Question

Which of the following compounds has the highest boiling point?

• A. Methanol (CH$_3$OH)
• B. Ethanol (C$_2$H$_5$OH)
• C. Water (H$_2$O)
• D. Propanol (C$_3$H$_7$OH)

Hint:

Boiling point depends on intermolecular forces.
Hydrogen bonding in water is exceptionally strong due to its structure,
often leading to a higher boiling point than alcohols of similar or higher molecular weight.

Answer 1

The Correct Option is C

To determine which compound has the highest boiling point,
we need to consider the intermolecular forces that affect boiling points, 
such as hydrogen bonding, molecular weight, and molecular structure. 
Step 1: Identify intermolecular forces 
- All the given compounds (methanol, ethanol, propanol, and water) 
are capable of forming hydrogen bonds because they have an -OH group, 
which allows hydrogen bonding between molecules. 
- Boiling point increases with stronger intermolecular forces 
and higher molecular weight, but hydrogen bonding strength depends 
on the molecule's structure. 
Step 2: Compare molecular weights and hydrogen bonding 
- Methanol (CH$_3$OH): Molecular weight $\approx$ 32 g/mol, 1 carbon. 
- Ethanol (C$_2$H$_5$OH): Molecular weight $\approx$ 46 g/mol, 2 carbons. 
- Propanol (C$_3$H$_7$OH): Molecular weight $\approx$ 60 g/mol, 3 carbons. 
- Water (H$_2$O): Molecular weight $\approx$ 18 g/mol, no carbon, 
but two hydrogen atoms available for hydrogen bonding per molecule. 
Step 3: Analyze hydrogen bonding 
- Water has a unique structure with two hydrogen atoms 
and two lone pairs on oxygen, allowing it to form an extensive network of hydrogen bonds. 
This results in a higher boiling point despite its lower molecular weight. 
- Alcohols (methanol, ethanol, propanol) also form hydrogen bonds, 
but the effect is less extensive due to the presence of the alkyl group, 
which reduces the hydrogen bonding efficiency per molecule as the carbon chain length increases. 
Step 4: Compare boiling points 
- Methanol: Boiling point $\approx$ 64.7°C 
- Ethanol: Boiling point $\approx$ 78.4°C 
- Propanol: Boiling point $\approx$ 97.4°C 
- Water: Boiling point $\approx$ 100°C 
The boiling point increases with molecular weight among alcohols 
due to stronger van der Waals forces, but water’s extensive hydrogen bonding network 
gives it the highest boiling point. 
Step 5: Conclusion 
Despite having a lower molecular weight, water has the highest boiling point (100°C) 
due to its strong and extensive hydrogen bonding network, 
making (c) Water (H$_2$O) the correct answer.