Advices For Studying Organic Chemistry

2.14D VAN DER WAALS FORCES

1. van der Waals Forces (or London forces or dispersion forces):
   
   
   1. The attractive intermolecular forces between the molecules are responsible for
      the formation of a liquid and a solid of a nonionic, nonpolar substance.
   
   
   2. The average distribution of charge in a nonpolar molecule over a period of time
      is uniform.
   
   
   3. At any given instant, because electrons move, the electrons and therefore the
      charge may not be uniformly distributed ⇒ a small temporary dipole will
      occur.
   
   
   4. This temporary dipole in one molecule can induce opposite (attractive) dipoles
      in surrounding molecules.
   
   
   
   

Figure 2.8 Temporary dipoles and induced dipoles in nonpolar molecules
resulting from a nonuniform distribution of electrons at a given
instant.

5) These temporary dipoles change constantly, but the net result of their existence

is to produce attractive forces between nonpolar molecules.

2. Polarizability:
   
   
   1. The ability of the electrons to respond to a changing electric field.
   
   
   2. It determines the magnitude of van der Waals forces.
   
   
   3. Relative polarizability depends on how loosely or tightly the electrons are held.
   
   
   4. Polarizability increases in the order F < Cl < Br < I.
   
   
   5. Atoms with unshared pairs are generally more polarizable than those with only
      bonding pairs.
   
   
   6. Except for the molecules where strong hydrogen bonds are possible, van der
      Waals forces are far more important than dipole-dipole interactions.