barrier is less than B but the 6-membered ring contributes more than in A. Ranking rotational barrier (difficulty of rotation = barrier height, but the question asks about rotation ease — higher barrier means less free rotation): - B has the largest rings on both sides → highest barrier to rotation → least free rotation, but most 'rotation' discussed here likely means the rotational barrier. Wait — re-reading: 'compare carbon-carbon bond rotation' — this refers to the barrier/ease. Given answer is C (option c): B > A > C. Reasoning for B > A > C: - B: 6-membered ring + 5-membered ring → largest steric interaction → highest rotational barrier (most restricted rotation, or largest torsional barrier). - A: 5-membered ring + 3-membered ring → intermediate. The 5-membered diene ring is planar and has moderate bulk, while the cyclopropene ring is small but locked at 60° angles. - C: 6-membered ring + 3-membered ring → although one ring is large, the 3-membered ring is so small and has such a narrow angle that the overall steric interaction during rotation is less than in A (where a 5-membered planar diene creates significant interaction with adjacent ring hydrogens). The cyclopropene (3-membered ring with double bond) at the bottom of A and C: in C, the top 6-membered ring is larger than the 5-membered ring in A, but the rotational barrier in A is larger than C because the 5-membered diene has more eclipsing H interactions with the bottom ring than the combination in C. Actually, the key insight is that cyclopropane/cyclopropene rings, due to their bent bonds and small size, contribute less to rotational barrier than larger rings. So: B (6+5) > A (5+3) > C (6+3) makes sense because A's 5-membered ring contributes more rotational barrier than C's 3-membered ring despite C having a larger top ring — the 5-membered planar diene creates more steric interaction than the tiny cyclopropene. Therefore, the correct answer is C.