Reagents · 9 min read

Oxidation and reduction in organic chemistry: a reagent map

Organic oxidation and reduction are about bonds to carbon: more bonds to oxygen or other electronegative atoms mean oxidation, and more bonds to hydrogen mean reduction. Once you can classify a change, choosing the reagent is mostly a matter of how far you want to go and what else is in the molecule.

How to tell what has been oxidised

Count the bonds from carbon to heteroatoms (O, N, halogen) and to hydrogen. Moving up this ladder is oxidation, and moving down is reduction:

alkane → alcohol → aldehyde or ketone → carboxylic acid → CO₂

An alcohol to a ketone is an oxidation (a C–H bond is replaced by a second C–O bond). A ketone to an alcohol is a reduction. Adding H₂ across a C=C is a reduction, and adding Br₂ to it is an oxidation of the carbons.

Oxidising alcohols

• Mild, stops at the aldehyde: Swern (DMSO, oxalyl chloride, then Et₃N), Dess–Martin periodinane, PCC-type reagents and Collins reagent. They work in the absence of water, so the aldehyde is not hydrated and cannot be oxidised further.
• Strong, goes to the acid: Jones reagent (CrO₃ in aqueous H₂SO₄) and permanganate. In water the aldehyde forms a hydrate, which is oxidised like an alcohol.

The common mechanistic theme is a leaving group on the heteroatom (a chromate ester, an activated sulfonium) and an E2-like removal of the C–H proton that forms the C=O. Compare the sulfur-based version in Pfitzner–Moffatt and the chromium version in the Jones page.

Reducing carbonyl compounds

• NaBH₄ is a mild hydride donor. It reduces aldehydes and ketones to alcohols in protic solvents but usually leaves esters, amides and carboxylic acids alone. See NaBH₄ reduction of a ketone.
• LiAlH₄ is much stronger. It reduces aldehydes, ketones, esters (to primary alcohols), carboxylic acids, amides (to amines) and nitriles. It reacts violently with water, so it is used in dry ether or THF followed by a careful workup. See LiAlH₄ reduction of an ester to a primary alcohol.
• DIBAL at low temperature stops at the aldehyde stage from esters or nitriles because a stable tetrahedral intermediate forms and survives until workup.
• Luche reduction (NaBH₄ with CeCl₃) gives 1,2-reduction of enones to allylic alcohols: Luche reduction of an enone.
• Meerwein–Ponndorf–Verley reduction transfers a hydride from isopropoxide in a six-membered transition state, and works with ketones in the presence of other reducible groups: Meerwein-Ponndorf-Verley reduction.

What matters mechanistically: a hydride adds to the carbonyl carbon, the π electrons move onto oxygen, and a proton arrives later (in the workup or from the solvent).

Reducing C=C, C≡C and other groups

• Catalytic hydrogenation (H₂ with Pd, Pt or Ni) adds H₂ across C=C and C≡C, with both hydrogens usually delivered to the same face ("syn"). See Catalytic hydrogenation of an alkene (Pd/C, H₂).
• Lindlar catalyst stops an alkyne at the cis-alkene: Lindlar semihydrogenation of an alkyne. Dissolving-metal (Birch-type) reduction gives trans-alkenes and, for arenes, 1,4-dienes: Birch reduction of benzene.
• Nitro groups are reduced to amines by metals in acid or by hydrogenation: Catalytic reduction of a nitro group to an amine. Nitriles are reduced to primary amines: LiAlH₄ reduction of a nitrile to a primary amine.
• Deoxygenation of a carbonyl to CH₂ uses either strongly acidic conditions (Clemmensen reduction) or strongly basic ones (Wolff-Kishner reduction). Choose the one your molecule can survive.

Oxidising alkenes

• Epoxidation with a peracid such as mCPBA gives an epoxide in a single concerted step.
• Dihydroxylation with OsO₄ (or cold, dilute KMnO₄) gives a syn diol: see Sharpless asymmetric dihydroxylation for the asymmetric version.
• Ozonolysis cleaves the C=C bond to give two carbonyl compounds. A reductive workup (Me₂S or Zn) gives aldehydes and ketones, while an oxidative workup (H₂O₂) gives acids: Ozonolysis.
• Hydroboration–oxidation gives the anti-Markovnikov alcohol with syn addition: Hydroboration-oxidation of propene.
• Wacker oxidation turns a terminal alkene into a methyl ketone with Pd(II) and Cu(II): Wacker oxidation.

Choosing a reagent: three questions

1. How far do I want to go? Aldehyde or acid, alcohol or alkane, alkene or alkane. Mild reagents stop earlier.
2. What else is in the molecule? A ketone and an ester in the same molecule: NaBH₄ reduces the ketone and not the ester. An alkene and a ketone: hydride reagents reduce the ketone and leave the alkene.
3. What conditions can it tolerate? Acid-sensitive groups rule out the Clemmensen reduction, and base-sensitive ones rule out Wolff–Kishner.

Selectivity of this kind is also behind the use of protecting groups: see the protecting groups guide.

Practise in the interactive app   Browse all 345 reactions

More study guides

• How to learn reaction mechanisms (without memorising everything)A practical method for learning organic reaction mechanisms: understand electron flow, group reactions into a few patterns, and practise by drawing.
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• SN1, SN2, E1 or E2? How to decide which mechanism winsA clear decision guide for substitution versus elimination: substrate, nucleophile or base strength, solvent and temperature, with examples.
• Nucleophiles, electrophiles and leaving groups: what makes a reaction goWhat decides how reactive a nucleophile, an electrophile or a leaving group is, and how solvent, charge, size and resonance change reactivity.