CH334

The concepts of "acid" and "base" permeate chemistry and give us an understanding of reactivity.  In general, acids and bases react with each other to neutralize the acidic and basic tendencies of each other.  In aqueous inorganic chemistry, acid + base always gives a salt (and usually water).

The Bronsted definition involves transfer of a proton (H⁺):  an acid is a proton donor, a base is a proton acceptor.  It is a convenient definition because it is experimentally easy to measure the propensity of a molecule to give up a proton, defined by the pK_a.

The Lewis definition involves electon pairs:  an acid is an electron pair acceptor, and an acid is an electron pair donor.  This is a much broader definition, but more difficult to provide a qualtitative measure for.  However, getting a sense of Lewis acids and bases gives you an extremely powerful predictive model of reactivity.

A proton is a specific case of a Lewis acid.  Because of this, all Bronsted bases are also Lewis bases.

We can visualize acidity or basicity using "electrostatic potential" maps on the surface of a molecule.  Areas of high positive electrostatic potential (I will use blue to represent this) will attract electrons, and are regions of Lewis basicity.

Compound	Jmol structure	ESP map
Trimethylamine
Boron trifluoride
Acetic acid
Ethylene
Acetophenone