Collins reagent is the complex of
chromium(VI) oxide with
pyridine in
dichloromethane.
This
metal-pyridine complex, a red solid, is used to
oxidize primary alcohols to the corresponding
aldehydes and
secondary alcohols to the corresponding
ketones.
This complex is a
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...
orange solid.
[
]
Synthesis and structure
The complex is produced by treating chromium trioxide with pyridine.[ The complex is diamagnetic. According to ]X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
, the complex is 5-coordinate with mutually trans pyridine ligands. The Cr-O and Cr-N distances are respectively 163 and 215 picometers.
In terms of history, the complex was first produced by Sisler et al.
Reactions
Collins reagent is especially useful for oxidations of acid sensitive compounds. Primary and secondary alcohols are oxidized respectively to aldehydes and ketones in yields of 87-98%.
Like other oxidations by Cr(VI), the stoichiometry of the oxidations is complex because the metal undergoes 3e reduction and the substrate is oxidized by 2 electrons:
:3 RCH2OH + 2 CrO3(pyridine)2 → 3 RCHO + 3 H2O + Cr2O3 + 4 pyridine
The reagent is typically used in a sixfold excess. Methylene chloride is the typical solvent, with the solubility of 12.5 g/100 ml.
The application of this reagent to oxidations was discovered by G. I. Poos, G. E. Arth, R. E. Beyler and L.H. Sarett in 1953. It was popularized by J. C. Collins several years later.
Other reagents
* Sarett oxidation
* Oxidation with chromium(VI)-amine complexes
Collins reagent can be used as an alternative to the Jones reagent and pyridinium chlorochromate (PCC) when oxidizing secondary alcohols to ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
s. PCC and pyridinium dichromate (PDC) oxidations have largely supplanted Collins oxidation.[
]
Safety and environmental aspects
The solid is flammable.[ Generally speaking chromium (VI) compounds are carcinogenic.
]
References
{{Organic reactions
Oxidizing agents
Chromium(VI) compounds