Chromyl chloride

Chromyl chloride is a chemical compound with the formula CrO₂Cl₂. This compound is a red liquid at room temperature and pressure. It is tetrahedral, somewhat like SO₂Cl₂. CrO₂Cl₂ is similar to the most commonly encountered chromium(VI) derivative chromate, [CrO₄]²⁻; both are tetrahedral Cr(VI) compounds. They differ in physical properties, one is a liquid and the other forms salts. And they differ in oxidizing power as well.

CrO₂Cl₂ is a neutral molecular species. This means that in the liquid and solid forms, the individual CrO₂Cl₂ entities interact purely via van der Waals bonding. Such weak bonding leads to low melting and boiling points, which is related to the fact that it is a distillable liquid.

The diminished oxidizing power of [CrO₄]²⁻ vs. CrO₂Cl₂ can be ascribed to its anionic nature, which diminishes its electron affinity. Also, chloride is a poorer pi-donor ligand than is oxide.

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Preparation

CrO₂Cl₂ is prepared by the action of HCl on CrO₃. K₂CrO₄ is treated with concentrated HCl followed by the addition of H₂SO₄ as a dehydrating agent. The heavy CrO₂Cl₂ separates as an immiscible, dense liquid. It then can be separated using a separatory funnel or by simple distillation, which is normally performed.^[1] CrO₂Cl₂ can also be prepared using other chlorinating agents such as PCl₅ and TiCl₄.

Chemical properties

CrO₂Cl₂ is highly electrophilic and an aggressive oxidizing agent. Its electrophilicity is demonstrated by its rapid hydrolysis to give chromic and hydrochloric acids:

CrO₂Cl₂ + 2H₂O → H₂CrO₄ + 2HCl

Its high reactivity toward water is further indicated by the fact that CrO₂Cl₂ fumes in moist air.

Chromyl chloride test for chloride

The chromyl chloride test entails heating a sample suspected of containing chloride is with potassium dichromate and concentrated sulfuric acid. If chloride is present, chromyl chloride is formed and red fumes of CrO₂Cl₂ are evident. If there is no chloride present, no red fumes are produced. No analogous compounds are formed with fluorides, bromides, iodides and cyanides, so this test is therefore specific for chlorides. The test is related to the synthesis shown above, exposure of CrO₄²⁻ to HCl.

Reagent for oxidation of alkenes

Depending on solvent, CrO₂Cl₂ oxidizes terminal alkenes to aldehydes. Internal alkenes give alpha-chloroketones or related derivatives.^[2] It will also attack benzylic methyl groups to give aldehydes via what is called the Etard Reaction.

Compatible solvents

CrO₂Cl₂ is such an aggressive reagent that solvents must be chosen judiciously. In light of its high reactivity toward water, CrO₂Cl₂ can be expected to decompose upon exposure to alcohols, similar to the behavior of other highly electrophilic chlorides such as VOCl₃, TiCl₄, and SO₂Cl₂. Typical for other electrophilic chlorides, chlorocarbons are excellent solvents, especially dichloromethane

As a further practical complication, CrO₂Cl₂ attacks most greases.

Safety considerations

Cr(VI) is considered carcinogenic. HCl can be acutely lethal. CrO₂Cl₂ reacts with water to release both. Thus, CrO₂Cl₂ should be carefully handled in a well ventilated area. CrO₂Cl₂ is so aggressive that its storage can be problematic as it attacks most plastics as well as greases.

References

1. ^ Sisler, H. H. "Chromyl Chloride" Inorganic Synthesis McGraw-Hill: New York, 1946; Vol. 2, pp 205–207.
2. ^ F. Freeman, R. H. DuBois, T. G. McLaughlin. "Aldehydes by Oxidation of Terminal Olefins with Chromyl Chloride: 2,4,4-Trimethylpentanal". Org. Synth.; Coll. Vol. 6: 1028. 

• F. Freeman "Chromyl Chloride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.