Chromium compounds pyridinium chlorochromate

A better reagent for the limited oxidation of primary alcohols to aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HC1. PCC oxidizes most primary alcohols to aldehydes in excellent yields. Unlike most other oxidants, PCC is soluble in nonpolar solvents such as dichloromethane (CH2C12), which is an excellent solvent for most organic compounds. PCC can also serve as a mild reagent for oxidizing secondary alcohols to ketones. 

Dissolve 12 g of chromium trioxide in 22 mL of 6 N hydrochloric acid, and add 9.5 g of pyridine during a lO-min period while maintaining the temperature at 45 C. Cool the mixture to 0°C and collect the crystalline yellow-orange pyridinium chlorochromate on a sintered glass funnel and dry it for 1 h in vacuo. The compound is not hygroscopic and is stable at room temperature. 

Because the reagent is slightly acidic, it cannot be used to oxidize acid-sensitive compounds [605. For easier isolation of the products, the complex may be formed in the presence of alumina. After the reagent is stirred with the alcohol at room temperature for 2 h, the aldehyde is isolated by filtration and evaporation of the filtrate [604. Another solid-support oxidant of pyridinium chlorochromate is prepared by treatment of cross-linked poly(vinylpyridine) with chromium trioxide and hydrochloric acid (equation 212) [612]. 

Secondary alcohols are oxidized by H5IO6 in the presence of various chromium catalysts to ketones [1321-1325], while primary alcohols can be oxidized to aldehydes or to carboxylic acids depending on the catalyst. Primary alcohols in the presence of pyridinium chlorochromate (PCC)[1322] or chromium (III) acetylacetonate, Cr(acac)3 [1321] are oxidized to aldehydes or ketones in excellent yields, while the use of CrOj [1326,1327] or pyridinium fluorochromate [1323] as catalysts results in the oxidation to carboxylic acids. The periodic acid promoted oxidation of primary and secondary alcohols to carbonyl compounds can also be catalyzed by Cu(II) derivatives [1328,1329], by bromide anion [1330] and by TEMPO [1331]. 

One reason for the success of oxidation with pyridinium chlorochromate is that the oxidation can be carried out in a solvent such as CH2CI2, in which PCC is soluble. Aldehydes themselves are not nearly so easily oxidized as are the aldehyde hydrates, RCH(OH)2, that form (Section 16.7A) when aldehydes are dissolved in water, the usual medium for oxidation by chromium compounds ... 

When we come to study synthesis, we will find that oxidation of alcohols by chromium(VI) compounds is an important reaction. Primary and secondary alcohols can both be oxidized under appropriate conditions, and by controlling the conditions, we can oxidize primary alcohols either to aldehydes or carboxylic acids (Figure 10.42). Pyridinium chlorochromate (PCC) [pyHj+lCrOjCl] can be, and often is, used instead of the CrOj/pyridine combination. Because this reaction is an elimination, it is necessary that there is a hydrogen atom attached to the carbon bearing the OH group to be oxidized. Therefore, tertiary alcohols are not oxidized under these conditions. 

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