Question

Which isomer of $\text{C}_6\text{H}_{14}$ has highest boiling point?

• A. Hexane
• B. 3-Methylpentane
• C. 2-Methylpentane
• D. 2,2-Dimethylbutane

Hint:

When sorting alkane isomers by boiling point, you can instantly find the answer with this simple visual rule:
$$\text{Highest Boiling Point} = \text{Straightest Chain (No branches)}$$
$$\text{Lowest Boiling Point} = \text{Most Spherical Chain (Most branches)}$$
Since Hexane has zero branches, it easily takes the top spot.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to identify which structural isomer of hexane ($\text{C}_6\text{H}_{14}$) exhibits the highest boiling point among the given options.

Step 2: Key Formula or Approach:
The boiling point of volatile organic alkanes is governed by the intensity of their intermolecular London dispersion forces.
For a group of structural isomers containing the same number of carbons:
$$\text{Boiling Point} \propto \text{Strength of London Forces} \propto \text{Surface Area} \propto \frac{1}{\text{Branching}}$$
Linear, straight-chain molecules pack closely together over a larger surface area, maximizing van der Waals attractions and raising the boiling point. Introducing branching makes the molecule more spherical, decreasing its effective surface area and dropping the boiling point.

Step 3: Detailed Explanation:
Let's classify the structural features of each option:
Hexane ($n$-hexane) is a completely straight, unbranched 6-carbon chain. It has the maximum possible surface area for contact.
2-Methylpentane and 3-Methylpentane are mono-branched isomers containing a 5-carbon main chain and one methyl branch, making them more compact than $n$-hexane.
2,2-Dimethylbutane is a highly branched di-substituted isomer with a 4-carbon main chain and two branches, making it the most spherical and compact option.
Since $n$-hexane is entirely unbranched, its molecules can align closely alongside one another, maximizing intermolecular attractions. Consequently, breaking these strong dispersion forces requires the highest thermal energy, giving it the highest boiling point of the group.

Step 4: Final Answer:
The isomer with the highest boiling point is the straight-chain alkane, Hexane, which corresponds to option (A).