Question

A/B Trans steroidal hydrogen in the 5th position

• A. Beta configuration
• B. Eclipsed conformation
• C. Alpha configuration
• D. Gauche conformation

Hint:

Remember: Trans A/B junction = H at C-5 in alpha position; Cis A/B junction = H at C-5 in beta position.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Steroid rings A and B can be fused in either a cis or a trans arrangement, and this depends on which face of the ring the hydrogen at carbon 5 sits on.

Step 2: Key Formula or Approach:
In steroid nomenclature, alpha means a group points below the plane of the ring system, away from the angular methyl groups, while beta means it points above, on the same side as those methyl groups.

Step 3: Detailed Explanation:
The angular methyl groups at C10 and C13 are conventionally drawn pointing up, or beta, and act as the reference for the rest of the molecule.
When the hydrogen at C5 also points up, or beta, it sits on the same face as the nearby ring B substituents, forcing rings A and B to bend into a cis fusion.
When the hydrogen at C5 instead points down, or alpha, it sits on the opposite face from the angular methyls, letting rings A and B lie in a flatter, trans arrangement.
Eclipsed and gauche are conformational terms describing rotation about a bond, and do not describe this fixed ring fusion geometry at all.

Step 4: Final Answer:
The C5 hydrogen is in the alpha configuration when the A/B ring fusion is trans.