Brønsted-Lowry
Summary
A Brønsted-Lowry acid is a proton (H⁺) donor, and a Brønsted-Lowry base is a proton acceptor. The most testable point in organic chemistry is that a stronger acid has a more stable conjugate base, so a lower pKa means a stronger acid.
Detail
Meaning: A Brønsted-Lowry acid donates a proton (H⁺); a base accepts one, usually through a lone pair. Each reaction involves conjugate pairs: an acid becomes its conjugate base by losing H⁺. Acid strength tracks conjugate base stability, which rises with a more electronegative atom, a larger atom down a group, resonance delocalization, nearby electron-withdrawing groups, and greater s character in the orbital holding the charge. This is why carboxylic acids are more acidic than alcohols.
Equation: HA + B ⇌ A⁻ + BH⁺, with Ka = [H⁺][A⁻]/[HA] and pKa = −log Ka. HA is the acid, B the base, A⁻ the conjugate base, BH⁺ the conjugate acid. Concentrations are in mol/L, and Ka and pKa are treated as unitless.
On the MCAT: Expect questions that rank compounds by acidity, label conjugate pairs, or predict which side of an equilibrium is favored. The equilibrium lies toward the side with the weaker acid, which is the one with the higher pKa. A base deprotonates an acid effectively when its conjugate acid has a higher pKa than the acid.
Common confusion: Do not mix it up with the Lewis definition, where an acid accepts an electron pair and a base donates one. Also, a stronger acid has a weaker conjugate base, not a stronger one.
Sources
- Organic Chemistry
- Organic Chemistry: Structure and Function
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.