Collins reagent is the complex of chromium(VI) oxide with

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...

. This metal-pyridine complex, a red solid, is used to oxidize primary alcohols to the corresponding

aldehydes In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...

and

secondary alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...

to the corresponding

ketones In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...

. This complex is a

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

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 determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, 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

and

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 Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...

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 The Sarett oxidation is an organic reaction that oxidizes primary and secondary alcohols to aldehydes and ketones, respectively, using chromium trioxide and pyridine. Unlike the similar Jones oxidation, the Sarett oxidation will not further ox ...

Oxidation with chromium(VI)-amine complexes Oxidation with chromium(VI) complexes involves the conversion of alcohols to carbonyl compounds or more highly oxidized products through the action of molecular chromium(VI) oxides and salts. The principal reagents are Collins reagent, PDC, and PCC. ...

Collins reagent can be used as an alternative to the

Jones reagent Jones may refer to: People *Jones (surname), a common Welsh and English surname *List of people with surname Jones *Jones (singer), a British singer-songwriter Arts and entertainment *Jones (Animal Farm), Jones (''Animal Farm''), a human charact ...

and

pyridinium chlorochromate Pyridinium chlorochromate (PCC) is a yellow-orange salt (chemistry), salt with the chemical formula, formula 5H5NH rO3Cl��. It is a reagent in organic synthesis used primarily for organic redox reaction, oxidation of Alcohol (chemistry), al ...

(PCC) when oxidizing secondary alcohols to

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

s. PCC and

pyridinium dichromate The pyridinium dichromate (PDC) or Cornforth reagent is a pyridinium salt of dichromate with the chemical formula 5H5NHsub>2 r2O7 This compound is named after the Australian-British chemist Sir John Warcup Cornforth (b. 1917) who introduced it ...

(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