Question

Which among the following isomers of $\mathrm{C_4H_9OH}$ has lowest boiling point?

• A. Isobutyl alcohol
• B. n-Butyl alcohol
• C. tert-Butyl alcohol
• D. sec-Butyl alcohol

Hint:

For any set of structural isomers, increased branching always drives the molecule toward a more compact, spherical shape. Spheres have the smallest possible surface-area-to-volume ratio, which minimizes van der Waals attractions and lowers the boiling point.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The problem presents four structural isomers of butyl alcohol ($\mathrm{C_4H_9OH}$) and asks us to determine which of these structural configurations results in the absolute lowest boiling point.

Step 2: Key Formula or Approach:
All four compounds are isomeric alcohols, meaning they share identical molecular weights and possess an identical functional group capable of forming intermolecular hydrogen bonds. For structural isomers with the same functional group, the boiling point depends on the degree of branching: $$\text{Boiling Point} \propto \text{Surface Area} \propto \frac{1}{\text{Branching}}$$

Step 3: Detailed Explanation:
Let's analyze the structural geometry of each isomer: n-Butyl alcohol ($\mathrm{CH_3CH_2CH_2CH_2OH}$) is a completely straight-chain primary alcohol. This linear configuration provides a maximized molecular surface area, allowing for strong, extensive intermolecular van der Waals dispersion forces. It has the highest boiling point. Isobutyl alcohol and sec-butyl alcohol possess moderate single branching along their carbon framework, which lowers their effective surface area and boiling points. tert-Butyl alcohol ($\mathrm{(CH_3)_3COH}$) is a highly symmetric, tertiary branched alcohol. This dense branching causes the molecule to adopt a compact, nearly spherical shape. This spherical contraction significantly minimizes the exposed molecular surface area, reducing the strength of the surrounding van der Waals interactions. Additionally, the steric crowding around the hydroxyl ($-\mathrm{OH}$) group limits the accessibility and overall strength of its hydrogen bonds.

Step 4: Final Answer:
The isomer with the lowest boiling point is tert-Butyl alcohol, which corresponds to option (C).