(Butanol) is likely to have the higher normal melting point.
Suggest, giving reasons and using Lewis structures, whether C2H5OC2H5 (diethyl ether) or C4H9OH (butanol) is likely to have the higher normal melting point.
The Correct Answer and Explanation is:
Correct Answer: Butanol (C₄H₉OH) is likely to have the higher normal melting point than diethyl ether (C₂H₅OC₂H₅).
Explanation
To determine which compound has a higher normal melting point—butanol or diethyl ether—we must consider the types of intermolecular forces present in each compound, as these directly affect melting and boiling points.
1. Lewis Structures:
- Butanol (C₄H₉OH)
Lewis structure: CH₃–CH₂–CH₂–CH₂–OH
It contains a hydroxyl (-OH) group, which is capable of hydrogen bonding. - Diethyl ether (C₂H₅OC₂H₅)
Lewis structure: CH₃–CH₂–O–CH₂–CH₃
It has an ether group (R–O–R’), with oxygen contributing lone pairs, but cannot donate hydrogen bonds, only accept them.
2. Intermolecular Forces:
- Butanol exhibits:
- Hydrogen bonding (due to the –OH group),
- Dipole-dipole interactions,
- London dispersion forces.
- Diethyl ether exhibits:
- Dipole-dipole interactions (due to the oxygen atom),
- London dispersion forces.
Hydrogen bonding is significantly stronger than dipole-dipole and dispersion forces. This allows molecules of butanol to pack more tightly and require more energy (higher temperature) to break apart from the solid structure.
3. Molecular Size and Shape:
Both compounds have similar molar masses (butanol ≈ 74 g/mol, diethyl ether ≈ 74 g/mol), but butanol has a more linear structure, aiding crystalline packing. Diethyl ether, being more branched and lacking hydrogen bonding, has weaker intermolecular cohesion.
4. Empirical Data Support:
- Melting point of butanol: ~–89°C
- Melting point of diethyl ether: ~–116°C
Though both are low-melting substances due to weak intermolecular forces overall, butanol’s hydrogen bonding results in a noticeably higher melting point.
Conclusion:
Butanol has the higher normal melting point due to its ability to form hydrogen bonds, which significantly strengthens its intermolecular attractions compared to diethyl ether.
