Carboxylic acids

Carboxylic acids are the most acidic organic compounds you meet in this course: stronger than phenols and alcohols, though weaker than mineral acids. When R–COOH loses H⁺, the carboxylate ion R–COO⁻ shares its negative charge equally over two oxygen atoms, by resonance. In phenoxide the charge is spread less well, partly onto carbon atoms. A more stable anion means a stronger acid.

Groups on the chain change the strength:

• Electron-withdrawing groups (F, Cl, –NO₂, –CF₃) pull charge away from the carboxylate and increase acidity. The more of them, and the closer they are to the COOH, the stronger the effect: CF₃COOH is far stronger than CH₃COOH, and an F on the carbon next to COOH matters more than one further away.
• Electron-releasing alkyl groups decrease acidity: methanoic acid is stronger than ethanoic acid.

Reactions of the –COOH group:

• With NaHCO₃: CO₂ is given off (a test for acids).
• With alcohols (and a little conc. H₂SO₄): esters form. The reaction is reversible; removing the water as it forms drives it forward.
• With PCl₅, PCl₃ or SOCl₂: the OH is replaced by Cl, giving an acid chloride. SOCl₂ is preferred because its by-products are gases.
• With ammonia: an ammonium salt, which on heating gives an amide. Strong heating with P₂O₅ removes water from two acid molecules to give an anhydride.

Reduction: LiAlH₄ or diborane (B₂H₆) reduce the acid all the way to a primary alcohol; NaBH₄ does not.

Hell-Volhard-Zelinsky (HVZ) reaction: with Cl₂ or Br₂ and a little red phosphorus, an acid with α-hydrogens is halogenated at the α-carbon.

Decarboxylation: heating the sodium salt with soda-lime (NaOH + CaO) removes CO₂, giving an alkane with one carbon fewer.

The benzene ring of benzoic acid. –COOH pulls electrons out of the ring, so it is deactivating and meta-directing. The ring is too deactivated for Friedel-Crafts reactions, and the AlCl₃ catalyst also binds to the COOH group.