Illustrated Glossary of Organic Chemistry

Acid: (1) A Bronsted acid or a Lewis acid. In common chemistry usage, the term 'acid' usually refers to a Bronsted acid, whereas when a Lewis acid is under discussion the terms 'Lewis acid' or 'electrophile' are used. Acidity refers to the strength of an acid (its ability to donate a proton to another molecule). A weakly acidic molecule does not donate a proton readily, and has a high pK_a (example: water; pK_a 15.7), whereas a strongly acidic molecule donates a proton readily and has a low pK_a (example: sulfuric acid; pK_a -10).

Sulfuric acid (H₂SO₄) is a strong Bronsted acid (pK_a -10). When sulfuric acid is under discussion it is called an acid or a Bronsted acid.


Aluminum chloride (AlCl₃) is a Lewis acid because the aluminum atom has an open valence shell. When aluminum chloride is under discussion it is called a Lewis acid or an electrophile.