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Collins reagent is the complex of chromium(VI) oxide with pyridine in dichloromethane. It is used to selectively oxidize primary alcohols to the aldehyde, and will tolerate many other functional groups within the molecule.

It can be used as an alternative to the Jones reagent and pyridinium chlorochromate (PCC) when oxidizing secondary alcohols to ketones. Moreover, the Collins reagent is especially useful for oxidations of acid sensitive compounds.

This complex is both difficult and dangerous to prepare, as it is very hygroscopic and can inflame during preparation. It is typically used in a sixfold excess in order to complete the reaction. Nowadays, PCC or PDC oxidation have largely supplanted Collins oxidation for these very reasons.

History
In 1948 several scientists H.H Sisler, J.D. Bush and O. E. Accountius noted an isolation in the compound with the empirical composition CrO8•2C5H5N, a brick-red complex, from a reaction of anhydrous chromium trioxide with pyridine. G. I. Poos, G. E. Arth, R. E. Beyler and L.H. Sarett in 1953 found the complex in pyridine solution to be an effective reagent for the oxidation of primary and secondary alcohols to aldehydes and ketones. In 1968 J. C. Collins, W. W. Hess, and F. J. Frank found the anhydrous complex is moderately soluble in polar chlorocarbons. They found the solvent of choice was methylene chloride with the solubility of 12.5 g/100 mL. Under those conditions yields of 87-98% of primary and secondary alcohols were oxidized to aldehydes and ketones. .

Collins reagent is derived from Sarett Reagent (CrO3•2 PY). Sarett reagent allows the oxidation of various primary alcohols to aldehydes due to the non-aqueous conditions. The complex is highly hygroscopic. The preparation of this reagent is not without risk because the solvent can catch fire during preparatation. Since pyridine is used as the solvent, the oxidation of base-sensitive substrates is not permitted with this reagent. Collins Reagent is made when the Sarett reagent is diluted in dichloromethane. This is a more convenient oxidation reagent that can be more easily prepared and allows the oxidation of a broader substrate scope. .

Properties
Molecular formula: CrO3 2C5H5N (CrO3 2 Py)

Molecular weight: 179.09 g/mol

Collins reagent is a dark red crystal. It easily absorbs moisture. It is soluble in dichloromethane. .

Mechanism
The mechanism for forming Collins reagent is as follows: (2)  (3)  Collins reagent will follow path B (4)

Synthesis
To make Collins reagent, one equivalent of chromium trioxide is added to a stirred solution of two equivalents of pyridine in methylene chloride. This allows for a safe and convenient preparation of the reagent. In addition, the use of methylene chloride as solvent and stoichiometric amounts of pyridine makes the Collins Reagent less basic than the Sarett Reagent. Thus, most acid and base-sensitive substrates can be oxidized with Collins Reagent. .

This reagent is made via Jones oxidation. This oxidation is rapid, exothermic, and typically results in high yields. This mechanism begins by the formation of a mixed ester.
 * CrO3(OH)– + RCH2OH → CrO3(OCH2R)– + H2O

The chromate ester formation is accelerated by the presence of the acid. These esters can be isolated when the alcohol lacks α-C-H bonds. The chromate esters degrade, releasing the carbonyl product and a Cr(IV) product:
 * CrO3(OCH2R)– → "CrO2OH–" + O=CHR

Uses
The oxidant is especially useful for the oxidation of primary alcohols to aldehydes where traces of water can lead to overoxidation. . This reagent can be used as an alternative to the Jones reagent and pyridinium chlorochromate (PCC) when oxidizing secondary alcohols to ketones. Moreover, the Collins reagent is especially useful for oxidations of acid sensitive compounds.

Chromium (VI)
Chromium (VI) is a known carcinogen. The genotoxicity of chromium (VI) has been studied in depth. There are currently three mechanisms that have been proposed to describe the genotoxicity. The first mechanism highly reactive hydroxyl radicals and other reactive radicals. These radicals are the by-products of the reduction of chromium (VI) to chromium (III). The second mechanism consists of the binding of chromium (V) to DNA. The third mechanism consists of the binding of chromium (III) to DNA.

The amount of chromium (VI) necessary to be considered toxic when ingested orally is between 50 and 150 µg/kg. The proposed beneficial effects of chromium(III) and the use as dietary supplements yielded some controversial results, but recent reviews suggest that moderate uptake of chromium(III) through dietary supplements poses no risk. When chromium (VI) is ingested, it is almost immediately reduced to chromium (III) in the blood before it enters cells. Chromium (III) is excreted from the body. Chromium (VI) is highly toxic because of its strong oxidation properties. Chromium (VI) can cause damage to the kidneys, liver, and blood cells through oxidation reactions. This can cause problems like renal and liver failure. Aggressive dialysis is one way to remedy renal and liver failure.