Pyridinium chlorochromate

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Pyridinium chlorochromate
Chemical structure of the Pyridinium Chlorochromate
Ball-and-stick model of the chlorochromate anion
IUPAC name Pyridinium chlorochromate
Other names PCC
Identifiers
CAS number [26299-14-9]
Properties
Molecular formula C5H5NHClCrO3
Molar mass 215.56 g/mol
Appearance orange crystalline powder
Melting point

205 °C
Solubility in other solvents soluble in dichloromethane,
benzene, diethyl ether,
sol acetone, acetonitrile,
THF
Hazards
MSDS external MSDS sheet
NFPA 704
 
3
3
 
R-phrases 49-8-43-50/53
S-phrases 53-45-60-61
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or PCC, will not 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.[1]

Pyridinium dichromate is a similar oxidizing agent, which has the advantage of being less acidic.

Contents

[edit] Preparation

The original preparation by Corey[1] involves adding one equivalent of pyridine to a solution of one equivalent of chromium(VI) trioxide and concentrated hydrochloric acid:

C5H5N + HCl + CrO3 → [C5H5NH][CrO3Cl]

Agarwal et al. presented an alternative synthesis that avoids the harmful side product chromyl chloride (CrO2Cl2).[2] Chromium(VI) oxide is treated with pyridinium chloride:

[C5H5NH+]Cl− + CrO3 → [C5H5NH][CrO3Cl]

[edit] 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.[3][4][5] A sample reaction would be:

C5H5NHCrO3Cl + R2CHOH → C5H5NHCl + H2CrO3 + R2C=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.

[edit] 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:

[edit] References

  1. ^ a b Corey, E.J.; Suggs, W. (1975). "Pyridinium Chlorochromate. An Efficient Reagent for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds". Tetrahedron Lett. 16: 2647-2650. doi:10.1016/S0040-4039(00)75204-X. 
  2. ^ Agarwal, S; Tiwari, H. P.; Sharma, J. P. (1990). "Pyridinium Chlorochromate: an Improved Method for its Synthesis and use of Anhydrous acetic acid as catalyst for oxidation reactions". Tetrahedron 46: 4417-4420. doi:10.1016/S0040-4020(01)86776-4. 
  3. ^ Paquette, L. A.; Earle, M. J.; Smith, G. F. (1998). "(4R)-(+)-tert-Butyldimethylsiloxy-2-cyclopenten-1-one". Org. Synth.; Coll. Vol. 9: 132. 
  4. ^ Tu, Y.; Frohn, M.; Wang, Z.-X.; Shi, Y. (2003). "Synthesis of 1,2:4,5-Di-‘’O’’-Isopropylidene-D-erythro-2,3-hexodiulo-2,6-pyranose. A highly Enantioselective Ketone Catalyst for Epoxidation". Org. Synth. 80: 1. 
  5. ^ White, J. D.; Grether, U. M.; Lee, C.-S. (2005). "(R)-(+)-3,4-Dimethylcyclohex-2-en-1-one". Org. Synth. 82: 108. 

[edit] Further reading

  • G. Tojo and M. Fernâandez (2006). Oxidation of alcohols to aldehydes and ketones : a guide to current common practice. Springer. ISBN 0387236074. 

[edit] External links