Illustrated Glossary of Organic Chemistry

Polar: A structure having an uneven distribution of electron density. A structure in which electron density is more or less evenly distributed is called nonpolar. Polarity comes in various forms (shown below), and influences many physical, chemical, and biological properties of molecules. For example, polar molecules such as methanol tend to be more soluble in polar solvents (such as water) and less soluble in nonpolar solvents (such as hexane).

In water, oxygen is significantly more electronegative than hydrogen resulting in uneven electron density distribution on an O-H bond (O-H ΔEN = 1.4). The oxygen has a slight electron density excess (as indicated by the δ^- charge) and the hydrogen has a slight electron density deficiency (as indicated by the δ⁺ charge). Attraction of the opposite charges of O-H bond dipoles between water molecules results in an electrostatic attraction called hydrogen bonding. Because the electron density in water is uneven, and because the directions of bond polarity do not cancel out, water is a labeled as polar molecule. A molecule's dielectric constant (ε) can be used as measure of the molecule's polarity, with higher ε indicating higher polarity. For water, a very polar molecule, ε = 80. The polarity of water can also be seen in its electrostatic potential map. Hydrogen bonding is widely important in biology, for example, in base pairing in DNA.

1-Octanol Nonpolar bonds in blue, polar bonds in red.		Electrostatic potential map for 1-octanol. Red = higher negative charge. Green = neutral. Blue = higher positive charge.