Question

Total Number of Carbon atoms in pregnane is

• A. 18
• B. 19
• C. 21
• D. 27

Hint:

Remember: Pregnane = 21-carbon steroid backbone; Androstane = 19-carbon; Estrane = 18-carbon.

Answer 1

The Correct Option is C

- Pregnane is a C-21 steroid nucleus. It consists of the basic steroid four-ring structure (cyclopentanoperhydrophenanthrene nucleus) with two additional carbon atoms in the side chain.
- The core steroid structure has 17 carbon atoms: 3 six-membered rings (A, B, and C) and 1 five-membered ring (D).
- In pregnane, a two-carbon side chain is attached at C-17, making the total carbon count: 17 (rings) + 2 (side chain) + 2 (additional substituents) = 21 carbon atoms.
- Pregnane serves as the parent structure for important hormones like progesterone and corticosteroids.

Answer 2

Pregnane is the parent steroid skeleton for hormones like progesterone and the corticosteroids, and the question asks for its total carbon count. Let's check each proposed number against how the steroid skeleton is actually built up.

1. 18: A carbon count of 18 matches the estrane skeleton, which is the parent structure of the estrogens and lacks the two-carbon side chain and one angular methyl group that pregnane carries.
2. 19: A carbon count of 19 matches the androstane skeleton, the parent structure of testosterone and other androgens, which has the four fused rings plus both angular methyl groups but no side chain at C17.
3. 21: The basic steroid nucleus, the four fused rings A, B, C and D, accounts for 17 carbons. Pregnane adds two angular methyl groups, at C10 and C13, bringing the count to 19, then adds a two-carbon side chain at C17, bringing the total to 21. This matches how pregnane is defined.
4. 27: A carbon count of 27 matches the cholestane skeleton, the parent structure of cholesterol, which carries a much longer eight-carbon side chain at C17 instead of pregnane's two-carbon chain.

Building up the rings, the two angular methyls, and the two-carbon side chain gives a running total of 21 carbons for pregnane.

So the correct answer is 21.