Pyridinium chlorochromate
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2008-06-25T17:51:35Z
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/* Controversy */
{{Chembox new
| Name = Pyridinium chlorochromate
| ImageFile = Pyridinium_chlorochromate.PNG
| ImageSize = 150px
| ImageName = Chemical structure of the Pyridinium Chlorochromate
| ImageFileL1 = Pyridinium-3D-balls.png
| ImageNameR1 = Ball-and-stick model of the pyridinium cation
| ImageFileR1 = Chlorochromate-3D-balls.png
| ImageNameR1 = Ball-and-stick model of the chlorochromate anion
| IUPACName = Pyridinium chlorochromate
| OtherNames = PCC
| Section1 = {{Chembox Identifiers
| CASNo = 26299-14-9
}}
| Section2 = {{Chembox Properties
| Formula = C<sub>5</sub>H<sub>5</sub>NHClCrO<sub>3</sub>
| MolarMass = 215.56 g/mol
| Appearance = orange crystalline powder
| MeltingPt = 205 °C
| Solvent = other solvents
| SolubleOther = soluble in [[dichloromethane]],<br />[[benzene]], [[diethyl ether]],<br />[[acetone|sol acetone]], [[acetonitrile]],<br />[[THF]]
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS = [http://www.setonresourcecenter.com/dtSearch/dtisapi6.dll?cmd=getdoc&DocId=21959&Index=D%3a%5cINDEX%5cMSDS&HitCount=21&hits=1+2+4+5+11+12+64+65+1cd+214+215+306+307+3a7+3a8+41f+420+49b+49c+4dd+4de+&SearchForm=D%3a%5ccrc%5cmsdss%5ccomply1%2ehtm external MSDS sheet]
| Main [[Worker safety and health|hazard]]
| oxidizing, toxic, flammable<br />carcinogenic, irritant
| NFPA-H = 3 | NFPA-R = 3
| RPhrases = 49-8-43-50/53
| SPhrases = 53-45-60-61}}
}}
'''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]].<ref name = corey>{{cite journal | author = Corey, E.J.; Suggs, W. | title = Pyridinium Chlorochromate. An Efficient Reagent for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds | journal = [[Tetrahedron Lett.]] | year = 1975 | volume = 16 | pages = 2647–2650 | doi = 10.1016/S0040-4039(00)75204-X}}</ref>
[[Pyridinium dichromate]] is a similar [[oxidizing agent]], which has the advantage of being less acidic.
==Preparation==
The original preparation by Corey<ref name = corey/> involves adding one equivalent of [[pyridine]] to a solution of one equivalent of [[Chromic acid|chromium(VI) trioxide]] and concentrated [[hydrochloric acid]]:
::C<sub>5</sub>H<sub>5</sub>N + HCl + CrO<sub>3</sub> → [C<sub>5</sub>H<sub>5</sub>NH][CrO<sub>3</sub>Cl]
Agarwal et al. presented an alternative synthesis that avoids the harmful side product [[chromyl chloride]] (CrO<sub>2</sub>Cl<sub>2</sub>).<ref>{{cite journal | author = Agarwal, S; Tiwari, H. P.; Sharma, J. P. | title = Pyridinium Chlorochromate: an Improved Method for its Synthesis and use of Anhydrous acetic acid as catalyst for oxidation reactions | journal = [[Tetrahedron (journal)|Tetrahedron]] | year = 1990 | volume = 46 | pages = 4417–4420 | doi = 10.1016/S0040-4020(01)86776-4}}</ref>
Chromium(VI) oxide is treated with [[pyridine|pyridinium]] chloride:
::[C<sub>5</sub>H<sub>5</sub>NH<sup>+</sup>]Cl<sup>−</sup> + CrO<sub>3</sub> → [C<sub>5</sub>H<sub>5</sub>NH][CrO<sub>3</sub>Cl]
==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]].<ref>{{OrgSynth | author = Paquette, L. A.; Earle, M. J.; Smith, G. F. | title = (4R)-(+)-tert-Butyldimethylsiloxy-2-cyclopenten-1-one | collvol = 9 | collvolpages = 132 | year = 1998 | prep = cv9p0132}}</ref><ref>{{OrgSynth | author = Tu, Y.; Frohn, M.; Wang, Z.-X.; Shi, Y. | title = Synthesis of 1,2:4,5-Di-‘’O’’-Isopropylidene-D-erythro-2,3-hexodiulo-2,6-pyranose. A highly Enantioselective Ketone Catalyst for Epoxidation | volume = 80 | pages = 1 | year = 2003 | prep = v80p0001}}</ref><ref>{{OrgSynth | author = White, J. D.; Grether, U. M.; Lee, C.-S. | title = (R)-(+)-3,4-Dimethylcyclohex-2-en-1-one | volume = 82 | pages = 108 | year = 2005 | prep = v82p0108}}</ref> A sample reaction would be:<br />
:: C<sub>5</sub>H<sub>5</sub>NHCrO<sub>3</sub>Cl + R<sub>2</sub>CHOH → C<sub>5</sub>H<sub>5</sub>NHCl + H<sub>2</sub>CrO<sub>3</sub> + R<sub>2</sub>C=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<!-- a tertiary alcohol cannot be oxidised, this makes no sense; maybe they are talking about the allyl susbstituent --> [[allyl]] alcohols, unsaturated [[aldehyde]]s 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.<br />
Another notable oxidative reaction of PCC is its efficient conversion of unsaturated alcohols or aldehydes to [[cyclohexenone]]s. This particular pathway is known as oxidative [[cation]]ic cyclization.
==Controversy==
PCC is controversial as it contains [[hexavalent chromium|chromium(VI)]]. Other methods for oxidizing [[alcohols]] using less toxic reagents have been introduced and are favored by [[green chemistry|green chemists]]:
*[[Dimethyl sulfoxide|DMSO]]-based oxidations ([[Swern oxidation]], [[Moffatt oxidation]])
*[[hypervalent iodine]] based oxidation (such as the [[Dess-Martin periodinane]])
==References==
{{reflist}}
==Further reading==
* {{cite book | author = G. Tojo and M. Fernâandez | title = Oxidation of alcohols to aldehydes and ketones : a guide to current common practice | year = 2006 | publisher = Springer | city = New York | isbn = 0387236074}}
==External links==
* [http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
* [http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Oxidation%20of%20Alcohols/PCCDevelopment.html History of PCC]
* [http://www.chemicalland21.com/arokorhi/lifescience/phar/PYRIDINIUM%20DICHROMATE.htm Datasheet PDC]
* [http://www.npi.gov.au/database/substance-info/profiles/25.html National Pollutant Inventory - Chromium VI and compounds fact sheet]
[[Category:Chromates]]
[[Category:Oxidizing agents]]
[[Category:IARC Group 1 carcinogens]]
[[de:Pyridiniumchlorochromat]]
[[fr:Chlorochromate de pyridinium]]
[[ja:クロロクロム酸ピリジニウム]]
[[pt:Clorocromato de piridínio]]
[[zh:氯铬酸吡啶盐]]