Question

Which of the following has the lowest boiling point?

• A. Butanol
• B. Propanol
• C. Ethanol
• D. Methanol

Hint:

The boiling point of alcohols increases as the size of the molecule increases, due to stronger intermolecular hydrogen bonding. Methanol, being the smallest, has the lowest boiling point.

Answer 1

The Correct Option is D

To determine which of the given alcohols has the lowest boiling point, we must consider the effects of molecular weight and intermolecular forces on boiling points. Alcohols exhibit hydrogen bonding, a strong type of dipole-dipole attraction due to the presence of the hydroxyl (-OH) group. Within a homologous series like alcohols, the boiling point generally increases with molecular weight because larger molecules have greater surface area, leading to stronger London dispersion forces.

Let's examine the alcohols in the list:

• Methanol (CH₃OH): Smallest and lightest, with the least molecular weight.
• Ethanol (C₂H₅OH): Slightly larger than methanol.
• Propanol (C₃H₇OH): Larger than ethanol.
• Butanol (C₄H₉OH): Largest among the options.

The trend in boiling points is influenced primarily by molecular weight and additional surface contact points as the alkyl chain length increases:

• Methanol: Approximately 65°C
• Ethanol: Approximately 78°C
• Propanol: Approximately 97°C
• Butanol: Approximately 118°C

From this analysis, methanol has the lowest boiling point because it has the smallest molecular structure with the weakest London dispersion forces despite hydrogen bonding being present in all options. Therefore, methanol's combination of the lowest molecular weight and relatively weaker intermolecular attractions leads to the lowest boiling point among the given alcohols.