Question

Explain, why propanol has higher boiling point than that of hydrocarbon, butane?

Answer 1

Physical properties like melting point, boiling point, physical state etc. depends on intermolecular attraction forces. There are mainly two types of intermolecular attraction forces, van der waals forces and hydrogen bonding. Hydrogen bonding is stronger than all types of van der waals forces.

Complete step by step answer:
London dispersion force: Dispersion forces are a type of van der Waals force and are the weakest of all intermolecular forces. London dispersion forces are the intermolecular attraction forces that occur between atoms and nonpolar molecules because of the formation of instantaneous dipole.
The electron cloud of a butane molecule between two atoms, which can normally be expected to be equally distributed spatially around them, at any given moment, may be uneven because of some external factor, resulting in an instantaneous dipole. This weak and temporary dipole subsequently influences neighboring butane molecules through electrostatic attraction and repulsion. It induces a dipole on nearby butane molecules and an induced dipole-induced dipole attraction force acts between molecules which is called London dispersion forces.
Hydrogen bonding: When hydrogen is bonded to a high electronegative atom like F, O or N, these highly electronegative atoms pull the shared electrons slightly towards them and generate partial positive charge on hydrogen and partial negative charge on themselves. Now these partially positively charged hydrogen creates a weak bond with a partially negatively charged atom of the adjacent molecule and this is called hydrogen bonding.
Hydrogen Bonding is stronger intermolecular force than London dispersion forces and that is the reason why propanol has a higher boiling point than butane.

Additional information:
1-propanol is a clear low viscosity neutral liquid with a characteristic alcohol odor. Butane is a colorless gas with a faint petroleum-like odor.

Note: There are two types of hydrogen bonding:
- Intermolecular hydrogen bonding: when hydrogen bonding exists between two molecules, it is called as intermolecular hydrogen bonding.
- Intramolecular hydrogen bonding: when hydrogen bonding exists between the atoms of a molecule, it is called as intermolecular hydrogen bonding.