Question

Out of the following isomeric alcohols containing five carbon atoms, the alcohol that exhibits optical isomerism is

• A. 1-pentanol
• B. 2-pentanol
• C. 3-pentanol
• D. 2-methyl-2-butanol
• E. 2,2-dimethyl-1-propanol

Hint:

A carbon must have four different groups to be chiral. Symmetry destroys optical activity.

Answer 1

The Correct Option is B

Concept: Optical isomerism arises when a molecule contains a chiral (asymmetric) carbon atom. A carbon atom is said to be chiral when it is attached to four different groups. Such molecules exist as non-superimposable mirror images (enantiomers).

Step 1: Write structures of all given alcohols.
• (A) 1-pentanol: \(CH_3-CH_2-CH_2-CH_2-CH_2OH\)
• (B) 2-pentanol: \(CH_3-CH(OH)-CH_2-CH_2-CH_3\)
• (C) 3-pentanol: \(CH_3-CH_2-CH(OH)-CH_2-CH_3\)
• (D) 2-methyl-2-butanol: \(CH_3-C(OH)(CH_3)-CH_2-CH_3\)
• (E) 2,2-dimethyl-1-propanol: highly branched structure

Step 2: Check for chiral carbon in each. (A) 1-pentanol: OH is at terminal carbon → that carbon has two H atoms → not chiral → no optical isomerism. (B) 2-pentanol: Focus on carbon 2: \[ CH_3-CH(OH)-CH_2-CH_2-CH_3 \] Substituents on this carbon:
• \(H\)
• \(OH\)
• \(CH_3\)
• \(C_3H_7\) All four are different → carbon is chiral → optically active. (C) 3-pentanol: Structure is symmetric: \[ CH_3-CH_2-CH(OH)-CH_2-CH_3 \] Left and right sides identical → not chiral. (D) 2-methyl-2-butanol: Central carbon has two identical \(CH_3\) groups → not chiral. (E) 2,2-dimethyl-1-propanol: Highly symmetric → no chiral carbon. Final Conclusion: \[ \boxed{\text{Only 2-pentanol shows optical isomerism}} \]