To convert from anti-conformation to gauche, through which conformation do the largest groups pass?

The transition from anti-conformation to gauche involves a specific sequence of molecular orientations as the larger substituents on a molecule adjust their positions relative to one another. In an alkane, the anti-conformation places the bulky groups opposite each other, while the gauche conformation places them closer together, creating steric strain.

As the molecule moves from the anti to gauche conformation, the largest groups must rotate past one another. This rotation typically involves passing through an eclipsed conformation. In the eclipsed state, the larger groups are aligned with one another, which maximizes steric hindrance and torsional strain. Despite being less stable, this conformation is a necessary intermediate when transitioning between the anti and gauche arrangements.

By understanding molecular conformations, it’s evident that during this specific transformation, the eclipsed conformation represents the key point where the largest substituents temporarily experience the highest degree of interaction before settling into the more stable gauche arrangement. Thus, the correct answer identifies the necessary transition state through which these groups must pass.