What determines the strength of a base?

Just as we did with acids in the last section, this section will focus on what determines the strength of a base.

To determine the basicity of a molecule, one must look at the atom in question; however, we now want to focus on how much electron density is focused on this atom.  The less stable the molecule is (which happens when lots of electron density is concentrated on one atom), the more it will want to give some of its electron density by accepting a proton; therefore, the strongest base will be the one with the highest energy electrons (or the one with the most electron density concentrated on it, as this causes more electron-electron repulsion that is relieved after protonation).

What determines the strength of a base? The key is looking at how concentrated electron density is around the atom of focus. The more dense, the stronger the base.













All of the conditions presented in the “Determining the Acidity” section are just reversed:

  1. The less electronegative an atom (the later it appears in the periodic table), the more basic it likely is.
  2. If electron density can be delocalized by resonance, the molecule is a weaker base as it is less interested in losing electrons and accepting a proton.  Resonance weakens a base because it makes a molecule more stable.
  3. An electronegative atom might reduce basicity due to inductive effects.  This is because such an atom stabilizes electron density, reducing the need for sharing it with a proton.

This leads to an important point: acids and bases are inverses.  If a molecule is a strong acid, then it is a weak base.  If it is a weak acid, then it is a strong base.