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Pyridinium chlorochromate

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Pyridinium chlorochromate
Chemical structure of the Pyridinium Chlorochromate
Ball-and-stick model of the chlorochromate anion
Ball-and-stick model of the chlorochromate anion
Names
IUPAC name Pyridinium chlorochromate
Other names PCC
Identifiers
CAS Number
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.043.253 Edit this at Wikidata
CompTox Dashboard (EPA)
InChI
  • InChI=1S/C5H5N.ClH.Cr.3O/c1-2-4-6-5-3-1;;;;;/h1-5H;1H;;;;/q;;+1;;;-1Key: LEHBURLTIWGHEM-UHFFFAOYSA-N
  • InChI=1/C5H5N.ClH.Cr.3O/c1-2-4-6-5-3-1;;;;;/h1-5H;1H;;;;/q;;+1;;;-1/rC5H5N.ClCrO3/c1-2-4-6-5-3-1;1-2(3,4)5/h1-5H;/q;-1/p+1Key: LEHBURLTIWGHEM-YOEUSAHMAN
SMILES
  • O=(=O)()Cl.1ccccc1
Properties
Chemical formula C5H5NHClCrO3
Molar mass 215.56 g/mol
Appearance orange crystalline powder
Melting point 205 °C (401 °F; 478 K)
Solubility in other solvents soluble in dichloromethane,
benzene, diethyl ether,
acetone, acetonitrile,
THF
Hazards
Occupational safety and health (OHS/OSH):
Main hazards oxidizing, toxic, flammable
carcinogenic, irritant
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability (red): no hazard codeInstability 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g. hydrogen peroxideSpecial hazards (white): no code
3 3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). checkY verify (what is  ?) Infobox references
Chemical compound

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 a primary 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 involves adding one equivalent of pyridine to a solution of one equivalent of chromic acid and concentrated hydrochloric acid:

C5H5N + HCl + CrO3 →

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

Cl + CrO3 →

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:

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.

In addition to simple oxidations of hydroxyl groups, rearrangements are possible. For example, tertiary alcohols cannot be oxidized directly. However, in the Babler oxidation, the chromate ester formed with PCC and an allylic tertiary alcohol can isomerize via a -sigmatropic reaction before the carbonyl-forming oxidation step. Other common oxidants usually lead to simple dehydration rather than any oxidation reaction at tertiary hydroxyl centers.

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), a known carcinogen. Other methods for oxidizing alcohols using less toxic reagents have been introduced and are favored by green chemists:

References

  1. ^ Corey, E.J.; Suggs, W. (1975). "Pyridinium Chlorochromate. An Efficient Reagent for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds". Tetrahedron Lett. 16 (31): 2647–2650. doi:10.1016/S0040-4039(00)75204-X.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  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 (12): 4417–4420. doi:10.1016/S0040-4020(01)86776-4.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. Paquette, L. A.; Earle, M. J.; Smith, G. F. (1998). "(4R)-(+)-tert-Butyldimethylsiloxy-2-cyclopenten-1-one". Organic Syntheses{{cite journal}}: CS1 maint: multiple names: authors list (link); Collected Volumes, vol. 9, p. 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". Organic Syntheses. 80: 1{{cite journal}}: CS1 maint: multiple names: authors list (link).
  5. White, J. D.; Grether, U. M.; Lee, C.-S. (2005). "(R)-(+)-3,4-Dimethylcyclohex-2-en-1-one". Organic Syntheses. 82: 108{{cite journal}}: CS1 maint: multiple names: authors list (link).

Further reading

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

External links

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