Oxidation & Reduction
Aldehydes and ketones both contain the carbonyl group (C=O), but they behave very differently when subjected to oxidation and reduction reactions. Aldehydes, with their hydrogen atom attached directly to the carbonyl carbon (R–CHO), are far more easily oxidised than ketones (R–CO–R'), which have two carbon substituents flanking the carbonyl. This distinction underpins a suite of classic chemical tests and synthetic transformations that chemists use to identify and modify these compounds. Oxidation of aldehydes converts them into carboxylic acids, and this process can be detected using mild reagents such as Tollens' reagent (ammoniacal silver nitrate) and Fehling's solution (alkaline copper(II) tartrate). Because ketones cannot be further oxidised under mild conditions without breaking carbon–carbon bonds, these tests serve as a reliable way to distinguish between the two functional groups. The elegant visual results — a silver mirror or a brick-red precipitate — make these reactions favourites in the laboratory. Reduction reactions, by contrast, add hydrogen across the C=O bond, converting carbonyl compounds into alcohols (using hydride reagents such as NaBH₄ or LiAlH₄) or all the way to alkanes (using Clemmensen or Wolff–Kishner reductions). Understanding which reagent to choose is a critical skill in organic synthesis: each method has specific conditions, selectivity, and mechanistic features that make it suitable for particular situations.
- Tollens' Test (Silver Mirror Test)
- Fehling's Test (Brick-Red Precipitate)
- Reduction with NaBH₄ and LiAlH₄
- Clemmensen Reduction (C=O → CH₂)
- Wolff–Kishner Reduction (C=O → CH₂)
Tollens' Test (Silver Mirror Test)
Tollens' reagent is prepared by dissolving AgNO₃ in aqueous NaOH, then adding dilute ammonia until the brown precipitate of Ag₂O just dissolves, forming the diamminesilver(I) complex [Ag(NH₃)₂]⁺. When an aldehyde is added and the mixture is warmed gently, the aldehyde is oxidised to a carboxylate ion while Ag⁺ is reduced to metallic silver, which deposits as a bright mirror on the inside of the glass tube. The half-reaction for the aldehyde is: RCHO + H₂O → RCOOH + 2H⁺ + 2e⁻. Ketones give no reaction because they lack the oxidisable C–H bond at the carbonyl carbon.
Fehling's Test (Brick-Red Precipitate)
Fehling's solution consists of two parts mixed before use: Fehling's A (CuSO₄ solution) and Fehling's B (alkaline sodium potassium tartrate). The tartrate acts as a ligand, keeping Cu²⁺ in solution as a deep blue complex at high pH. On heating with an aldehyde, Cu²⁺ is reduced to Cu⁺, which precipitates as brick-red copper(I) oxide (Cu₂O). The aldehyde is simultaneously oxidised to the carboxylate: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O↓ + 3H₂O. Fehling's test is particularly sensitive to aliphatic aldehydes; aromatic aldehydes (e.g. benzaldehyde) are not reliably detected. Ketones do not react.
Reduction with NaBH₄ and LiAlH₄
Sodium borohydride (NaBH₄) and lithium aluminium hydride (LiAlH₄) are nucleophilic hydride-ion (H⁻) donors. The H⁻ attacks the electrophilic carbonyl carbon, and subsequent protonation of the alkoxide intermediate yields an alcohol. Aldehydes → primary alcohols; ketones → secondary alcohols. NaBH₄ is mild: it can be used in protic solvents (water, ethanol) and is selective — it reduces aldehydes and ketones but not esters or carboxylic acids. LiAlH₄ is much more powerful and reduces virtually all carbonyl-containing functional groups, including esters, acids, and amides. It must be used in dry ether (THF or diethyl ether) because it reacts violently with water. Workup involves careful addition of water or dilute acid to destroy the aluminium alkoxide complex.
Clemmensen Reduction (C=O → CH₂)
The Clemmensen reduction converts a carbonyl group directly to a methylene group (–CH₂–) using amalgamated zinc (Zn/Hg) and concentrated hydrochloric acid. The conditions are strongly acidic and aqueous, making it suitable for acid-stable, base-sensitive substrates. The mechanism is not fully understood but is believed to involve the zinc surface acting as an electron source. A common application is the conversion of an aryl ketone (made by Friedel–Crafts acylation) into an alkylbenzene: PhCOCH₃ → PhCH₂CH₃. The reaction fails with compounds that are sensitive to acid.
Wolff–Kishner Reduction (C=O → CH₂)
The Wolff–Kishner reduction achieves the same overall transformation as Clemmensen (C=O → CH₂) but under strongly basic, high-temperature conditions, making it ideal for acid-sensitive substrates. The carbonyl compound is first treated with hydrazine (H₂NNH₂) to form a hydrazone (R₂C=NNH₂). The hydrazone is then heated with a strong base (KOH) in a high-boiling solvent such as ethylene glycol. Base abstracts a proton from the hydrazone nitrogen, triggering a series of proton transfers and loss of N₂ gas, ultimately yielding the alkane. The Huang Minlon modification simplifies the procedure by running both steps in a single pot.
The Key Equation
- \ce{RCHO} — Aldehyde (mol): The organic compound being oxidised; R is any carbon chain or aromatic group.
- \ce{[Ag(NH3)2]+} — Diamminesilver(I) ion (mol): The active oxidising species in Tollens' reagent, formed by dissolving Ag₂O in excess ammonia.
- \ce{Ag(s)} — Metallic silver (mol): The reduction product that deposits as a shiny silver mirror on the glass surface.
- \ce{RCOO-} — Carboxylate ion (mol): The oxidised product of the aldehyde, equivalent to the conjugate base of a carboxylic acid under alkaline conditions.
Worked Example
Problem. A student has two unlabelled bottles, one containing pentan-2-one (a ketone) and one containing pentanal (an aldehyde). Describe the chemical tests the student could use to distinguish between them, and explain what the expected observations are for each compound. Then state what product would be formed if pentanal were reduced with NaBH₄, followed by aqueous workup.
Answer. Pentanal gives a positive Tollens' test (silver mirror) and a positive Fehling's test (brick-red precipitate); pentan-2-one gives negative results in both tests. Reduction of pentanal with NaBH₄ yields pentan-1-ol.
Origins · 1 minBernhard Tollens and how this was discovered
Bernhard Christian Gottfried Tollens was a German chemist who developed the silver mirror test for aldehydes in the 1880s while working at the University of Göttingen. His reagent — ammoniacal silver nitrate — provided chemists with a simple, visually striking method to distinguish aldehydes from ketones, and it remains in use in teaching laboratories worldwide. Tollens also made important contributions to carbohydrate chemistry, including work on the structure of pentoses. The spectacular silver mirror produced by Tollens' test is not just a chemical curiosity — before synthetic coatings, a similar silvering reaction was used commercially to manufacture mirrors and vacuum flasks (Thermos bottles).
Related Concepts
- Nucleophilic Addition to Aldehydes and Ketones — Reduction by NaBH₄ and LiAlH₄ proceeds via nucleophilic addition of H⁻ to the carbonyl carbon — understanding this mechanism is essential for understanding selectivity and stereochemical outcomes.
- Carboxylic Acids: Structure and Properties — Oxidation of aldehydes produces carboxylic acids; knowing the structure, acidity, and further reactions of carboxylic acids is the natural next step after studying aldehyde oxidation.
- Oxidation of Alcohols — Reduction of aldehydes and ketones produces alcohols, and the reverse process — oxidation of primary and secondary alcohols — connects directly to the same oxidation-state ladder covered in this topic.
- Friedel–Crafts Acylation — Clemmensen and Wolff–Kishner reductions are classically used after Friedel–Crafts acylation to introduce alkyl groups into aromatic rings — a two-step sequence central to aromatic synthesis.