Pyridinium chlorochromate
| Pyridinium chlorochromate | |
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Pyridinium chlorochromate |
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Other names
PCC |
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| Identifiers | |
| CAS number | 26299-14-9 |
| ChemSpider | 10608386  |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C5H5NHClCrO3 |
| Molar mass | 215.56 g/mol |
| Appearance | orange crystalline powder |
| Melting point |
205 °C, 478 K, 401 °F |
| Solubility in other solvents | soluble in dichloromethane, benzene, diethyl ether, acetone, acetonitrile, THF |
| Hazards | |
| MSDS | external MSDS sheet |
| R-phrases | R49, R8, R43, R50/53 |
| S-phrases | S53, S45, S60, S61 |
| Main hazards | oxidizing, toxic, flammable carcinogenic, irritant |
| NFPA 704 |
 3
3
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 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox 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 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.[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 chromic acid 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.
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 [3,3]-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.[clarification needed]
[edit] 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:
- DMSO-based oxidations (Swern oxidation, Moffatt oxidation)
- hypervalent iodine based oxidation (such as the Dess-Martin periodinane)
[edit] References
- ^ 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 (31): 2647–2650. doi:10.1016/S0040-4039(00)75204-X.
- ^ 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.
- ^ Paquette, L. A.; Earle, M. J.; Smith, G. F. (1998), "(4R)-(+)-tert-Butyldimethylsiloxy-2-cyclopenten-1-one", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv9p0132; Coll. Vol. 9: 132
- ^ 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, http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=v80p0001
- ^ White, J. D.; Grether, U. M.; Lee, C.-S. (2005), "(R)-(+)-3,4-Dimethylcyclohex-2-en-1-one", Org. Synth. 82: 108, http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=v82p0108
[edit] 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.
