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Sarrett identified the adduct of pyridine and chromium(VI) oxide ([[Collins reagent]]) as a selective compound for the oxidation of primary and secondary alcohols to carbonyl compounds.<ref>Poos, G. I.; Arth, G. E.; Beyler, R. E.; Sarrett, L. H. ''J. Am. Chem. Soc.'', '''1953''', ''75'', 422.</ref> Despite its selectivity, Collins reagent suffers from difficulties associated with its preparation, stability, and efficiency. The less reactive adducts pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC) are more easily handled and more selective than Collins reagent in oxidations of alcohols. These reagents, as well as other, more exotic adducts of nitrogen heterocycles with chromium(VI), facilitate a number of oxidative transformations of organic compounds, including cyclization to form [[tetrahydrofuran]] derivatives and allylic transposition to afford enones from [[allyl]]ic alcohols.
Oxidation with chromium(VI) amines has two primary limitations. Operationally, the tarry byproducts of chromium oxidations cause reduced yields and product sequestration.<ref>Ratcliffe, R.; Rodehorst, R. ''J. Org. Chem.'', '''1970''', ''35'', 4000.</ref> In addition, Cr(VI)-amines (particularly PCC) may react with [[acid]]-labile functionality. Thus, these agents have been employed in oxidations of relatively simple substrates, often in excess to account for reagent trapping and decomposition. The use of adsorbents such as Celite® or silica gel facilitates the removal of chromium byproducts and eliminates many of the operational difficulties associated with chromium-mediated oxidations.
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(1)'''''</span><center>[[File:ChroGen.png]]</center>
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==Experimental Conditions and Procedure==
===Typical Conditions===
Reagent-grade pyridine is usually sufficient for the preparation of PDC and PCC. Although these reagents may darken over time, their loss in activity is minimal. Isolated reagents should be stored in a desiccator in the dark. Care should be taken when adding chromium trioxide to pyridine, as ignition of pyridine has been known to occur. Celite® or silica gel may be used to facilitate the removal of polymeric chromium byproducts from the reaction mixture; these adsorbents also serve as convenient buffer systems and dessicants.
Reduced chromium residues can be removed from glassware with concentrated HCl or 10-15% aqueous HF. Solid chromium waste should never be thrown away, as residual CrO<sub>3</sub> may ignite. Chromium(VI) reagents are toxic and should be handled with care in a well-ventilated fume hood.
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