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.

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. 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. 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.