93. Which one of the following compounds is the simplest alkane that is optically active?
Simplest Optically Active Alkane
Correct Answer
✅ Correct Option: (B)
Understanding Optical Activity in Alkanes
An alkane becomes optically active only if it contains at least one chiral (asymmetric) carbon atom. A carbon atom is chiral when it is bonded to four different groups. Such a carbon atom gives rise to two non-superimposable mirror images called enantiomers, which rotate plane-polarized light in opposite directions.
If any two groups attached to the carbon are identical, the carbon is not chiral and the molecule remains optically inactive.
Analysis of Each Option
Option (A)
The branching carbon is attached to two identical alkyl groups. Since two substituents are the same, the carbon atom is not stereogenic. The molecule therefore possesses no chiral center and is optically inactive.
Option (B)
This molecule contains a carbon atom bonded to four different substituents:
- A methyl group (–CH₃)
- An ethyl group (–CH₂CH₃)
- A propyl group (–CH₂CH₂CH₃)
- A hydrogen atom (–H)
Because all four groups are different, this carbon is a stereogenic center. Consequently, the molecule exists as two enantiomers and is optically active.
Among the given compounds, this is the simplest optically active alkane.
Option (C)
Although this structure contains a chlorine atom, it is not an alkane because it is a haloalkane. Since the question specifically asks for the simplest alkane, this option is not applicable.
Option (D)
The central carbon is attached to two identical methyl groups. Since two substituents are identical, the carbon atom is not chiral. Therefore, the molecule is optically inactive.
Why Option (B) is Correct
Option (B) is the only alkane in which the stereogenic carbon is attached to four different groups. This satisfies the necessary condition for chirality, allowing the molecule to exist as a pair of non-superimposable mirror images. Hence, it is optically active.
Concept Behind the Question
This question examines the student’s understanding of chirality and stereochemistry. A common mistake is to count the number of carbon atoms without checking whether the central carbon actually has four different substituents. In stereochemistry, the identity of the groups attached to the carbon is more important than the total number of carbon atoms.
Final Answer
The only alkane containing a carbon atom bonded to four different substituents is Option (B). Therefore, it is the simplest optically active alkane among the given structures.
✅ Correct Answer: Option (B)
