Alcohol Reactions: Carboxylic Acid and Ketone Formation from Alcohols using Chromate

Introduction

In acidic water (or acetone/water, “Jones conditions”), Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃ (with acid), and H₂CrO₄ generate acidic aqueous Cr(VI) species in equilibrium, commonly represented as chromic acid. These reagents oxidise primary alcohols to carboxylic acids and secondary alcohols to ketones through a chromate-ester → E2-like elimination pathway. The aldehyde produced from a primary alcohol is not isolated: in water it forms a gem-diol (hydrate) that is immediately re-oxidised to the carboxylic acid. Simple tertiary alcohols resist this carbonyl-forming oxidation because they lack a hydrogen on the alcohol-bearing carbon; acid-driven side reactions can still occur. Chromate reagents will also oxidise aldehydes rapidly to acids.

Use this family when you want acids from primary alcohols. If you need aldehydes (stop one step earlier), switch to aldehyde-stop Cr(VI) protocols (e.g., Collins or PDC in CH₂Cl₂; PDC in DMF can give acids) or to PCC or Dess–Martin periodinane, or run the Swern oxidation.

Quick Summary

• Reagents/conditions (unified behaviour): Na₂Cr₂O₇/H₂SO₄/H₂O, K₂Cr₂O₇/H₂SO₄/H₂O, CrO₃/H₂SO₄/H₂O (or acetone/water; “Jones”), H₂CrO₄ generated in situ.
• Outcomes: primary alcohols → carboxylic acids (via aldehyde hydrate); secondary alcohols → ketones; simple tertiary alcohols → no ordinary carbonyl-forming oxidation (acid-driven side reactions may occur); aldehydes (if present) → carboxylic acids.
• Mechanism class: chromate ester formation → E2-like oxidative elimination to carbonyl; subsequent hydrate oxidation for primary alcohols. This is a formal teaching pathway; the full chromium redox network contains additional steps.
• Aldehyde-stop alternatives: For anhydrous oxidations that halt at aldehydes, see PCC/DMP (CH₂Cl₂) or the low-temperature Swern oxidation.
• Safety: hexavalent chromium (Cr(VI)) is toxic and carcinogenic; strict PPE and hazardous-waste protocols are mandatory.

Mechanisms — Secondary (4 Steps) and Primary (7 Steps)

Secondary alcohol → ketone (4 frames)

Step C‑S1 — Chromate-ester formation

Step C‑S2 — E2-like oxidative elimination

Step C‑S3 — Acid–base equilibration

Step C‑S4 — Ketone released

Primary alcohol → carboxylic acid (7 frames)

Step C‑P1 — Chromate-ester formation

Step C‑P2 — First oxidative elimination → aldehyde

Step C‑P3 — Acid-catalyzed aldehyde hydration

Step C‑P4 — Second chromate-ester formation on the gem-diol

Step C‑P5 — Second oxidative elimination → carboxylic acid

Step C‑P6 — Acid–base equilibration

Step C‑P7 — Carboxylic acid released

Aldehydes present at the start—or generated transiently—are consumed to acids under these aqueous chromate conditions.

Mechanistic Checklist

• Primary → acid; secondary → ketone; simple tertiary alcohols resist ordinary carbonyl-forming oxidation (no alcohol-carbon H).
• Aldehydes oxidise to acids in aqueous chromate; aldehyde isolation requires nonaqueous protocols (PCC/DMP, Swern, Collins).
• Key intermediate: neutral chromate ester; E2-like oxidative elimination gives C=O.
• Primary path requires water: aldehyde → hydrate → re-oxidation → acid.
• Do not introduce carbocation shifts into the standard chromate-ester elimination. The full inorganic redox network is not depicted.
• Spectator functionality: alkenes generally survive; acid-labile groups (acetals) can be hydrolysed in the medium.

Worked Examples

Na₂Cr₂O₇ / H₂SO₄ ⟶

Na₂Cr₂O₇/H₂SO₄

Butan-1-ol → butanoic acid (via aldehyde hydrate; carbon connectivity retained).

K₂Cr₂O₇ / H₂SO₄ ⟶

K₂Cr₂O₇/H₂SO₄

Cyclohexanol → cyclohexanone (neutral carbonyl from chromate-ester elimination).

H₂CrO₄ (Jones) ⟶

H₂CrO₄ (Jones)

Benzyl alcohol → benzoic acid (benzylic hydrates oxidise rapidly).

CrO₃ / H₂O ⟶

CrO₃/H₂O

tert-Butyl alcohol → no ordinary carbonyl-forming oxidation (lacks an alcohol-carbon H; acid-driven dehydration or other side reactions can occur).

Edge Cases & Exam Traps

• “Can I stop a primary at the aldehyde?” Not with these aqueous chromate systems. They over-oxidise to acids via the hydrate. Use PCC/DMP, the Swern oxidation, or Collins for aldehydes.
• Aldehyde in the flask? Expect carboxylic acid at the end (Jones/dichromates).
• Diols: individual OH sites can oxidise, but some vicinal diols also undergo C–C cleavage with chromic acid. Do not assume the carbon skeleton always survives; periodate is the standard selective glycol-cleavage comparison.
• Acid-sensitive groups: acetals, some protecting groups, and basic heterocycles may not survive H₂SO₄.
• Colour cue: orange Cr(VI) turns green (Cr(III)) as the oxidation proceeds—common lab check.
• Selectivity: benzylic/allylic alcohols often react rapidly. Simple tertiary alcohols lack the alcohol-carbon H needed for ordinary carbonyl formation; do not extend this rule to every polyol or strained substrate.

Compare oxidation with permanganate

Potassium permanganate (KMnO4) is another alcohol oxidant. In aqueous alkaline conditions, propan-1-ol gives propanoate; separate acidification gives propanoic acid. The carboxylate charge belongs in the basic-stage product.

This is a reagent and endpoint comparison, not the chromium-ester mechanism above. Other oxidizable groups can also react with permanganate, so check the entire substrate before selecting it.

Practical Tips

• Jones reagent: CrO₃ in H₂SO₄/acetone is convenient; run cold-to-rt, then quench with isopropanol before workup.
• Stoichiometry: Sufficient aqueous oxidant and reaction time favor full primary-alcohol conversion to acid; use the stoichiometry specified by the substrate’s procedure.
• Workup: Filter chromium sludge, neutralise properly, and dispose as hazardous Cr waste.
• Choose the right tool: Need aldehyde? Use PCC/DMP or the Swern oxidation. Need the acid? Use these chromate systems.

Exam-Style Summary

All four reagent sets (Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃/H₂SO₄, H₂CrO₄) support chromate-ester formation and E2-like oxidative elimination. With sufficient aqueous oxidant, primary alcohols → carboxylic acids (via aldehyde hydrate), secondary → ketones; simple tertiary alcohols resist the ordinary carbonyl-forming pathway. Aldehydes are normally oxidised onward to acids.

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