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Difficulty: MediumStructural Isomerism and Stereoisomerism

For an organic compound to exhibit optical isomerism, it must possess a chiral carbon atom bonded to four different groups or atoms. Which of the following compounds possesses a chiral carbon atom and exhibits optical activity?

  1. 2-chlorobutaneAnswer
  2. B
    1-chlorobutane
  3. C
    2-chloropropane
  4. D
    2-methylpropan-2-ol

Answer

2-chlorobutane is the correct answer because its second carbon atom is attached to four distinct groups, forming a chiral center that exhibits optical isomerism.
The correct answer, 2-chlorobutane, has a central carbon atom (C-2) bonded to four completely distinct substituents: a hydrogen atom, a chlorine atom, a methyl group, and an ethyl group. Because all four attached groups are unique, the molecule lacks internal symmetry and displays optical isomerism (enantiomerism).

Step-by-Step Solution

1
Recall the condition for optical isomerism
A molecule must contain at least one chiral (asymmetric) carbon atom, which is a carbon atom bonded to four completely different atoms or groups.
Chirality prevents the mirror image of the molecule from being superimposable on the original structure.
2
Analyze the carbon environments for each compound
For 2-chlorobutane: CH3−C∗H(Cl)−CH2CH3\text{CH}_3-\text{C}^* \text{H(Cl)}-\text{CH}_2\text{CH}_3. The starred carbon (C∗\text{C}^*) is attached to −H-\text{H}, −Cl-\text{Cl}, −CH3-\text{CH}_3, and −CH2CH3-\text{CH}_2\text{CH}_3.
All four groups attached to carbon-2 are unique, creating a chiral center.
3
Check the remaining options for symmetry
1-chlorobutane has −CH2−-\text{CH}_2- groups; 2-chloropropane has two identical −CH3-\text{CH}_3 groups on C-2; 2-methylpropan-2-ol has three identical −CH3-\text{CH}_3 groups on C-2.
Molecules containing identical groups attached to the same carbon atom are achiral and optically inactive.

Key Concept

Optical Isomerism and Chirality
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Topics

Structural Isomerism and Stereoisomerism

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