Question

Which of the following alkanes is optically active?

• A. 2-Methylbutane
• B. 2,3-Dimethylbutane
• C. 2,3-Dimethylpentane
• D. 2-Methylpropane

Hint:

To quickly find an asymmetric carbon in an alkane, look for a branching point that sits off-center along an asymmetric main chain. In 2,3-dimethylpentane, the 5-carbon chain provides an ethyl side and a methyl side, ensuring that the middle branched carbon gets four unique groups.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The objective is to identify which of the given branched alkanes is chiral, meaning it contains at least one asymmetric carbon atom (a stereocenter bonded to four entirely different groups) that renders the molecule optically active.

Step 2: Key Formula or Approach:
Draw the structural formulas of the options and check each carbon atom to see if it is bonded to four distinct chemical groups ($-\text{R}_1$, $-\text{R}_2$, $-\text{R}_3$, and $-\text{H}$).

Step 3: Detailed Explanation:
Let's analyze the carbon environments for each compound: 1.

2-Methylbutane: $\text{CH}_3\text{-CH(CH}_3\text)- \text{CH}_2\text{-CH}_3$. The second carbon ($\text{C}2$) is attached to a hydrogen, an ethyl group, and two identical methyl groups. Since it has two identical groups, it is achiral.
2.

2,3-Dimethylbutane: $\text{(CH}_3)_2\text{CH-CH(CH}_3)_2$. The structure is completely symmetrical, and both internal carbons are attached to two identical methyl groups, making it achiral.
3.

2,3-Dimethylpentane: Let's write out its structure and analyze the third carbon ($\text{C}3$): $$ \text{CH}_3- \overset{2}{\text{CH}}(\text{CH}_3)- \overset{3}{\text{CH}}(\text{CH}_3)- \overset{4}{\text{CH}}_2- \overset{5}{\text{CH}}_3 $$ Look closely at the substituents bound to $\text{C}3$: A hydrogen atom ($-\text{H}$) A methyl group ($-\text{CH}_3$) An ethyl group ($-\text{CH}_2\text{CH}_3$) An isopropyl group ($-\text{CH(CH}_3)_2$) Since all four attached groups are unique, $\text{C}3$ is a true asymmetric/chiral center, which makes the entire molecule optically active. 4.

2-Methylpropane: $\text{CH(CH}_3)_3$. The central carbon is bonded to three identical methyl groups, so it is achiral.

Step 4: Final Answer:
The optically active alkane is 2,3-Dimethylpentane, corresponding to option (C).