Question

why does CH3CH2CH3 boil at -42 celsius but CH3CH2CH2CH3 boils at -0.5 celsius ?

Answer 1

There are some attractive forces between molecules called secondary forces which operate over long distances of 25-50 nm and ma have lower energyvalues (1-10 kcal/mol) . These secondary forces are called as vander waals forces. These forces increase cumulatively as one go from methane to higher alkanes. Thesesecondary forces can be classified as weak london forces or or dispersion forces (about 2 Kcal/mol), dipole-dipole interactions (2-6 Kcal/mol) and hydrogen bonding (10 Kcal/mol). Since these forces are acting together , secondary energies and boiling points increase as one go from methane to higher alkanes. By keeping these facts, one can understand that why propane has has lower boiling point than butane.