Vapor Pressure: H-bonding vs. dipole-dipole attraction
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.
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Keywords
vapor pressure, atmospheric pressure, barometric pressure, barometer, intermolecular forces, hydrogen bond, gases and liquids, organic, phase changes, physical properties, polar molecules
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The molecular formulas for butanol and diethyl ether are the
same. However, their structures are very 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.