Question

Which of the following is optically active? (1) \(n\) -Propanol (2) 2-Chlorobutane (3) \(n\) -Butanol (4) 3-Hydroxy pentane

Short answer

2-Chlorobutane

Step-by-step solution

Understand Optical Activity

A compound is optically active if it can rotate plane-polarized light. This usually occurs when the molecule has a chiral center, which is a carbon atom bonded to four different groups.

Analyze each Compound

Examine each given compound to identify if it has a chiral center.

Step 2.1: Analyze n-Propanol

The structure of n-Propanol is CH3-CH2-CH2-OH. There is no carbon atom in this structure that is attached to four different groups. Thus, n-Propanol is not optically active.

Step 2.2: Analyze 2-Chlorobutane

The structure of 2-Chlorobutane is CH3-CH(Cl)-CH2-CH3. Here, the second carbon (C2) is attached to four different groups: a hydrogen (H), a chlorine (Cl), a methyl group (CH3), and an ethyl group (C2H5). Therefore, it has a chiral center and is optically active.

Step 2.3: Analyze n-Butanol

The structure of n-Butanol is CH3-CH2-CH2-CH2-OH. Again, no carbon atom in this structure is bonded to four different groups. So, n-Butanol is not optically active.

Step 2.4: Analyze 3-Hydroxy Pentane

The structure of 3-Hydroxy Pentane is CH3-CH2-CH(OH)-CH2-CH3. The third carbon in the chain (C3) is attached to a hydroxyl group (OH), a hydrogen (H), a methyl group (CH3), and a propyl group (C3H7), making it a chiral center. Therefore, 3-Hydroxy Pentane is optically active.

Conclusion

From the analysis, both 2-Chlorobutane and 3-Hydroxy Pentane have chiral centers and are optically active. However, as the exercise here directs us to consider single choice, the most commonly recognized optically active compound in the context of introductory problems is 2-Chlorobutane.

Key concepts

chiral center

A molecule is said to have a chiral center when it contains a carbon atom bonded to four different groups or atoms. This results in non-superimposable mirror images, known as enantiomers. Enantiomers have unique 3D arrangements that can't be aligned by simple rotation. The chiral center is crucial for optical activity because it enables the molecule to interact with plane-polarized light, rotating it in a specific direction. To identify a chiral center:

• Look for a carbon atom bonded to four distinct groups (