Concept 07 / 07Aldehydes, Ketones and Carboxylic Acids
Chemical Reactions and Uses of Carboxylic Acids
Reactions of carboxylic acids involving the O-H bond, the C-OH bond (anhydrides, Fischer esterification, acyl chlorides, amides), the COOH group as a whole (reduction, decarboxylation, Kolbe electrolysis), the alkyl chain (Hell-Volhard-Zelinsky reaction) and the aromatic ring, plus uses of aldehydes, ketones and carboxylic acids.
Reactions of carboxylic acids (see figure).
Cleavage of O-H (acidic behaviour): with metals (), alkalis (salts) and hydrogencarbonates ().
Cleavage of C-OH (acyl substitution).
Anhydride formation: heating with a dehydrating agent ( or concentrated ): .
Esterification (Fischer): heating with an alcohol and a little concentrated or HCl gas: . Mechanism: protonation of the carbonyl oxygen activates the carbon; the alcohol adds; a proton transfers and water leaves (isotope labelling shows the O of water comes from the acid). The reaction is reversible; excess alcohol or removal of water drives it forward.
Acyl chlorides: with , or (preferred, as and HCl escape): .
Amides: with ammonia, first the ammonium salt forms, which on heating loses water: . Phthalic acid gives phthalimide on strong heating with ammonia.
Reactions of the COOH group.
Reduction: or diborane reduces acids to primary alcohols (). Diborane does not easily reduce ester, nitro or halo groups; does not reduce COOH.
Decarboxylation: sodium salts heated with soda lime (NaOH + CaO) lose : . Kolbe electrolysis of aqueous sodium or potassium salts gives alkanes with twice the alkyl groups at the anode.
Substitution in the alkyl chain: Hell-Volhard-Zelinsky (HVZ) reaction. Acids with -hydrogens react with chlorine or bromine in the presence of a little red phosphorus to give -halo acids: .
Ring substitution. COOH is deactivating and meta-directing: benzoic acid gives 3-nitrobenzoic acid (nitration) and 3-bromobenzoic acid. Benzoic acid does not undergo Friedel-Crafts reactions (the ring is deactivated and the COOH binds the ).
Uses.
Methanal: formalin ( solution) preserves biological specimens; used for bakelite (with phenol), urea-formaldehyde resins.
Ethanal: starting material for ethanoic acid, ethyl acetate, vinyl acetate, polymers and drugs.
Benzaldehyde: perfumes and dyes. Acetone and ethyl methyl ketone: industrial solvents.
Methanoic acid: rubber, textile, dyeing, leather and electroplating industries. Ethanoic acid: vinegar (food); solvent. Hexanedioic acid (adipic acid): nylon-6,6. Esters of benzoic acid: perfumery; sodium benzoate: food preservative. Higher fatty acids: soaps and detergents.
Key Takeaways:
- Esterification: acid + alcohol, , reversible.
- Acyl chlorides from ; amides from ammonium salts on heating.
- or reduces COOH to ; soda lime decarboxylates.
- HVZ: -halogenation with /red P.
- COOH is meta-directing; benzoic acid gives no Friedel-Crafts reaction.
- Pitfall: in Fischer esterification, the OH lost as water comes from the acid, not the alcohol.
Brain-Teaser 1: Ethanoic acid is esterified with ethanol whose oxygen is the isotope . Where does the label appear?
Solution: In the ester, ethyl ethanoate (), not in the water. The alcohol's oxygen attacks the protonated carbonyl carbon and is retained; the acid's OH leaves as water.
Brain-Teaser 2: Give the products: (a) propanoic acid + /red P, then water; (b) sodium propanoate + soda lime, heat; (c) benzoic acid + , then .
Solution: (a) 2-Bromopropanoic acid. (b) Ethane. (c) Benzyl alcohol, .
Brain-Teaser 3: What volume of at STP () is released when of ethanoic acid reacts with excess ?
Solution: acid releases : .
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