Everything about Pyridinium Chlorochromate totally explained
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Pyridinium chlorochromate is a reddish orange solid reagent used to
oxidize primary
alcohols to
aldehydes and secondary
alcohols to
ketones. Pyridinium chlorochromate, or
PCC, won't fully oxidize the alcohol to the
carboxylic acid as does the
Jones reagent. A disadvantage to using PCC is its toxicity. PCC was developed by
Elias James Corey and William Suggs in
1975.
Pyridinium dichromate is a similar
oxidizing agent, which has the advantage of being less acidic.
Preparation
The original preparation by Corey
Chromium(VI) oxide is treated with
pyridinium chloride:
» :[C
5H
5NH
+]Cl
− + CrO
3 → [C
5H
5NH][CrO
3Cl]
Properties and uses
PCC is primarily used as an
oxidant. In particular, it has proven to be highly effective in oxidizing primary and secondary
alcohols to
aldehydes and
ketones, respectively. Rarely does over-oxidation occur (whether intentionally or accidentally) to form
carboxylic acids. A typical PCC oxidation involves addition of the alcohol to a suspension of PCC in
dichloromethane. A sample reaction would be:
» : C
5H
5NHCrO
3Cl + R
2CHOH → C
5H
5NHCl + H
2CrO
3 + R
2C=O
In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate matters. Addition of an inert adsorbent such as crushed molecular sieves or silica gel allows the sticky byproduct to adsorb to the surface, and makes workup easier.
PCC is also remarkable for its high selectivity. For example, when oxidizing tertiary
allyl alcohols, unsaturated
aldehydes are observed as the sole product. This reaction is known as the Babler oxidation. Otherwise such oxidations commonly afford dienes as by-products resulting from dehydration.
Another notable oxidative reaction of PCC is its efficient conversion of unsaturated alcohols or aldehydes to
cyclohexenones. This particular pathway is known as oxidative
cationic cyclization.
Controversy
PCC is controversial as it contains
chromium(VI). More environmentally friendly
oxidants listed above are therefore favored by
green chemists. Other methods for oxidizing
alcohols using less toxic reagents have been introduced:
Further Information
Get more info on 'Pyridinium Chlorochromate'.
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