Oxygen Containing Compounds - Carboxylic Acids


  • nomenclature
    • Suffix: -oic acid, carboxylic acid, -dioic acid.
  • physical properties and solubility
    • High boiling point due to hydrogen bonding.
    • Soluble in water.
  • infrared absorption
    • C=O at 1700 cm-1
    • -OH at 3100 cm-1

Important reactions

  • carboxyl group reactions
    • nucleophilic attack
    • COOH nucleophilic attack mechanism
      • Nucleophilic attack occurs on the electrophilic carbon of C=O.
      • Nucleophilic attack occurs by the nucleophilic oxygen of COOH.
    • reduction
      • LiAlH4: COOH -> alcohol.
    • decarboxylation: occurs for beta-keto acids
    • beta-keto decarboxylation mechanism
    • esterification: COOH + ROH under acidic conditions = ester.
    • COOH esterification mechanism
  • reactions at 2 position
    • halogenation: RCOOH + X2 -> halogenation at the alpha carbon (2 position).
    • halogenation at 2 position of carboxylic acid
    • substitution reactions: RCOOH + E+ -> substitution at the alpha carbon (2 position).
      1. Carboxylic acid converted to Acyl Halide, which can enolize.
      2. Acyl Halide tautomerizes to its enol form by abstraction of acidic alpha hydrogen.
      3. Halogen (or some other E+) gets attacked by alpha position.
      4. Revert back to carboxylic acid. The net effect is that the alpha H get substituted by an electrophile.

General principles

  • H bonding: COOH has high boiling point because of H bonding.
  • dimerization: Hydrogen bonding causes dimerization of carboxylic acids.
  • hydrogen bonding and dimerization of carboxylic acids
  • acidity of the carboxyl group: pKa of COOH is about 5. pKa of H+ is 0 while the pKa of water is 16. So, COOH can be classified as a weak acid. Vinegar is dilute acetic acid, which is CH3COOH.
  • inductive effect of substituents: electron withdrawing groups makes the acid stronger.
  • inductive effect
    • electron withdrawing groups attached to positions close to the COOH helps to distribute the charge of the COO- and stabilize it.
    • A more stabilized carboxylate ion makes a stronger acid.
  • resonance stability of carboxylate anion: the reason why COOH is a good acid is because the conjugate base (carboxylate ion) is stabilized by resonance.
  • resonance of carboxylate