Mild oxidation with PCC: practice and mechanism
PCC oxidises mildly and without water: primary alcohol → aldehyde (no further), secondary alcohol → ketone.
Example
PCC is a mild, water-free oxidant: primary alcohols stop at the aldehyde (not the carboxylic acid), secondary alcohols give ketones. Tertiary alcohols do not react.
Mechanism step by step
- 1
The alcohol forms a chromate ester with chromium(VI).
- 2
A base removes the H on the carbinol carbon, chromium is reduced and the C=O bond forms (E2-like).
- 3
Without water the aldehyde cannot form a hydrate, so it is not oxidised further.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Strong oxidants in water (e.g. K₂Cr₂O₇/H₂SO₄) go all the way to the acid. Water-free PCC stops at the aldehyde.
Often confused with: Strong oxidation (chromic acid) · Acid-catalysed dehydration (E1)
Frequently asked questions
Why don't tertiary alcohols react?
There is no hydrogen on the carbinol carbon that could be removed in the oxidation.
Are there chromium-free alternatives?
Yes, e.g. the Swern oxidation or Dess–Martin periodinane, both also stopping at the aldehyde.