Question

What intermolecular force(s) is/are present in acetone that it would be a liquid at room temperature? Is there more than one force? If yes, which is stronger and which is weaker?

Answer 1

Dipole-dipole forces are the primary intermolecular forces present in acetone.

There are three intermolecular forces that occur in covalent compounds:

• Dipole-dipole forces occur when polar molecules are attracted to one another. Basically, if one atom on the molecule is more electronegative than the others, it will pull electrons toward itself, giving it a partial negative charge. Likewise, the atoms that have had electrons pulled away from it will have a partial positive charge. When two molecules of this compound come in contact with each other, the partial positive charge on one will be attracted to the partial negative charge on the other - this is a dipole-dipole force. Because acetone is polar but doesn't have the H-F, H-O, and H-N bonds needed for hydrogen bonding (see below), it experiences dipole-dipole forces.

• Hydrogen bonds occur in molecules that have H-F, H-O, and H-N bonds. Basically, this strong intermolecular force is due to a strong dipole-dipole force as described above, plus the fact that the F, O, and N lone pairs are able to reach in closer to the hydrogen atom on another molecule.

• London dispersion forces occur from the attraction between temporarily polarized nonpolar molecules. Here's what happens: The electrons in a molecule are pretty much randomly and evenly distributed. However, through random movement, sometimes the electron density (i.e. negative charge) is higher on one side than the other. This temporary polarity, in turn, causes an adjacent molecule to become polarized, and they are attracted to one another.

The London dispersion force is the weakest, followed by dipole-dipole forces and then hydrogen bonding.