Aldehydes, Ketones and Carboxylic Acids
What this domain tests
Nucleophilic addition, oxidation and reduction, distinguishing tests, aldol and Cannizzaro reactions, and the factors controlling carboxylic acid acidity.
Core rules
- Reactivity towards nucleophilic addition: HCHO > RCHO > R₂CO, because alkyl groups are electron-releasing and sterically hindering.
- Tollens' reagent oxidises both aliphatic and aromatic aldehydes to acids, giving a silver mirror. Fehling's solution oxidises aliphatic aldehydes but not aromatic ones such as benzaldehyde. 2,4-DNP detects the carbonyl group generally.
- Aldol condensation needs at least one α-hydrogen and dilute NaOH; the β-hydroxy carbonyl dehydrates on heating. Benzaldehyde has no α-hydrogen, so with concentrated NaOH it undergoes Cannizzaro disproportionation.
- Iodoform test is positive for CH₃CO– groups and for CH₃CH(OH)– groups.
- Clemmensen (Zn-Hg/HCl) and Wolff–Kishner (NH₂NH₂, KOH) both reduce C=O to CH₂.
- Acidity of carboxylic acids depends on inductive electron withdrawal, resonance and the ortho effect. Chloroacetic acid is stronger than acetic acid; trichloroacetic acid is stronger still; benzoic acid is stronger than acetic acid.
Worked example
Benzaldehyde with concentrated NaOH gives benzyl alcohol and sodium benzoate: one molecule is reduced while another is oxidised, which is Cannizzaro. Acetaldehyde under the same conditions cannot do this because its α-hydrogens are removed as an enolate, so with dilute NaOH it gives the aldol product 3-hydroxybutanal instead.
Traps that cost marks
Fehling's test fails for benzaldehyde; Tollens' succeeds. Ketones do not reduce Tollens' or Fehling's reagent. Boiling point order is carboxylic acid > alcohol > aldehyde of similar mass, owing to hydrogen bonding. Do not apply Cannizzaro to aldehydes that possess α-hydrogens.