Chapter 8 · Chemistry
Esterification and the C–OH Swap: Esters, Acid Chlorides, Anhydrides, Amides
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Fruity esters from an acid and an alcohol, with the acid-catalysed mechanism drawn arrow by arrow, and three more ways to replace the –OH of –COOH: acid chlorides (best with SOCl₂), anhydrides and amides.
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Losing CO₂: Decarboxylation, Kolbe Electrolysis and ReductionHow the –COOH group itself changes: soda lime removes it as CO₂ (one carbon less), Kolbe electrolysis joins two alkyl groups, and LiAlH₄ or diborane reduces it to a primary alcohol, while NaBH₄ cannot.Chemistry
Key points
- RCOOH + R′OH ⇌ RCOOR′ + H₂O (conc. H₂SO₄): reversible; remove water or use excess alcohol.
- Mechanism: protonate C=O, the alcohol O adds to C, a proton shift makes –OH₂⁺, water leaves, −H⁺ gives the ester.
- The water's oxygen comes from the acid.
- SOCl₂ is the best reagent for RCOCl: SO₂ and HCl escape as gases (PCl₅ and PCl₃ also work).
- P₂O₅ dehydrates two acids to an anhydride; NH₃ gives an ammonium salt that, heated, gives an amide.
The mechanism
- H⁺ protonates the carbonyl O: the C becomes more δ⁺.
- The alcohol's O lone pair adds to that C; the C=O π pair moves onto O.
- A proton shift turns one –OH into –OH₂⁺.
- An –OH lone pair re-forms C=O⁺ as H₂O leaves.
- −H⁺: the ester. The water's O came from the acid.
Replacing the OH
| Reagent | Product | By-products |
|---|---|---|
| R′OH, H⁺ | ester RCOOR′ | H₂O |
| PCl₅ | acid chloride RCOCl | POCl₃ + HCl |
| PCl₃ | acid chloride RCOCl | H₃PO₃ |
| SOCl₂ | acid chloride RCOCl | SO₂ + HCl (gases) |
| P₂O₅ or conc. H₂SO₄, heat | anhydride (RCO)₂O | H₂O |
| NH₃, then heat | amide RCONH₂ | H₂O (via RCOO⁻NH₄⁺) |
Quick quiz
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