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Organic Chemistry · Aldehydes and Ketones: Nucleophilic Addition Reactions

Oxidation of Aldehydes and Ketones

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Oxidation is where aldehydes and ketones part ways for good. An aldehyde carries a hydrogen on its carbonyl carbon, and that C–H bond is the handle oxidation grabs: aldehydes oxidize easily to carboxylic acids with mild reagents — Tollens' silver-ammonia reagent, Fehling's/Benedict's copper solutions, chromium(VI), even oxygen in air. A ketone has no such hydrogen; its carbonyl carbon is already flanked by two C–C bonds at a higher oxidation level, so it resists these conditions. Only brutal treatment (hot KMnO4, hot nitric acid) fragments a ketone by cleaving C–C bonds. The difference is so clean that mild oxidants are used as chemical tests to tell the families apart — the silver mirror of Tollens' test, the brick-red Cu2O precipitate of Fehling's/Benedict's. This topic explains the oxidation-level logic, walks through the hydrate mechanism, and connects to biology (aldehyde dehydrogenase, reducing-sugar tests).

Why this matters

The aldehyde–ketone contrast is one of the most exam-tested ideas in carbonyl chemistry. The same copper chemistry powers the Benedict's test historically used to detect glucose in urine (glucose's open-chain aldehyde form reduces Cu2+ to red Cu2O). In metabolism, aldehyde dehydrogenase oxidizes acetaldehyde (from ethanol) to acetate; when slow or inhibited, acetaldehyde accumulates and causes flushing. Understanding which carbonyls oxidize — and why — lets you predict products and design tests.

The college version

Core Concepts

Oxidation states at the carbonyl carbon

The carbonyl carbon's oxidation state is set by its bonds (C–H = −1, C–C = 0, each C–O bond in C=O = +1):

  • Aldehyde carbon, R-CHO: C=O (+2) + C–H (−1) + C–C (0) = +1
  • Carboxylic acid carbon, R-COOH: C=O (+2) + C–OH (+1) + C–C (0) = +3
  • Ketone carbon, R-CO-R': C=O (+2) + 2 C–C (0) = +2

Aldehyde → acid is a two-electron oxidation (+1 → +3). The ketone carbon at +2 could in principle reach +3 — but that would require replacing a C–C bond with a C–O bond, i.e., breaking the carbon skeleton. Mild oxidants cannot do that, which is why ketones survive.

Why aldehydes oxidize so easily: the hydrate mechanism

In water, an aldehyde exists in equilibrium with its hydrate (gem-diol), formed by nucleophilic addition of water:

R-CHO + H2O ⇌ R-CH(OH)2

The hydrate carries an O–H on the carbonyl carbon — exactly the pattern oxidizing agents attack; oxidizing it removes two electrons and two hydrogens, giving the carboxylic acid. Ketones hydrate far less, and their hydrate would still need to become an acid. That is why Tollens', Fehling's, and Cr(VI) all convert aldehydes to acids under conditions where ketones are untouched.

The classic tests

  • Tollens' test: Ag(NH3)2+ in aqueous ammonia. An aldehyde reduces Ag+ to metallic silver, which plates the glass as a mirror. Balanced (basic conditions):

R-CHO + 2 Ag(NH3)2+ + 3 OH- ⟶ R-COO- + 2 Ag(s) + 4 NH3 + 2 H2O

  • Fehling's/Benedict's test: Cu2+ complexed with tartrate or citrate in strong base; aldehydes reduce it to brick-red Cu2O:

R-CHO + 2 Cu2+ + 5 OH- ⟶ R-COO- + Cu2O(s) + 3 H2O

Ketones give no reaction in either test (except α-hydroxy ketones, which can isomerize and respond).

Strong oxidants: forcing ketones

Hot, concentrated oxidants (KMnO4, HNO3, CrO3/H2SO4) oxidize ketones by C–C bond cleavage, giving mixtures of carboxylic acids (plus CO2 from cleaved terminal carbons). Cyclic ketones open to dicarboxylic acids — cyclohexanone → hexanedioic (adipic) acid, a nylon precursor. Because the products depend on which bond breaks, ketone oxidation is rarely a synthesis tool — it's a degradation reaction for structure determination.

Common Confusions

Do not confuseWithDifference
Aldehyde oxidation (easy, to acid)Ketone oxidation (needs C–C cleavage)Aldehydes have a C–H on the carbonyl; ketones don't — mild oxidants distinguish them instantly
Tollens' test (Ag+ → Ag mirror)Fehling's/Benedict's (Cu2+ → Cu2O)Both oxidize aldehydes; they differ in metal, color change, and basic conditions
Hydrate formation (addition of water)Oxidation (removal of H / addition of O)Hydration is reversible addition, not oxidation; it enables oxidation by creating the C–OH handle
Reducing an aldehyde (NaBH4 → alcohol)Oxidizing an aldehyde (Tollens' → acid)Opposite directions: reduction adds H (alcohol); oxidation adds O (acid)
"Ketones don't oxidize""Ketones are inert to everything"Strong oxidants do cleave ketones (hot KMnO4, HNO3) — they just need much harsher conditions
Autoxidation (slow, air)Controlled oxidation (reagent-driven)Same chemistry, different rate and purpose: shelf spoilage vs deliberate test
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

An aldehyde has a "handle" — a hydrogen stuck to its double-bonded carbon — and oxidizers grab that handle and turn it into an acid. A ketone has no handle, just two carbon arms, so gentle oxidizers leave it alone. That's why the silver test works: aldehydes make a shiny mirror, ketones do nothing.

Worked example

Example 1: Oxidation state bookkeeping

Problem. Show that aldehyde → acid is a two-electron oxidation and that a ketone carbon cannot reach the acid oxidation state without C–C cleavage.

Aldehyde carbon. CH3-CHO: C=O (+2), C–H (−1), C–C (0) → +1.

Acid carbon. CH3-COOH: C=O (+2), C–OH (+1), C–C (0) → +3.

so the change is Δ= +3 - (+1) = +2 electrons removed per carbon.

Ketone carbon. CH3-CO-CH3: C=O (+2), two C–C (0) → +2. Reaching +3 requires converting a C–C bond (0) into a C–O bond (+1) — that is C–C cleavage, which mild oxidants cannot do.

Example 2: Stoichiometry of the Tollens' silver mirror

Problem. How many grams of silver deposit when 0.0500 mol of butanal is completely oxidized by excess Tollens' reagent? M(Ag) = 107.87 g mol-1.

Balanced equation (1:2 aldehyde-to-Ag+):

R-CHO + 2 Ag(NH3)2+ + 3 OH- ⟶ R-COO- + 2 Ag(s) + 4 NH3 + 2 H2O

Moles of Ag:

n(Ag) = 0.0500 mol RCHO × 2 mol Ag1 mol RCHO = 0.100 mol

Mass of Ag:

m(Ag) = 0.100 mol × 107.87 g mol-1 = 10.8 g

Interpretation. 0.0500 mol butanal (≈ 3.6 g) deposits 10.8 g of silver — the 2:1 Ag+ stoichiometry means the silver mass exceeds the aldehyde mass.

Follow-up scenario: predicting test outcomes. Butanal, butan-2-one, and D-glucose are each treated with Benedict's reagent. Butanal (open aldehyde) gives brick-red Cu2O; butan-2-one (ketone) gives no reaction. Glucose gives a positive test even though it is mostly a cyclic hemiacetal: the small equilibrium fraction of open-chain aldehyde is consumed by oxidation, pulling the ring–chain equilibrium forward — the definition of a reducing sugar and the basis of urine-glucose screening.

Key takeaways

  • Aldehyde → acid: 2-electron oxidation (carbon +1 → +3); ketones inert to mild oxidants (no C–H on the carbonyl carbon).
  • Tollens': Ag(NH3)2+ → silver mirror; 1 mol aldehyde uses 2 mol Ag+.
  • Fehling's/Benedict's: Cu2+ → brick-red Cu2O; basis of classic glucose-in-urine testing.
  • Mechanism: aldehyde + H2O ⇌ hydrate (gem-diol); the hydrate is oxidized.
  • Strong oxidants (hot KMnO4, HNO3) cleave ketones at C–C bonds → acid mixtures; cyclic ketones → dicarboxylic acids (cyclohexanone → adipic acid).
  • Aldehydes autoxidize in air — store under inert gas.
  • Biological: aldehyde dehydrogenase converts acetaldehyde → acetate; slow ALDH2 causes flushing.
  • Safety principles: prepare Tollens' reagent fresh and never store it (ammoniacal silver solutions can form explosive residues); follow institutional disposal rules. Strong oxidants and organics are a fire hazard — keep them separate.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Why does a mild oxidant convert an aldehyde to a carboxylic acid but leave a ketone untouched?

    Show answer

    The aldehyde's carbonyl carbon carries a C–H; in water it forms a hydrate whose C–OH is readily oxidized (+1 → +3). The ketone carbon (+2) has only C–C bonds; reaching +3 means breaking a C–C bond — impossible for mild oxidants.

  2. What visual result confirms an aldehyde in Tollens' test, and what species is reduced?

    Show answer

    A silver mirror plates onto the glass as Ag+ is reduced to metallic silver (one electron per Ag, two per aldehyde).

  3. Write the balanced Tollens' equation for one mole of RCHO and state the aldehyde-to-Ag mole ratio.

    Show answer

    RCHO + 2 Ag(NH3)2+ + 3 OH- → RCOO- + 2 Ag(s) + 4 NH3 + 2 H2O; 1 mol aldehyde : 2 mol Ag+.

  4. Cyclohexanone is heated with strong KMnO4. What class of product forms, and why is this not useful for synthesis?

    Show answer

    A dicarboxylic acid (cyclohexanone → hexanedioic/adipic acid). Products depend on which bonds break, so yields and selectivity are poor — it's a degradation probe, not a synthesis.

  5. Glucose gives a positive Benedict's test even though it exists mostly as a cyclic hemiacetal. Explain.

    Show answer

    The tiny equilibrium fraction of open-chain aldehyde is consumed by oxidation; Le Châtelier's principle pulls the ring–chain equilibrium toward the open form, so the test runs to completion.

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Oxidation (organic)
Gain of oxygen or loss of hydrogen at carbon; rise in oxidation state
Hydrate (gem-diol)
The R–CH(OH)2 adduct of an aldehyde with water
Tollens' reagent
Ag(NH3)2+ in aqueous ammonia
Fehling's/Benedict's reagent
Cu2+ complexed with tartrate/citrate in base
Reducing sugar
A carbohydrate whose open-chain aldehyde form reduces Cu2+
Autoxidation
Slow reaction of an aldehyde with O2 in air
C–C cleavage
Breaking a carbon–carbon bond, as in strong oxidant attack on ketones

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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