Chapter 8 · Chemistry
HVZ Reaction and Why Benzoic Acid Goes Meta
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Two places a carboxylic acid can be substituted: the α-carbon (HVZ: X₂ and red phosphorus, then water) and the benzene ring of an aromatic acid, where –COOH is deactivating and meta-directing and blocks Friedel-Crafts reactions.
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Vinegar to Nylon: Uses of Carboxylic AcidsWhere carboxylic acids are used: formic acid in industry, acetic acid as a solvent and vinegar, adipic acid for nylon-6,6 through amide links, sodium benzoate as a food preservative, fatty-acid salts as soap, and aspirin as a medicine.Chemistry
Key points
- HVZ: R–CH₂–COOH + X₂ / red P, then H₂O → R–CHX–COOH (an α-halo acid).
- HVZ needs at least one α-hydrogen: CH₃COOH → ClCH₂COOH.
- On a benzene ring –COOH pulls π electrons: positive charge at ortho and para, so it is deactivating and meta-directing.
- Benzoic acid → m-nitrobenzoic acid (nitration), m-bromobenzoic acid (bromination); no Friedel-Crafts (AlCl₃ bonds to COOH).
The two substitutions
| Where | Reagent | Product |
|---|---|---|
| α-carbon (HVZ) | Cl₂ or Br₂ / red P, then H₂O | α-halo acid, e.g. ClCH₂COOH |
| Ring: nitration | conc. HNO₃ + conc. H₂SO₄ | m-nitrobenzoic acid |
| Ring: bromination | Br₂ / FeBr₃ | m-bromobenzoic acid |
| Ring: Friedel-Crafts | R–Cl or RCOCl / AlCl₃ | no reaction |
Why meta
- A ring C=C pair moves towards the ring–C bond; the C=O π pair moves onto O.
- The resonance structures put + charge at the ortho and para carbons.
- The meta carbon is least electron-poor, so the electrophile attacks there.
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