Van der Waals force

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The Van der Waals force is a intermolecular force caused by electrostatic attraction. Some molecules (polar molecules) have a more negative end and a more positive end. This means that a weak electromagnetic force is present between the positive end of one molecule and the negative end of another. This is a Van der Waals force. They are far weaker than intramolecular bonding such as ionic, polar or covalent bonding. Non-polar substances also have very weak Van der Waals forces, called the London dispersion force.

The effect of Van der Waals' forces

Van der Waals' forces play a pivotal role in determining melting (but not boiling or equivalence) points. The greater the molecular mass of a compound, the more protons it possesses, and hence is able to attract electrons in orbitals towards the nucleus to a greater degree. Consequently, electrons are promoted to higher energy levels, and are hence more likely to induce dipoles in adjacent, neighbouring molecules. With molecules of lower molecular mass, the electrons are of lower energy, and are hence less controlled by the constituent atoms; as a result, repulsion between the electrons of two adjacent molecules tends to dominate over the electrostatic attraction of the induced dipoles. This would explain why water has a lower melting point than 5,5,6-trimethyldodecanoate.

Van der Waals' forces tend to become more dominant in hydrocarbons with a polar functional group and a significantly larger hydrotrophic, insoluble region (c.f. methyl ethanoate and ethyl methanoate).