ELECTRONEGATIVITY

Electronegativity In Complex Molecules

When looking at complex molecules, molecules with many atoms and more than one bond, individual bonds within the molecule are considered in exactly the same way as they were when looking at simple molecules. However, whereas in simple molecules like HCl when if the bond is polar then the molecule is polar, all the bonds in the molecule and its symmetry must be considered before we can say whether it is polar or not.

Now lets look at a few examples:

BF₃ has a trigonal planar geometry, it is a flat molecule with 3 planes of symmetry and 3 polar bonds. However, the molecule is not polar. This is because all 3 bonds have equal dipole moments and the symmetry of the molecule means that the dipoles cancel each other out.

CCl₄ has a tetrahedral geometry, it is a 3D molecule with 3 planes of symmetry and 4 polar bonds. However, this molecule is not polar either. All 3 bonds have equal dipole moments and the symmetry of the molecule means that the dipoles cancel each other out

CH₃Cl has a tetrahedral geometry, it is a 3D molecule with 3 planes of symmetry and 4 polar bonds. This molecule is polar though. This is because not all 4 bonds are the same, the dipole moment of the C-Cl bond is different to that of the C-H bond. This means that despite the symmetry, there is an imbalance in the dipole moment over the whole molecule, so the molecule itself has a dipole moment. Carbon is more electronegative than Hydrogen, Chlorine is more electronegative than Carbon, so the molecular dipole goes from having positive sign at the Hydrogen end to negative sign at the Chlorine end.

NH₃ has a tetrahedral geometry but appears to be a triangle based pyramid because a lone pair occupies the other arm of the tetrahedral structure. It is a 3D molecule with 3 planes of symmetry and 3 polar bonds. This molecule is polar. This is because although all the bonds are the same and have the same dipole moment, and even though the molecule is symmetrical, there is an overall dipole across the molecule. This is because the molecule is raised and not flat, so it is not symmetrical in the vertical direction. Thus the dipoles do not cancel each other out, indeed they combine to make a bigger overall dipole than each of the bond dipoles. Nitrogen is more electronegative than Hydrogen, so the molecular
dipole has positive sign at the Hydrogen end and negative sign at the Nitrogen end.

Electronegativity in Organic Molecules

Organic molecules are generally even more complicated than those above and can be comprised of lots of different atoms. As the number of atoms in the molecule increases it becomes increasingly unlikely that the molecule will not be polar, simply due to the number of polar bonds and the shape of the molecule, although the molecular dipole may only be small.

Electronegativity is very important in organic chemistry, for example, the high electronegativity of Oxygen and Nitrogen (higher than Carbon) means that when they are bonded to Carbon in organic molecules, the bond formed is polar. This means that the Carbon is d⁺ and the Oxygen or Nitrogen is d ^-, this makes the Carbon more suscepticable to nucleophilic attack and so makes it more reactive. Many organic reactions are started by the nucleophilic attack of a d⁺ Carbon. You also need to know about and understand the concept of induction.
Look at the picture of propanoic acid. Carbon 3 is bonded to 2 Oxygens, both bonds are polar and the Carbon has a large d⁺ on it. Because of this, Carbon 3 withdraws more electron density from the bond with Carbon 2 than it normally would, in order to try and reduce the size of d⁺. This makes the bond between Carbons 3 and 2 polar. So now Carbon 2 has a small d⁺, so it withdraws slightly more electron density from the bond with Carbon 1 than it normally would. This makes the bond between Carbons 2 and 1 slightly polar. This is the process of induction.

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