Oxidation Methods Compared: Choosing the Right Reagent
The commonest exam error in oxidation is choosing a reagent that goes one step too far.
The alcohol oxidation problem
| Substrate | Mild, anhydrous conditions | Strong, aqueous conditions |
|---|---|---|
| Primary alcohol | Aldehyde | Carboxylic acid |
| Secondary alcohol | Ketone | Ketone |
| Tertiary alcohol | No reaction | No reaction under normal conditions |
Tertiary alcohols resist oxidation because the carbon bearing the hydroxyl has no hydrogen. Oxidation at that carbon would require breaking a carbon–carbon bond, which needs far harsher conditions.
The main reagents
| Reagent type | Conditions | Primary alcohol gives | Notes |
|---|---|---|---|
| Chromium(VI) in anhydrous medium | Mild, non-aqueous | Aldehyde | Stops cleanly; tolerates alkenes |
| Chromium(VI) in aqueous acid | Strong | Carboxylic acid | Good for secondary alcohols to ketones too |
| Permanganate | Strong, often basic then acidified | Carboxylic acid | Also cleaves alkenes; not selective |
| Activated dimethyl sulphoxide | Very mild, low temperature | Aldehyde | Excellent functional group tolerance |
| Manganese dioxide | Mild, selective | Aldehyde, only if allylic or benzylic | Leaves ordinary alcohols untouched |
The last row is worth noting: its selectivity for allylic and benzylic alcohols means it can oxidise one hydroxyl in a molecule containing several, which is a genuinely useful piece of chemoselectivity.
Oxidations of other functional groups
Alkene cleavage
Ozonolysis cleaves a double bond to give two carbonyl fragments, and the workup determines which. A reductive workup gives aldehydes and ketones; an oxidative workup takes any aldehyde on to the acid.
Because the fragments reveal where the double bond was, ozonolysis is a structure-determination tool as much as a synthetic one. Hot concentrated permanganate cleaves alkenes similarly but with less control.
Epoxidation
A peroxy acid converts an alkene to an epoxide, retaining alkene stereochemistry — a cis alkene gives a cis epoxide. The epoxide is then a versatile intermediate, opening with nucleophiles at either carbon depending on conditions.
Dihydroxylation
Osmium tetroxide adds two hydroxyls across a double bond on the same face, giving the syn diol. Cold dilute permanganate does the same less cleanly. The syn stereochemistry follows from the cyclic intermediate, and it contrasts with the anti addition seen when an epoxide is opened with water.
Being asked to obtain a specific diol stereochemistry and to choose between these routes is a standard question.
A decision procedure
- Identify the functional group and the oxidation level wanted.
- For a primary alcohol, decide aldehyde or acid, and choose anhydrous or aqueous accordingly.
- Check what else is in the molecule — an alkene rules out reagents that cleave or dihydroxylate.
- Where selectivity between similar groups is needed, use a reagent whose scope is restricted.
- State the workup where it changes the product, as with ozonolysis.
Frequently asked questions
Why does an anhydrous reagent stop at the aldehyde?
Because further oxidation proceeds through the hydrate, which cannot form without water. No hydrate means no route onward.
Why are tertiary alcohols not oxidised?
Because the carbinol carbon bears no hydrogen, so oxidation there would require breaking a carbon–carbon bond, which needs much harsher conditions.
How does the ozonolysis workup change the product?
A reductive workup preserves aldehydes; an oxidative workup converts them to carboxylic acids. Ketone fragments are unaffected either way.
How do I get an anti diol rather than a syn one?
By epoxidising the alkene and then opening the epoxide with water under acid catalysis, which proceeds with inversion at one carbon and gives the anti product.
Preparing for a chemistry entrance exam?
ABC Chemistry runs focused IIT-JAM, CSIR-NET, GATE and CUET-PG Chemistry coaching at our centre and through live online classes for students across India.
Call / WhatsApp: +91 92121 42427