To ascertain the compound with the highest boiling point, an analysis of intermolecular forces is required, specifically hydrogen bonding, molecular weight, and molecular structure.
Step 1: Identify intermolecular forces
- All compounds (methanol, ethanol, propanol, and water) exhibit hydrogen bonding potential due to the presence of an -OH group.
- Boiling points are elevated by stronger intermolecular forces and increased molecular weight. Hydrogen bond strength is influenced by molecular structure.
Step 2: Compare molecular weights and hydrogen bonding
- Methanol (CH$_3$OH): Molecular weight $\approx$ 32 g/mol, 1 carbon atom.
- Ethanol (C$_2$H$_5$OH): Molecular weight $\approx$ 46 g/mol, 2 carbon atoms.
- Propanol (C$_3$H$_7$OH): Molecular weight $\approx$ 60 g/mol, 3 carbon atoms.
- Water (H$_2$O): Molecular weight $\approx$ 18 g/mol. It lacks carbon but possesses two hydrogen atoms per molecule available for hydrogen bonding.
Step 3: Analyze hydrogen bonding
- Water's structure, featuring two hydrogen atoms and two lone pairs on oxygen, facilitates an extensive hydrogen bonding network, resulting in a higher boiling point despite its lower molecular weight.
- Alcohols (methanol, ethanol, propanol) also form hydrogen bonds, but the alkyl group's presence diminishes the efficiency of hydrogen bonding per molecule as 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
Among alcohols, boiling points increase with molecular weight due to enhanced van der Waals forces. However, water's comprehensive hydrogen bonding network results in the highest boiling point.
Step 5: Conclusion
Water (100°C) exhibits the highest boiling point, not due to molecular weight, but owing to its robust and extensive hydrogen bonding network. Therefore, (c) Water (H$_2$O) is the correct selection.