Vapor Pressure: H-bonding vs. dipole-dipole attraction

The vapor pressures of butanol and diethyl ether are compared using barometers to show the effect of hydrogen-bonding on vapor pressure.

The measurement of pressure exerted by a vapor is demonstrated using barometers. The vapor pressures of butanol and diethyl ether are compared.

Diethyl ether has a much higher vapor pressure than butanol. In addition to the London forces, which are about the same for both molecules, diethyl ether has only intermolecular dipole-dipole forces, much weaker than the hydrogen-bond intermolecular forces of the butanol.

Discussion: 

The molecular formulas for butanol and diethyl ether are the same. However, their structures are different. Butanol molecules form hydrogen bonds, while diethyl ether molecules can only form dipole-dipole interactions. Which force is stronger?

When butanol is injected into the open end of the barometer, the butanol rises to the top and vaporizes. The mercury column is depressed to 721 millimeters. When diethyl ether is injected, the diethyl ether rises to the top and vaporizes. The mercury column is depressed to 264 millimeters.

Diethyl ether has a much higher vapor pressure than butanol. In addition to the London forces which are about the same for both molecules, diethyl ether has only intermolecular dipole-dipole forces, much weaker than the hydrogen-bond intermolecular forces of the butanol.

Credits: 
  • Design and Demonstration
    • Kristin Johnson University of Wisconsin - Madison, Madison, WI 53706
  • Text
    • Kristin Johnson University of Wisconsin - Madison, Madison, WI 53706
    • John W. Moore University of Wisconsin - Madison, Madison, WI 53706
  • Video Production
    • Jerrold J. Jacobsen University of Wisconsin - Madison, Madison, WI 53706
  • Graphics
    • Kristin Johnson University of Wisconsin - Madison, Madison, WI 53706