Concept 05 / 07Aldehydes, Ketones and Carboxylic Acids
Carboxylic Acids: Nomenclature, Preparation and Physical Properties
Structure and naming of carboxylic acids, preparation from primary alcohols and aldehydes, alkylbenzenes, nitriles and amides, Grignard reagents with carbon dioxide, acyl halides, anhydrides and esters, and physical properties explained by hydrogen-bonded dimers.
Carboxylic acids contain the carboxyl group, (carbonyl plus hydroxyl). Many have common names from their natural sources: formic acid (HCOOH, from ants; methanoic acid), acetic acid (, vinegar; ethanoic acid), butyric acid (rancid butter; butanoic acid). Aromatic: benzoic acid. Dicarboxylic: oxalic (ethanedioic), adipic (hexanedioic) acids. The carbon of COOH is C1 in IUPAC names; when attached to a ring, the suffix is -carboxylic acid.
Structure: the carboxyl carbon is ; resonance involving the OH lone pair makes the carbonyl carbon less electrophilic than in aldehydes and ketones.
Preparation.
From primary alcohols and aldehydes by oxidation: .
From alkylbenzenes: vigorous oxidation (alkaline , then acid; or chromic acid) converts the whole side chain (of any length, if it has a benzylic hydrogen) to COOH: toluene and ethylbenzene both give benzoic acid. Tert-butylbenzene (no benzylic H) resists.
From nitriles and amides: hydrolysis with acid or base, (mild conditions stop at the amide).
From Grignard reagents: (dry ice) gives , and acid hydrolysis gives RCOOH (one more carbon than RX).
From acyl halides, anhydrides and esters: hydrolysis. Acyl halides with water give acids; anhydrides with water give acids; esters are hydrolysed by acid or, better, by base (saponification) followed by acidification.
Physical properties. Lower aliphatic acids (up to ) are colourless liquids with unpleasant odours; higher ones are waxy solids. Boiling points are higher than those of aldehydes, ketones and even alcohols of comparable mass, because of extensive hydrogen bonding; carboxylic acids exist as cyclic dimers held by two hydrogen bonds, even in the vapour phase and in aprotic solvents (see figure). The first four aliphatic acids are miscible with water (hydrogen bonding with water); solubility falls as the hydrocarbon chain lengthens. Benzoic acid is nearly insoluble in cold water. Carboxylic acids are soluble in less polar solvents such as benzene, ether and alcohol.
Key Takeaways:
- Oxidation of alcohols, aldehydes and alkylbenzenes (side chain → COOH).
- Nitrile hydrolysis; Grignard + adds one carbon.
- Hydrolysis of acyl halides, anhydrides and esters.
- Hydrogen-bonded dimers give high boiling points.
- Pitfall: any alkyl side chain with a benzylic H is oxidised all the way to a single COOH on the ring; a tertiary side chain is not.
Brain-Teaser 1: Which Grignard reagent and which reaction give (a) propanoic acid, (b) benzoic acid?
Solution: (a) Ethylmagnesium bromide with , then . (b) Phenylmagnesium bromide with , then .
Brain-Teaser 2: What product forms when 4-ethyltoluene (1-ethyl-4-methylbenzene) is oxidised with hot alkaline followed by acid?
Solution: Both side chains have benzylic hydrogens and are oxidised to COOH: benzene-1,4-dicarboxylic acid (terephthalic acid).
Brain-Teaser 3: Ethanoic acid (M ) boils at , while propan-1-ol (M ) boils at . Explain.
Solution: Both form hydrogen bonds, but carboxylic acid molecules pair into cyclic dimers held by two strong hydrogen bonds (the C=O oxygen is a good acceptor and the O-H is more acidic). The dimer behaves like a larger molecule, so more energy is needed to vaporise the acid.
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