Polarity is an indication of the charge distribution in the molecule. Nonpolar molecules have a completely symmetrical charge distribution; there is no excess charge anywhere in the molecule. Polar molecules have a partial charge separation. This different from ionic compounds like NaCl in which there is a full separation of charge to create ions (in the case of NaCl, the ions are Na+ and Cl-). HCl is an example of a polar molecule. Here the hydrogen and chlorine attract electrons differently; the Cl has a stronger attraction for electrons than H. This means that Cl will have a partial negative charge whereas H will have a partial positive charge. Again, this is not a full separation of charge; the atoms are still covalently bound. It's just that the electron density is more concentrated around Cl. We represent partial positive and negative charges with d+ and d-, respectively. The HCl molecule may be written as
The image below shows the space-filling structure (relative atomic sizes are shown) of HCl.
You can identify nonpolar molecules by their symmetry - they are completely symmetrical. Polar molecules are less symmetrical. This is because different types of atoms will attract electrons differently, leading to uneven distribution of electrons. (We will learn how to assess polarity in more detail later in the semester.) Hydrocarbons tend to have low polarity.
For purposes of solubility, we will note that nonpolar molecules dissolve in nonpolar solvents and polar molecules dissolve in polar solvents ("like dissolves like"). Consequently, a nonpolar solvent can extract a nonpolar compound from a polar solvent. The in-class demonstration illustrated this; we extracted I2 (nonpolar) from water (polar) with CCl4 (nonpolar).
A quantitative measure of a molecule's polarity is its dipole moment. The dipole moment is strictly defined as the product of the partial charge and the separation of charge
Dipole Moment = partial charge x distance
Molecules that are completely nonpolar have a dipole moment of zero. A more polar molecule has a larger dipole moment. The table below lists the dipole moments of a few common molecules. (The units of dipole moment are Debye.)
means that the reaction is reversible; it goes in both
directions (I guess it can't make up its mind). Hence the acetic acid does not dissociate
completely and the solution contains a mixture of CH3COOH molecules and CH3COO-
(acetate) anions. Acetic acid is an example of an organic acid called a carboxylic acid.
All carboxylic acids, which are characterized by the -COOH group, are weak.