Answer to Question #132594 in Organic Chemistry for Steph Leo

Question

At 20°C, pentane (MW = 72.2 g/mol) has a vapor pressure of 434mmHg while butanol (MW = 74.1g/mol) has a vapor pressure of 6mmHg.

Explain this difference in vapor pressure in terms of specific of specific intermolecular forces for each molecule.

Answer 1

Pentane (C5H12) and butanol (C3H7OH) have very close molecular weights. The only forces holding together the molecules of pentate are London Dispersion forces. As there is OH group in the molecule of butane, which is responsible for hydrogen bonds, then the bonds which hold together molecules of butanol are London dispersion forces, dipole-diplole forces and hydrogen bonding, the last being the strongest ones. The stronger the forces holding together molecules the lower the vapour pressure, as a lot of energy is required to breake these bonds. So, for butanol intermolecular forces are stronger then the vapour pressure is lower.