Intermolecular Forces
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Summary

Why is the boiling point of ethanol higher than that of ethyl ether?

The higher boiling point of ethanol indicates stronger intermolecular forces compared to ethyl ether. Why are the intermolecular forces in ethanol stronger than those in ethyl ether?

To answer this question, we must look at the molecular structure of these two substances. The molecular structure of ethyl ether (C2H5OC2H5) is shown at right (red spheres represent oxygen atoms, grey spheres represent carbon atoms, and white spheres represent hydrogen atoms).

Is ethyl ether a polar molecule?

Yes No

Good! Ethyl ether is a polar molecule since the geometry does not cause the oxygen-carbon bond dipoles to cancel. This means that the electrons are not evenly distributed, resulting in regions of high and low electron density. The structure at right shows electron density. The red represents regions of high electron density and the blue represents regions of low electron density. As expected, a region of high electron density is centered on the very electronegative oxygen atom. This area of high electron density will carry a partial negative charge while the region of low electron density will carry a partial positive charge. These partial charges are represented by d+ and d- as shown in the structure below.

 




If two ethyl ether molecules are brought together, the opposite partial charges will be attracted to one another. This type of intermolecular force is called a dipole-dipole interaction or dipole-dipole attraction since it occurs in polar molecules with dipoles.

Remember that oxygen is more electronegative than carbon so the carbon-oxygen bonds in this molecule are polar bonds. Does the geometry of this molecule cause these bond dipoles to cancel each other?

The Review module has a page on polarity. The link on the right will open up this page in a separate window. When you are finished reviewing, closing the window will return you to this page.

Molecular Polarity











































Why are the dipole-dipole attractions in ethanol stronger than those in ethyl ether?

To understand the intermolecular forces in ethanol (C2H5OH), we must examine its molecular structure. The structure of ethanol is shown on the right.

Will there be dipole-dipole interactions in ethanol?

Yes No

Is ethanol a polar molecule? If you can't determine this, you should work through the review module on polarity. The link on the right will open up this page in a separate window. When you are finished reviewing, closing the window will return you to this page.

Molecular Polarity

Good! Like ethyl ether, ethanol is a polar molecule and will experience dipole-dipole interactions. Why are the dipole-dipole forces in ethanol stronger than those in ethyl ether?

The especially strong intermolecular forces in ethanol are a result of a special class of dipole-dipole forces called hydrogen bonds. This term is misleading since it does not describe an actual bond. A hydrogen bond is the attraction between a hydrogen bonded to a highly electronegative atom and a lone electron pair on a fluorine, oxygen, or nitrogen atom. Because the hydrogen atom is very small, the partial positive charge that occurs because of the polarity of the bond between hydrogen and a very electronegative atom is concentrated in a very small volume. This allows the positive charge to come very close to a lone electron pair on an adjacent molecule and form an especially strong dipole-dipole force. The image below shows the hydrogen bonds that form in ethanol.
























In which of the following compounds will hydrogen bonding occur?

In order for hydrogen bonding to occur, hydrogen must be bonded to a very electronegative atom. Carbon is only slightly more electronegative than hydrogen.

While methyl ether has hydrogen atoms and lone electron pairs on an oxygen atom, hydrogen must be bonded to a very electronegative atom in order for hydrogen bonds to form. Carbon is only slightly more electronegative than hydrogen.

Good! There are several places in this molecule where hydrogen bonds can form. There are hydrogens bonded to very electronegative atoms (both nitrogen and oxygen) and there are lone electron pairs on nitrogen and oxygen.






Hydrogen bonding is the intermolecular force responsible for water's unique properties discussed at the beginning of this module. Each water molecule has the ability to participate in four hydrogen bonds: two from the hydrogen atoms to lone electron pairs on the oxygen atoms of nearby water molecules, and two from the lone electron pairs on the oxygen atom to hydrogen atoms of nearby water molecules. In the crystal structure of ice, each oxygen does participate in these four hydrogen bonds. In order to do this, the oxygen atoms lie at the corners of six-sided rings with empty space in the center of each ring. When ice melts, approximately 15% of the hydrogen bonds are broken. This causes the rigid structure of ice to collapse and some H2O molecules are able to enter the previously empty space. This explains why ice is less dense than liquid water. The crystal structure of ice is shown on the right. Can you see the hexagonal rings and empty space? (Clicking on the structure and dragging with your mouse will rotate the structure.)

   

Intermolecular Forces