5.8: Bond polarity, inter- and intramolecular interactions

Atoms of each element have a characteristic "electronegativity", a measure of how tightly they hold onto their electrons. If the electronegativities of the two atoms in a covalent bond are equal or close, then the electrons are shared more or less equally between the two atoms. The resulting bond is said to be non-polar, meaning without direction. There are no stable regions of net negative or net positive charge on the resulting molecular surface due to the presence of the bond. When the electronegativities of the two bonded atoms are unequal the electrons of the bond will be shared un-equally. On average more electrons will spend more of their time around the more electronegative atom and fewer around the less electronegative atom. The result is what is known as a polar bond, with partially negatively and partially positively-charged regions – a polar bond has a direction and leads to an electrical field known as a dipole.

O and N atoms are more electronegative than C and H (see table, next page). When O or N form a bond with C or H, the O and N become partly negative and the C and H become partly positive - the bond is polarized. In contrast, there is no significant charge polarization in bonds between C and H; such bonds are termed non-polar. The presence of polar bonds can lead to electrostatic interactions between molecules that is stronger than "simple" van der Waals interactions. While stronger than non-polar LDF-mediated interactions they are weaker than covalent bonds. Like covalent bonds, polar interactions have a directionality to them – the three atoms involved are arranged more or less along a straight line. There is no such geometric constraint on simple LDF-mediated interactions. Since the intermolecular forces arising from polarized bonds often involve an H atom interacting with an O or an N atom, these are known generically, and perhaps unfortunately, as hydrogen or H-bonds (→). Why unfortunate? H atoms take part in covalent bonds, but H-bonds are not covalent, they are weaker. It takes much less energy to break an H-bond between molecules or within (generally macro-) molecules than it does to break a covalent bond involving a H atom.

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