What is the ranking of the following compounds (1 = strongest) on the strength of their intermolecular forces between molecules? And why?

1. (d) CH₃CH₂CH₂OH
2. (c) CH₃CH₂OH
3. (a) CH₃CHO
4. (b) CH₃OCH₃

All the four compounds are small polar molecules.

In (c) and (d), O is a very electronegative atom and thus H is rather bare in electrons opposite the O-H bond. Hence, the intermolecular forces between their molecules are mainly intermolecular hydrogen bonds. In (a) and (b), there are no hydrogen bonds and the polar molecules are held together by mainly dipole-dipole attractions. For small molecules, intermolecular hydrogen bonds are strong than dipole-dipole attractions. Therefore,
Intermolecular forces: (c)/(d) > (a)/(b)

Compare (a) and (b). Since C=O bond is much polar than C-O bond, CH₃CHO is polar than CH₃OCH₃. The more polar the small molecules, the stronger intermolecular forces between them are. Therefore,
Intermolecular forces: (a) > (b)

Compare (c) and (b). Both of them have intermolecular hydrogen bonds. However, CH₃CH₂CH₂OH molecule has a longer hydrocarbon chain than CH₃CH₂OH, and thus CH₃CH₂CH₂OH molecules have stronger van der Waals' and thus have stronger intermolecular forces. Therefore,
Intermolecular forces: (d) > (c)

Conclusively, intermolecular forces: (d) > (c) > (a) > (b)
1. (d) CH₃CH₂CH₂OH
2. (c) CH₃CH₂OH
3. (a) CH₃CHO
4. (b) CH₃OCH₃

All the four compounds are small polar molecules.

In (c) and (d), O is a very electronegative atom and thus H is rather bare in electrons opposite the O-H bond. Hence, the intermolecular forces between their molecules are mainly intermolecular hydrogen bonds. In (a) and (b), there are no hydrogen bonds and the polar molecules are held together by mainly dipole-dipole attractions. For small molecules, intermolecular hydrogen bonds are strong than dipole-dipole attractions. Therefore,
Intermolecular forces: (c)/(d) > (a)/(b)

Compare (a) and (b). Since C=O bond is much polar than C-O bond, CH₃CHO is polar than CH₃OCH₃. The more polar the small molecules, the stronger intermolecular forces between them are. Therefore,
Intermolecular forces: (a) > (b)

Compare (c) and (b). Both of them have intermolecular hydrogen bonds. However, CH₃CH₂CH₂OH molecule has a longer hydrocarbon chain than CH₃CH₂OH, and thus CH₃CH₂CH₂OH molecules have stronger van der Waals' and thus have stronger intermolecular forces. Therefore,
Intermolecular forces: (d) > (c)

Conclusively, intermolecular forces: (d) > (c) > (a) > (b)
This can be shown by their boilng points: (d) > (c) > (a) > (b)