Chromate ester oxidation
Jones oxidation
How the Jones oxidation works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: CrO₃, H₂SO₄, acetone (Jones reagent: H₂CrO₄). Conditions: Acetone, 0–25 °C, strongly acidic. Bonds formed: C=O; Cr(IV) → Cr(III); bonds broken: O–H, α C–H, Cr–O. Stereochemistry: Retention of configuration at neighbouring centres; carbinol centre destroyed. Rate and selectivity: Chromate ester formation fast; C–H cleavage rate-limiting (large kH/kD).
At a glance
Step 1: Chromate ester forms
An alcohol's oxygen attacks chromium(VI) (as chromic acid, H₂CrO₄), displacing water and forming a chromate ester — a new O–Cr bond that activates the alpha C–H for the next step.
Step 2: E2-like C–H cleavage
A base (often another equivalent of water or chromate) removes the alpha C–H in a concerted, E2-like elimination, with the electrons flowing directly into the Cr=O bond as chromium is reduced from +6 to +4.
Step 3: Ketone; over-oxidation of 1° alcohols
This delivers the carbonyl directly; a secondary alcohol stops cleanly at the ketone, but a primary alcohol's resulting aldehyde is itself hydrated and oxidized further by a second equivalent of chromium, driving all the way to the carboxylic acid — this over-oxidation of primary alcohols is exactly why the milder, chromium-free oxidants developed later (Swern, Dess-Martin) became preferred when stopping at the aldehyde matters.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- E2
- Bimolecular elimination. Concerted base removal of β-H and loss of the leaving group (anti-periplanar).
How it compares with related reactions
| Jones oxidation | Collins (Sarett) oxidation | Baeyer-Villiger oxidation of a ketone | |
|---|---|---|---|
| Reagents | CrO₃, H₂SO₄, acetone (Jones reagent: H₂CrO₄) | CrO₃·2 pyridine (Collins/Sarett reagent) | mCPBA or CH₃CO₃H (peracid) |
| Conditions | Acetone, 0–25 °C, strongly acidic | Anhydrous CH₂Cl₂, 25 °C | CH₂Cl₂, 25 °C; cat. acid helps |
| Stereochemistry | Retention of configuration at neighbouring centres; carbinol centre destroyed | Neighbouring stereocentres retained | Retention at the migrating carbon |
| Rate / selectivity | Chromate ester formation fast; C–H cleavage rate-limiting (large kH/kD) | Alpha C–H cleavage rate-limiting; use 6 equiv reagent | Migratory aptitude: tert > cyclohexyl ≈ sec > Ph > prim > Me |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Jones oxidation?
CrO₃, H₂SO₄, acetone (Jones reagent: H₂CrO₄). Conditions: Acetone, 0–25 °C, strongly acidic.
Which bonds are formed and which are broken?
Formed: C=O; Cr(IV) → Cr(III). Broken: O–H, α C–H, Cr–O.
What is the stereochemical outcome?
Retention of configuration at neighbouring centres; carbinol centre destroyed
What controls the rate or the selectivity?
Chromate ester formation fast; C–H cleavage rate-limiting (large kH/kD)
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go