Question

Identify the correct increasing order of boiling points of the given compounds:

• A. Propan-1-ol $<$ butan-1-ol $<$ butan-2-ol $<$ pentan-1-ol
• B. Pentan-1-ol $<$ butan-1-ol $<$ butan-2-ol $<$ propan-1-ol
• C. Propan-1-ol $<$ butan-2-ol $<$ butan-1-ol $<$ pentan-1-ol
• D. Butan-1-ol $<$ butan-2-ol $<$ propan-1-ol $<$ pentan-1-ol

Hint:

Boiling point trends in alcohols: Longer chain $\uparrow$ BP More branching $\downarrow$ BP All alcohols have hydrogen bonding, so size and branching dominate comparisons.

Answer 1

The Correct Option is C

To determine the correct increasing order of boiling points for the given alcohols, we need to consider the factors that affect boiling points in organic compounds, particularly alcohols.

1. Understanding Boiling Points:
   • Boiling point is the temperature at which a liquid's vapor pressure equals the atmospheric pressure.
   • For alcohols, hydrogen bonding plays a significant role in determining their boiling points. The strength of hydrogen bonding generally increases with the molecular size and surface area.
   • Among isomeric alcohols, usually the one with the more accessible hydroxyl group (in a less hindered position) will have a lower boiling point.
2. Analyzing the Given Alcohols:
   1. Propan-1-ol (C₃H₈O): Being the smallest molecule in the group, it has relatively weak intermolecular forces compared to others.
   2. Butan-1-ol (C₄H₁₀O): Larger than propan-1-ol, with greater surface area and stronger intermolecular forces.
   3. Butan-2-ol (C₄H₁₀O): Isomer of butan-1-ol but slightly more branched (secondary alcohol), which can lower the boiling point slightly compared to its straight-chain isomer.
   4. Pentan-1-ol (C₅H₁₂O): The largest molecule, thus expected to have the strongest intermolecular forces and the highest boiling point.
3. Determining the Order:
   • In general, as the carbon chain length increases, the boiling point increases due to increased van der Waals forces. Hence, pentan-1-ol should have the highest boiling point.
   • Between butan-1-ol and butan-2-ol, although butan-1-ol has a higher molecular weight, the branching in butan-2-ol reduces its boiling point compared to butan-1-ol.
   • Therefore, the correct increasing order of boiling points based on molecular structure and hydrogen bonding is: \(\text{Propan-1-ol} < \text{Butan-2-ol} < \text{Butan-1-ol} < \text{Pentan-1-ol}\)

Thus, the correct answer is Propan-1-ol $<$ Butan-2-ol $<$ Butan-1-ol $<$ Pentan-1-ol.