Among the following structural isomers of hexane (C6H14), which one has the highest boiling point under normal conditions?

Introduction / Context:
Boiling points of organic compounds depend on intermolecular forces and molecular shape. For alkanes with the same molecular formula, different structural isomers can have different boiling points. In general, less branched (more linear) isomers have higher boiling points because they have larger surface areas and stronger London dispersion forces. This question tests your understanding of how branching affects boiling point by asking which isomer of hexane has the highest boiling point.

Given Data / Assumptions:

• All compounds listed are structural isomers of hexane with the formula C6H14.
• The options include n-hexane (unbranched) and several branched isomers.
• We assume normal atmospheric pressure and compare relative boiling points.
• We apply general trends of boiling points in isomeric alkanes rather than exact numerical values.

Concept / Approach:
For isomeric alkanes, the key factor affecting boiling point is the strength of intermolecular London dispersion forces. A more extended, unbranched chain has a larger contact surface area with neighbouring molecules, allowing stronger dispersion interactions and leading to a higher boiling point. As branching increases, the molecule becomes more compact and spherical, reducing the effective surface area and hence decreasing the boiling point. Therefore, among structural isomers of hexane, n-hexane (the straight chain isomer) has the highest boiling point, and highly branched isomers such as 2,2-dimethylbutane have the lowest.

Step-by-Step Solution:
Step 1: Recognise that all listed compounds are isomers with the same molecular formula C6H14 but different branching. Step 2: Identify n-hexane as the completely unbranched, straight chain isomer. Step 3: Note that 2-methylpentane and 3-methylpentane each have one branch and are therefore more compact than n-hexane. Step 4: Observe that 2,2-dimethylbutane is the most highly branched of the group, with multiple methyl groups on the same carbon, making it the most compact. Step 5: Apply the rule that boiling point decreases with increasing branching and conclude that n-hexane has the highest boiling point among the options.

Verification / Alternative check:
If you look up typical boiling points, you will find that n-hexane has a higher boiling point than 2-methylpentane or 3-methylpentane, and both have higher boiling points than 2,2-dimethylbutane. The numerical values consistently show the pattern: unbranched > slightly branched > highly branched. These data confirm the general rule that more branching leads to lower boiling points for alkanes with the same number of carbon atoms. Therefore, n-hexane, being the least branched, has the highest boiling point among the listed isomers.

Why Other Options Are Wrong:
2-methylpentane and 3-methylpentane each contain a single methyl branch, making them more compact than n-hexane, so they have somewhat lower boiling points. 2,2-dimethylbutane is the most branched isomer listed, with two methyl groups on the same carbon, giving it the smallest surface area and the lowest boiling point in the group. While all of these are valid isomers, their increased branching reduces their boiling points compared to the unbranched n-hexane, so none of them can have the highest boiling point among the group.

Common Pitfalls:
Students sometimes think that more branches mean a larger or heavier molecule and therefore a higher boiling point, forgetting that all isomers have the same molecular formula and mass. Another pitfall is to rely on memorising specific names rather than understanding the underlying trend related to surface area and molecular shape. To avoid these mistakes, remember the simple rule: for isomeric alkanes, the straight chain isomer has the highest boiling point, and boiling point decreases as branching increases. Applying this rule makes many similar questions easy to answer.

Final Answer:
Among the isomers of hexane listed, the compound with the highest boiling point is n-hexane (unbranched).