The number of chiral carbons present in the molecule given below is _____ .

Solution

Chemistry ma’am · 4 steps

Look only at sp³ carbons with one H, then check that their four groups are all different.

1. Filter first. A chiral carbon needs four different groups, so it must be sp³. Skip every ring carbon of the aromatic rings, every CH₃ and every CH₂, because each of these has two identical groups or a double bond.
2. Check the CH carbons. Only five sp³ CH carbons are left. For each, read off its four groups.

   Carbon	Its four groups	Chiral?
   CH(OH)CH₃ on the quinoline	H, OH, CH₃, quinoline ring	yes
   CH carrying the O-aryl group	H, O-tolyl, quinoline ring, bicyclic ring	yes
   Ring CH next to N	H, N, the CH(O-aryl) carbon, ring CH₂	yes
   Ring CH carrying the ethyl	H, ethyl, CH₂N, bridgehead CH	yes
   Bridgehead CH	H, plus three ring paths: one through the ethyl-bearing CH, one through the substituted CH next to N, one through plain CH₂CH₂ to N	yes, the three paths differ

3. Bridgehead check. In an unsubstituted bicyclic ring the bridgehead would have identical paths. Here the ethyl group and the side chain sit on different bridges, so all three paths are different and the bridgehead carbon is chiral.
4. So. The molecule has 5 chiral carbons.

The common slip: 4. Skipping the bridgehead CH of the bicyclic ring gives 4, not 5. Its three bridges are made different by the ethyl group and the side chain.

The idea to keep. In bicyclic rings, a bridgehead CH is chiral whenever substituents make its bridges different. Trace each path until the bridges stop matching.

Answer from the NTA final answer key. Solution by PrepWiser.