Q.53 In the given molecule,
the number of chiral centers is __________.

The number of chiral centers in the given molecule is 4.

Detailed identification of chiral centers

The molecule shown is a polycyclic system with several saturated ring carbons, carbonyl groups, hydroxyl groups and an N,N‑dimethylamino substituent. Each potential chiral center must be a tetrahedral (sp³) atom attached to four different groups.

1. Chiral carbon bearing the left methoxy group

• This sp³ carbon is bonded to:

  • The methoxy group (–OCH₃).

  • A hydrogen atom (implicit).

  • One side of the fused ring system (toward the left aromatic/quinone portion).

  • The other side of the fused ring (toward the central ring).

• All four substituents differ, so this carbon is a chiral center (one stereocenter).

2. Chiral carbon bearing the left OH group (vicinal to the above)

• This sp³ carbon carries:

  • A hydroxyl group (–OH).

  • A hydrogen (shown with wedge).

  • The portion of the ring toward the left.

  • The portion of the ring toward the right (including the bridge that links to the carbonyl region).

• Each substituent is distinct, so this is the second chiral center.

3. Chiral carbon bearing the central OH group

• This ring carbon is drawn with wedges for the C–H and C–OH bonds.

• It is attached to:

  • An OH group.

  • A hydrogen.

  • Ring fragment leading toward the left double‑bonded carbonyl portion.

  • Ring fragment leading toward the right, including the nitrogen‑containing ring.

• Different connectivity on each side of the ring makes this carbon the third stereocenter.

4. Chiral nitrogen in the dimethylamino group

• The nitrogen is formally sp³ and attached to:

  • Two nonequivalent methyl groups (because each lies in a different environment within the fused system).

  • The polycyclic ring carbon.

  • A lone pair.

• In a rigid, bridged polycyclic framework where inversion at nitrogen is strongly hindered, such an amine nitrogen can be configurationally stable and treated as a chiral center. That gives the fourth chiral center in this molecule.

5. Why other positions are not chiral

• Carbonyl carbons are sp²-hybridized and therefore achiral.

• Aromatic carbons and explicitly double‑bonded carbons are sp² and cannot be chiral centers here.

• Remaining saturated ring carbons either:

  • Bear two identical ring paths (symmetry-related), or

  • Are not attached to four different substituents.

• Hence they do not qualify as chiral centers.

Therefore, counting only atoms that meet the stereochemical criteria, the molecule has 4 chiral centers.

SEO‑friendly introduction

Understanding how to count the number of chiral centers in the given molecule is a crucial stereochemistry skill for competitive exams like NEET, JEE and CSIR‑NET. This question features a fused polycyclic system with multiple functional groups, testing your ability to identify sp³ atoms with four different substituents and to recognise when atoms like nitrogen can also become stereogenic. By following a systematic method, you can reliably arrive at the correct answer, 4 chiral centers, without overlooking or overcounting any stereocenters.