Observe structures of the following compounds
The total number of structures/compounds which possess asymmetric carbon atoms is ______________.

Solution

Chemistry ma’am · 3 steps

For each carbon, ask one question: are all four groups on it different? Then count compounds, not carbons.

1. What to look for. An asymmetric carbon is an sp³ carbon with four different groups on it. Two identical groups (two H, or two CH₃) on the same carbon rule it out, and sp² carbons never qualify.

   Compound	Carbon to check	Groups on it	Asymmetric carbon?
   2-Chlorobutane	C bearing Cl	H, Cl, CH₃, C₂H₅	Yes (1)
   Glycine	α-carbon	H, H, NH₂, COOH	No, two H
   Alanine	α-carbon	H, CH₃, NH₂, COOH	Yes (1)
   Styrene	all carbons	every carbon is sp²	No
   Menthol	C–OH, C–isopropyl, C–methyl	each has H, a substituent and two different ring paths	Yes (3)

2. Menthol in detail. On the ring carbons bearing OH, isopropyl and methyl, the two ring directions lead to different neighbours, so each has four different groups. The CH of the isopropyl group carries two CH₃ groups, so it is not asymmetric.
3. So. 2-Chlorobutane, alanine and menthol have asymmetric carbons, so the answer is 3.

The common slip: 5. Counting asymmetric carbon atoms instead of compounds gives 1 + 1 + 3 = 5, because menthol alone has three.

The idea to keep. Check each sp³ carbon for four different groups, and in a ring compare the two ring directions as two separate groups. Read carefully whether the question counts compounds or carbons.

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