Strong oxidation (chromic acid): practice and mechanism
Chromic acid in water oxidises primary alcohols all the way to the carboxylic acid, secondary alcohols to the ketone.
Example
In aqueous acid, a primary alcohol is oxidised via the aldehyde (which reacts on as its hydrate) all the way to the carboxylic acid. Secondary alcohols give ketones, which are not oxidised further.
Mechanism step by step
- 1
As with PCC: chromate ester, then loss of the carbinol H; the aldehyde forms first.
- 2
In water the aldehyde adds water to give the hydrate R–CH(OH)₂.
- 3
The hydrate is again an "alcohol" and is oxidised to the carboxylic acid; Cr(VI) (orange) becomes Cr(III) (green).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Only water-free PCC stops at the aldehyde. In aqueous chromic acid the aldehyde forms a hydrate and is oxidised further to the acid.
- A secondary alcohol cannot become a carboxylic acid: that would require breaking a C–C bond. It is oxidised to the ketone.
- That would be an ester (acetate). Chromic acid is an oxidant and does not transfer an acyl group.
Often confused with: Mild oxidation with PCC · Acid-catalysed dehydration (E1)
Frequently asked questions
How can you see the reaction happening?
By the colour change from orange (dichromate) to green (Cr³⁺); once the basis of the breathalyser.
Why aren't ketones oxidised further?
That would require breaking a C–C bond; there is no H left on the carbonyl carbon.