Alcohol Oxidation with Chromic Acid Reagents (Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃, H₂CrO₄ in Acidic Aqueous Media)

Introduction

In acidic water (or acetone/water, “Jones conditions”), Na₂Cr₂O₇, K₂Cr₂O₇, CrO₃ (with acid), and H₂CrO₄ all generate the same active oxidant: 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. Tertiary alcohols do not react (no α-hydrogen). 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 nonaqueous Cr(VI) variants (Collins, PDC) 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; tertiary alcohols → no reaction (may dehydrate under strong acid, but no oxidation); aldehydes (if present) → carboxylic acids.
• Mechanism class: chromate ester formation → β-hydride (E2-like) elimination to carbonyl; subsequent hydrate oxidation for primary alcohols. Closed-shell; no radicals.
• 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 — β‑hydride elimination (E2-like)

Step C‑S3 — Bisulfate-mediated deprotonation

Step C‑S4 — Ketone released

Primary alcohol → carboxylic acid (7 frames)

Step C‑P1 — Chromate-ester formation

Step C‑P2 — First β‑hydride elimination → aldehyde

Step C‑P3 — Hydration to the oxonium gem-diol

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

Step C‑P5 — Second β‑elimination → protonated carboxylic acid

Step C‑P6 — Bisulfate-mediated deprotonation

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; tertiary → no oxidation (no α-H).
• Aldehydes oxidise to acids in aqueous chromate; aldehyde isolation requires nonaqueous protocols (PCC/DMP, Swern, Collins).
• Key intermediate: chromate ester; E2-like β-hydride elimination gives C=O.
• Primary path requires water: aldehyde → hydrate → re-oxidation → acid.
• No rearrangements: closed-shell ionic steps; no carbocation intermediates.
• Spectator functionality: alkenes generally survive; acid-labile groups (acetals) can be hydrolysed in the medium.

Worked Examples

Na₂Cr₂O₇ / H₂SO₄ ⟶

Na₂Cr₂O₇/H₂SO₄

Isobutanol → isobutyric acid (aldehyde hydrate never isolated).

K₂Cr₂O₇ / H₂SO₄ ⟶

K₂Cr₂O₇/H₂SO₄

Cyclohexanol → cyclohexanone (stops after bisulfate work-up).

H₂CrO₄ (Jones) ⟶

H₂CrO₄ (Jones)

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

CrO₃ / H₂O ⟶

CrO₃/H₂O

tert-Butyl alcohol → no oxidation (lacks an α-H; dehydration is the only acid-side reaction).

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: chromate does not perform glycol cleavage (that’s periodate or lead tetraacetate). It will oxidise secondary alcohol sites in a diol to ketones.
• 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 are particularly fast; tertiary alcohols do not oxidise (no α-H).

Practical Tips

• Jones reagent: CrO₃ in H₂SO₄/acetone is convenient; run cold-to-rt, then quench with isopropanol before workup.
• Stoichiometry: Use excess oxidant for primary alcohols to ensure full conversion to acid (invariably happens under standard Jones/dichromate conditions).
• 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) operate through chromate-ester formation and β-hydride elimination. In aqueous acid, primary alcohols → carboxylic acids (via aldehyde hydrate), secondary → ketones, tertiary → no reaction. Aldehydes do not survive; they are oxidised to acids.

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