Q.Explain why propanol has higher boiling point than that of the hydrocarbon, butane?
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The key is intermolecular forces: propanol has strong hydrogen bonding between its molecules, while butane only has weak London dispersion forces. This makes propanol's boiling point much higher ( vs ).
Why boiling point depends on intermolecular forces
Boiling happens when molecules have enough energy to overcome the forces holding them together in the liquid. The stronger these intermolecular forces, the more energy (higher temperature) needed to break them.
For small organic molecules, the main forces are:
- London dispersion forces — present in all molecules, increase with molecular size and surface area
- Dipole-dipole interactions — present in polar molecules
- Hydrogen bonding — a special, very strong dipole-dipole interaction when H is bonded to N, O, or F
The question asks why propanol () and butane () differ so much, even though they have similar molecular masses (propanol ≈ 60 g/mol, butane ≈ 58 g/mol).
Step-by-step reasoning
1. Identify the functional groups and molecular structure
Propanol has an -OH (hydroxyl) group at one end. This means an oxygen atom is bonded to a hydrogen — the classic setup for hydrogen bonding. The O-H bond is highly polar because oxygen is much more electronegative than hydrogen.
Butane is a straight-chain hydrocarbon with only C-C and C-H bonds. All bonds are nearly nonpolar (C-H is only slightly polar), so the molecule has no permanent dipole worth mentioning.
2. Compare the types of intermolecular forces present
For butane: only London dispersion forces exist. These arise from temporary fluctuations in electron distribution. Since butane has 4 carbons, it has a moderate surface area, giving moderate dispersion forces.
For propanol: three types of forces act together:
- London dispersion forces (from the 3-carbon chain)
- Dipole-dipole interactions (from the polar C-O and O-H bonds)
- Hydrogen bonding — the O-H group can form strong H-bonds with neighbouring propanol molecules
Hydrogen bonding strength:
London dispersion (for C4):
The hydrogen bond is roughly 2–3 times stronger than the dispersion forces in butane.
3. Consider the energy needed to separate molecules
To boil butane, you only need to overcome weak dispersion forces. That's why butane is a gas at room temperature (boiling point: ).
To boil propanol, you must break hydrogen bonds first. These bonds hold the molecules together in a loose network. Even though propanol has one fewer carbon (slightly smaller dispersion forces), the hydrogen bonding contribution is so large that it dominates.
A common mistake is to think that molecular mass alone determines boiling point. While mass correlates with dispersion forces, it does not account for hydrogen bonding. Propanol (60 g/mol) and butane (58 g/mol) have nearly the same mass, yet their boiling points differ by almost — proof that mass is not the deciding factor here.
4. Check the actual boiling points
| Compound | Formula | Molar mass (g/mol) | Boiling point () | …
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