Aldehydes, Ketones and Carboxylic Acids

The One Thing

The polarity of the carbonyl group, C=O, determines the characteristic reactions of aldehydes and ketones, especially nucleophilic addition. Structural differences, including the presence of a carbonyl hydrogen, alpha hydrogen, or carboxyl group, account for their oxidation, acidity, identification tests and synthetic transformations.

Reactions, Processes and Experiments

Key Terms

• Carbonyl group: The functional group C=O, present in aldehydes, ketones, carboxylic acids and several related compounds.
• Aldehyde: An organic compound containing the –CHO group; its general formula is R–CHO.
• Ketone: An organic compound in which the carbonyl group is bonded to two carbon groups; its general formula is R–CO–R′.
• Carboxylic acid: An organic compound containing the carboxyl group, –COOH; its general formula is R–COOH.
• Nucleophilic addition: A reaction in which a nucleophile attacks the electron-deficient carbonyl carbon, followed by addition across the C=O bond.
• Nucleophile: An electron-rich species that donates an electron pair to an electron-deficient atom.
• Electrophilic carbonyl carbon: The carbon atom of C=O carrying partial positive charge because oxygen attracts the shared electrons.
• Oxidation of aldehydes: Conversion of aldehydes into the corresponding carboxylic acids: R–CHO + [O] → R–COOH.
• Reduction of aldehydes: Conversion of aldehydes into primary alcohols: R–CHO + 2[H] → R–CH2OH.
• Reduction of ketones: Conversion of ketones into secondary alcohols: R–CO–R′ + 2[H] → R–CHOH–R′.
• Tollens’ test: A test in which aldehydes reduce ammoniacal silver nitrate to metallic silver, producing a silver mirror.
• Fehling’s test: A test in which many aliphatic aldehydes reduce Fehling’s solution to brick-red copper(I) oxide.
• Iodoform test: A test for compounds containing the CH3CO– group, or compounds oxidisable to it, producing yellow CHI3.
• Aldol condensation: Formation of a beta-hydroxy carbonyl compound by aldehydes or ketones containing alpha hydrogen in dilute base, followed potentially by dehydration.
• Cannizzaro reaction: Self-oxidation and self-reduction of aldehydes without alpha hydrogen in concentrated alkali, forming an alcohol and a carboxylate salt.
• Haloform reaction: Reaction of methyl ketones with halogen and alkali to produce a haloform and a carboxylate salt.
• Hell–Volhard–Zelinsky reaction: Alpha-halogenation of carboxylic acids having alpha hydrogen using halogen and phosphorus or a suitable phosphorus reagent.
• Esterification: Reaction of a carboxylic acid with an alcohol in concentrated sulphuric acid to form an ester and water.
• Decarboxylation: Removal of carbon dioxide from a carboxylic acid or its salt, commonly when sodium salts are heated with soda lime.
• Acidity of carboxylic acids: The ability of carboxylic acids to donate H+ because the conjugate carboxylate ion is resonance-stabilised.
• Resonance stabilisation: Delocalisation of the negative charge in a carboxylate ion over two oxygen atoms.
• Inductive effect: The influence of electron-withdrawing or electron-donating groups on electron distribution and carboxylic acid strength.
• Aromatic aldehyde: An aldehyde in which –CHO is directly attached to an aromatic ring, such as benzaldehyde.
• Aromatic ketone: A ketone containing an aromatic group attached to the carbonyl carbon, such as acetophenone.
• Clemmensen reduction: Reduction of an aldehyde or ketone carbonyl group to a methylene group using zinc amalgam and concentrated hydrochloric acid.
• Wolff–Kishner reduction: Reduction of an aldehyde or ketone carbonyl group to a methylene group using hydrazine and strong base on heating.
• Ester: A product of esterification, often characterised by a pleasant odour and used in perfumes, flavouring agents, solvents and plasticisers.
• Carbonyl carbon hybridisation: The carbonyl carbon is sp2-hybridised, giving a trigonal planar arrangement around the carbonyl group.

Easily Confused

• Aldehydes and ketones: Aldehydes contain –CHO and are generally more reactive because they have less steric hindrance and fewer electron-releasing alkyl groups; ketones contain R–CO–R′.
• Aldol condensation and Cannizzaro reaction: Aldol condensation requires alpha hydrogen, whereas Cannizzaro reaction occurs in aldehydes without alpha hydrogen.
• Tollens’ test and Fehling’s test: Tollens’ test gives a silver mirror, whereas Fehling’s test gives a brick-red precipitate of copper(I) oxide.
• Aldehyde oxidation and ketone oxidation: Aldehydes are readily oxidised to carboxylic acids; ketones resist mild oxidation and may undergo carbon–carbon bond cleavage only under vigorous conditions.
• Aldehyde reduction and ketone reduction: Aldehyde reduction produces a primary alcohol, whereas ketone reduction produces a secondary alcohol.
• Esterification and decarboxylation: Esterification forms an ester and water from a carboxylic acid and alcohol; decarboxylation removes carbon dioxide from a carboxylic acid or its salt.
• Aromatic aldehyde and aromatic ketone: Benzaldehyde has –CHO directly attached to an aromatic ring, whereas acetophenone has an aromatic group attached to the carbonyl carbon of a ketone.
• Resonance stabilisation and inductive effect: Resonance stabilisation delocalises charge within the carboxylate ion, whereas the inductive effect transmits electron-withdrawing or electron-donating influence through sigma bonds.
• Carboxylic acid hydrogen bonding and aldehyde or ketone polarity: Carboxylic acids form strong hydrogen-bonded dimers because they contain O–H, whereas aldehydes and ketones lack an O–H bond and do not generally hydrogen-bond strongly with one another.

What Gets Asked

• Questions may require identification of aldehydes, ketones and carboxylic acids from their general formulae and examples such as methanal, ethanal, propanone and ethanoic acid. A common mark-losing error is confusing R–CHO with R–CO–R′.
• Reaction-classification questions may ask why aldehydes and ketones undergo nucleophilic addition. The essential point is the electrophilic carbonyl carbon produced by the polar C=O bond.
• Test-based questions may ask for the observations in Tollens’ test, Fehling’s test and the iodoform test. The relevant observations are respectively a silver mirror, a brick-red precipitate and a yellow precipitate.
• Questions may distinguish aldol condensation from the Cannizzaro reaction. The deciding structural feature is the presence or absence of alpha hydrogen.
• Oxidation and reduction questions may require aldehydes to be converted to carboxylic acids, aldehydes to primary alcohols and ketones to secondary alcohols. Assigning a ketone reduction product as a primary alcohol loses the distinction.
• Acidity questions may require explanation using resonance stabilisation and inductive effects. Electron-withdrawing groups increase carboxylic acid strength, whereas electron-donating alkyl groups generally decrease it.