Deprotonation of Carboxylic Acids
Deprotonation of Carboxylic Acids: A carboxylic acid (–COOH) reacts with a base to form a carboxylate ion (–COO⁻).
What happens
A carboxylic acid (–COOH) reacts with a base to form a carboxylate ion (–COO⁻). The base simply removes the acidic proton from the O–H group; the carbon skeleton is unchanged and the carbonyl is retained. Carboxylic acids are relatively acidic for organic compounds (pKₐ ≈ 4–5), so many bases work: NaOH, KOH, NaHCO₃, Na₂CO₃ and NaH. The carboxylate is stabilized by resonance: the negative charge is shared equally between the two oxygens, which makes it much more stable than a typical alkoxide and explains why carboxylic acids readily lose H⁺.
Reaction summary
Carboxylic acid + base → carboxylate ion + conjugate acid (COOH → COO⁻)
Reagents and conditions
- Starting material: Acetic acid (carboxylic acid)
- Reagents: Sodium hydroxide (NaOH)
- Products: Acetate ion (carboxylate); Water
Mechanism
- Base Removes H: The base uses a lone pair to attack the acidic hydrogen of the –OH. The electrons from the O–H bond move onto oxygen. This produces R–COO⁻.
- Product: The carboxylate ion and the conjugate acid of the base (here water): R–COOH → R–COO⁻.
Common mistakes
- The proton is removed from the O–H, not from the carbon next to the carbonyl. Don't confuse this with enolate formation.