acidity
Summary
Acidity is how readily a compound donates a proton (H⁺), measured by Ka or pKa. A lower pKa means a stronger acid. The most testable point is that acidity is judged by the stability of the conjugate base: the more stable A⁻ is, the stronger the acid.
Detail
Meaning: Acidity is the tendency of a compound to give up H⁺ (Brønsted–Lowry sense). The stability of the conjugate base decides it. A more stable A⁻ forms more easily, so the acid is stronger. Factors that stabilize A⁻ include an electronegative atom carrying the charge, a larger atom (down a group), resonance delocalization, nearby electron-withdrawing groups (induction), and more s character in the orbital holding the lone pair (sp > sp² > sp³).
Equation: Ka = [H⁺][A⁻]/[HA] and pKa = −log Ka, where Ka is in mol/L and pKa has no units. Lower pKa means stronger acid.
On the MCAT: You rank compounds by acidity. A typical order is carboxylic acid (pKa about 4–5) > phenol (about 10) > alcohol (about 16) > terminal alkyne (about 25) > alkane (far higher). Passages also show substituent effects, such as chloroacetic acid being stronger than acetic acid. Questions may ask which proton in a molecule is most acidic, or whether a given base can deprotonate an acid. A base works well when its conjugate acid has a higher pKa than the acid being deprotonated.
Common confusion: Students look at the acid itself and not its conjugate base. Always ask which anion is more stable. Electron-donating alkyl groups destabilize A⁻ and lower acidity, so in solution a tertiary alcohol is less acidic than methanol.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.