Polar Bonds
Polar bonds are important because they dictate many of the properties of organic molecules you need to understand.
Polar Bonds
Recall that electronegativity is the tendency of atom to attract a bonded pair of electrons towards itself. A polar covalent bond (i.e. a polar bond) occurs between two different atoms with an electronegativity difference greater than 0.4.
We can denote a polar bond using a special dipole arrow or using delta positive/negative symbols.
Polar Molecules
For a molecule to be overall polar (posses a net dipole) it needs:
- At least one polar covalent bond
- Appropriate geometry
Having a difference in electronegativity (dipole) is not the only factor to consider when assigning a molecule as polar or non-polar. We must also consider molecular shape or geometry (VSEPR).
Example: Polar Bonds
Does BF3 or NH3 posses a greater overall dipole?
NH3 is more polar as it is trigonal pyramidal. The lone pair of electrons found on the central nitrogen atom contributes to the overall dipole.
BF3 is trigonal planar and all the B-F dipoles cancel out.
Example: Polar Bonds
Does structure (3) or structure (4) possess a greater overall dipole?
Both structures are planar, but in structure (4), the fluorine atoms are across from each other. This symmetry means the dipoles cancel each other out and the molecule has no overall dipole.