Aldehyde To Carboxylic Acid Chromate

Aldehyde → Carboxylic Acid with Chromate (Cr(VI))

Aldehyde Oxidation to Carboxylic Acids with Chromate (Cr(VI))

Under aqueous acidic conditions, Cr(VI) oxidants—Na₂Cr₂O₇/H₂SO₄, K₂Cr₂O₇/H₂SO₄, CrO₃/H₂SO₄ (Jones reagent), or preformed H₂CrO₄—rapidly convert aldehydes into carboxylic acids. In water an aldehyde hydrates to its gem-diol; one hydroxyl esterifies with chromic acid to give a chromate ester, and β-elimination (formally hydride transfer to Cr(VI)) generates the carboxylic acid while chromium is reduced to Cr(III). The four reagent buttons shown in the UI all converge on this shared mechanism; only the oxidant label/solvent overlay changes.


Quick Summary

  • Reagents/conditions: Na₂Cr₂O₇ or K₂Cr₂O₇ with H₂SO₄ (aq), CrO₃/H₂SO₄ in acetone–water (Jones), or H₂CrO₄ (aq); 0–25 °C with water present.
  • Outcome: Aldehyde → carboxylic acid (formaldehyde → formic acid; longer chains → corresponding acids).
  • Mechanistic spine: Hydrate formation → chromate ester → β-elimination / hydride transfer → carboxylic acid (Cr(VI) → Cr(III)).
  • Selectivity: Ketones do not over-oxidise to acids here; this page focuses on aldehydes.
  • Safety: Cr(VI) reagents are toxic and strongly oxidising; quench to Cr(III) (green) before disposal.

Mechanism — Hydrate First, Chromate Ester, β-Elimination (6 Steps)


Step 1: Water adds to the protonated aldehyde as the C=O π bond shifts to oxygen.
**Step 1 – Water addition to the protonated carbonyl:** Hydronium activates the C=O and water performs the 1,2-addition, pushing electron density onto oxygen.
Step 2: Proton transfers finish the gem-diol hydrate.
**Step 2 – Proton shuttles complete the hydrate:** Hydronium donates a proton back to the original carbonyl oxygen, and water deprotonates the new hydroxyl to give the gem-diol.
Step 3: Chromate adds to the protonated hydrate.
**Step 3 – Chromate ester formation:** The cationic hydroxyl attacks Cr(VI) as one oxo lone pair collapses, establishing the Cr–O bond that defines the chromate ester.
Step 4: Proton relays stabilise the chromate ester with a neutral bridge oxygen.
**Step 4 – Proton transfers within the chromate ester:** Hydronium reprotonates the leaving oxygen while water removes the bridging proton, leaving a neutral chromate ester ready to eliminate.
Step 5: β-hydride transfer breaks the Cr–O bond and reduces chromium.
**Step 5 – β-Elimination / hydride transfer:** Water removes the α-hydrogen as electrons flow into the C=O and the Cr–O bond breaks, reducing Cr(VI) toward Cr(III).
Step 6: Carboxylic acid appears after quench, chromium is now blue-green Cr(III).
**Step 6 – Carboxylic acid isolation:** The neutral carboxylic acid is present; workup or quench consumes the remaining oxidant, leaving characteristic green Cr(III) species.

Mechanistic Checklist (Exam Focus)

  • Hydrate-first logic: show gem-diol formation; chromate attacks the hydrate, not the bare carbonyl.
  • Chromate ester + β-elimination: depict the chromate ester before the E2-like step that forms the C=O and ejects Cr(VI).
  • Cr(VI) reduction: note orange → green colour change as Cr(VI) becomes Cr(III).
  • Aldehydes only: ketones do not advance to acids under these aqueous Cr(VI) conditions.
  • Formaldehyde oxidises fastest (nearly 100 % hydrate); aryl aldehydes are slower but still proceed.
  • Closed-shell pathway: no radicals in the accepted mechanism.

Worked Examples


Benzaldehyde → Benzoic acid (Na₂Cr₂O₇/H₂SO₄)

Benzaldehyde reactant structure Na₂Cr₂O₇/H₂SO₄ chromate reagent card Benzoic acid product structure

Aromatic aldehydes hydrate more slowly, but chromate ester formation still drives oxidation to benzoic acid; the classic orange → green shift signals Cr(VI) reduction.

Hexanal → Hexanoic acid (K₂Cr₂O₇/H₂SO₄)

Hexanal reactant structure K₂Cr₂O₇/H₂SO₄ reagent button Hexanoic acid product structure

Unbranched aliphatic aldehydes hydrate quickly, so the K₂Cr₂O₇ button delivers quantitative oxidation to the straight-chain carboxylic acid.

Cyclohexanecarboxaldehyde → Cyclohexanecarboxylic acid (CrO₃/H₂SO₄)

Cyclohexanecarboxaldehyde reactant structure CrO₃/H₂SO₄ (Jones reagent) button Cyclohexanecarboxylic acid product structure

Jones reagent oxidises benzylic and aliphatic aldehydes alike; cyclic aldehydes stay intact because the chromate pathway only modifies the side-chain carbonyl.


Scope & Limitations

  • Works best: Aliphatic, benzylic, heteroatom-activated aldehydes; formaldehyde is extremely fast.
  • Slower: Aryl aldehydes (hydrate fraction smaller) but still oxidise in Jones/dichromate solutions.
  • Not covered: Ketones (do not oxidise to acids here), aldehydes in strictly anhydrous media (hydrate suppressed).
  • Functional groups: Oxidises susceptible alcohols/allylic positions in the same pot; protect acid-sensitive groups if needed.

Practical Tips

  • Prepare chromate in aqueous acid, then add the aldehyde (often dissolved in acetone for Jones reagent).
  • Monitor colour: orange Cr(VI) fades to green Cr(III) as oxidation completes.
  • Quench excess Cr(VI) (NaHSO₃ or i-PrOH) before workup; collect chromium waste separately.
  • Ensure water is present—hydrate formation is required to reach the acid.

Exam-Style Summary

In water/acid, aldehydes hydrate to gem-diols that form chromate esters. β-Elimination (formally hydride transfer) produces the carboxylic acid and reduces Cr(VI) to Cr(III). Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃ (Jones), and H₂CrO₄ all converge on this pathway; ketones do not over-oxidise under these conditions.


Interactive Toolbox

  • Mechanism Solver — Use Mechanism Solver to see each step of the chromate oxidation mechanism along with descriptions of each step!
  • Reaction Solver — Quickly find the product of any aldehyde reacted with Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃, or H₂CrO₄!
  • IUPAC Namer — Learn the naming ins and outs of aldehyde starting materials and carboxylic acid products.

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