Aldehyde To Carboxylic Acid Chromate

Aldehyde → Carboxylic Acid 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 an E2-like elimination forms the carboxylic acid. The H lost is the former aldehydic C–H on the hydrate carbon. Chromium is reduced to Cr(III) through the overall multistep redox process.

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 → E2-like elimination → carboxylic acid (Cr(VI) → Cr(III)).
Selectivity: Ordinary ketones generally resist oxidation under these mild conditions; the illustrated hydrate pathway focuses on aldehydes.
Safety: Cr(VI) reagents are toxic and strongly oxidising; quench to Cr(III) (green) before disposal.

Mechanism — Hydrate, Chromate Ester and Redox Balance (6 Frames)

**Step 1 – Hydrate formation.** In aqueous acid, carbonyl activation, water addition and proton transfers establish the gem-diol equilibrium. The diagram summarizes hydration rather than a unique elementary sequence.

**Step 2 – The gem-diol.** Butanal hydrate retains a C–H bond on the carbon bearing both hydroxyls. This is the former aldehydic hydrogen that is removed during oxidation; it is not an α-H.

**Step 3 – Chromate ester formation.** A neutral hydrate hydroxyl oxygen attacks electrophilic Cr(VI). Proton transfer and water loss form a chromate ester with the connectivity shown.

**Step 4 – E2-like organic oxidation.** Water removes the H from the hydrate carbon as C=O forms and the O–Cr bond breaks. The carbon is oxidized by two electrons. No α-H on the neighboring carbon is required.

**Step 5 – Overall chromium reduction.** The equation balances aldehyde oxidation with conversion of Cr(VI) to Cr(III). Chromium turnover includes additional redox steps; one two-electron organic elimination is not itself a three-electron Cr(VI)→Cr(III) event.

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: a hydrate hydroxyl oxygen attacks electrophilic Cr(VI); the aldehyde is not oxidized by removal of an α-H.
Show the chromate ester before the E2-like elimination. The former aldehydic C–H electrons form C=O as O–Cr cleavage releases a reduced chromium fragment.
Cr(VI) reduction: note orange → green colour change as Cr(VI) becomes Cr(III).
Benzaldehyde oxidizes even though it has no α-H; ordinary ketones generally resist these mild oxidation conditions.
Formaldehyde is strongly hydrated, while aryl aldehydes generally have smaller hydrate fractions. Hydrate fraction alone does not establish a universal oxidation-rate ranking.
The conventional organic bond-change model is a chromate-ester elimination; it does not imply that every chromium redox step is one elementary closed-shell process.

Worked Examples

Benzaldehyde → Benzoic acid (Na₂Cr₂O₇/H₂SO₄)

Benzaldehyde can be oxidized through its hydrate even when the hydrate fraction is small; the classic orange → green shift signals Cr(VI) reduction.

Hexanal → Hexanoic acid (K₂Cr₂O₇/H₂SO₄)

Hexanal is oxidized to hexanoic acid under aqueous acidic dichromate conditions, retaining all six carbons. Conversion and yield depend on the experimental conditions.

Cyclohexanecarboxaldehyde → Cyclohexanecarboxylic acid (CrO₃/H₂SO₄)

Cyclohexanecarboxaldehyde reactant structure

Cyclohexanecarboxylic acid product structure

Jones reagent oxidises benzylic and aliphatic aldehydes alike; the illustrated conversion retains the cyclohexane ring while oxidizing the attached aldehyde carbon to CO₂H.

Scope & Limitations

Works best: Many aliphatic, benzylic and heteroaryl aldehydes; hydration and oxidation rates depend on the specific substrate.
Slower: Aryl aldehydes (hydrate fraction smaller) but still oxidise in Jones/dichromate solutions.
Not covered: Ordinary ketones under these mild conditions, and aldehydes in strictly anhydrous media where hydrate formation is 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. An E2-like chromate-ester elimination removes the former aldehydic H to form the acid. Overall chromium reduction reaches Cr(III) through additional redox steps. Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃ (Jones), and H₂CrO₄ all converge on this pathway; ordinary ketones generally resist these mild conditions.

Interactive Toolbox

Mechanism Solver — Explore supported reactions and compare the organic bond changes with the hydrate/chromate-ester model above.
Reaction Solver — Explore available oxidants; check the product against the examples and the need for aqueous acidic conditions above.
IUPAC Namer — Learn the naming ins and outs of aldehyde starting materials and carboxylic acid products.

Study Tools