Question

Which one of the following compounds is optically active?

• A. 2-Chloropentane
• B. 3-Chloropentane
• C. 2-Chloropropane
• D. 2-Chloro-2-methylbutane

Hint:

Optical activity arises only when a carbon atom is attached to four different substituents.

Answer 1

The Correct Option is A

Step 1: Recall condition for optical activity.
A compound is optically active if it contains at least one chiral carbon atom bonded to four different groups.

Step 2: Analyse 2-chloropentane.
In 2-chloropentane, the second carbon is attached to:
– \(\text{Cl}\)
– \(\text{H}\)
– \(\text{CH}_3\)
– \(\text{CH}_2\text{CH}_2\text{CH}_3\)
These four groups are different, making it a chiral center.

Step 3: Analyse other options.
3-Chloropentane: Has two identical ethyl groups → achiral.
2-Chloropropane: Has two identical methyl groups → achiral.
2-Chloro-2-methylbutane: Central carbon has identical alkyl groups → achiral.

Step 4: Conclusion.
2-Chloropentane is optically active due to the presence of a chiral carbon.