Oxygen Containing Compounds - Carboxylic Acids

Description

• 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
  • 
    • 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
  • 
  • esterification: COOH + ROH under acidic conditions = ester.
  • 
• reactions at 2 position
  • halogenation: RCOOH + X₂ -> halogenation at the alpha carbon (2 position).
  • 
  • 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.
• 
• 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.
• 
  • 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.
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