Chromyl chloride

Chromyl chloride is an inorganic compound with the formula CrO₂Cl₂ and is the formal dichloride of chromic acid. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal compounds.^[3]

Chromyl chloride

Wireframe model of chromyl chloride Ball and stick model of chromyl chloride
Chromyl chloride in vial
Names
Preferred IUPAC name Chromium(VI) dichloride dioxide
Systematic IUPAC name Dichloridodioxidochromium
Other names Chromic acid chloride Chromium oxychloride Chromic oxychloride Étard reagent Chlorochromic anhydride Chromium chloride oxide Chromium dioxide dichloride Chromium dioxychloride
Identifiers
CAS Number	14977-61-8 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:33038 Y
ChemSpider	21106426 Y
ECHA InfoCard	100.035.491
EC Number	239-056-8
Gmelin Reference	2231
PubChem CID	22150757
RTECS number	GB5775000
UNII	JQU316FZ5W Y
UN number	1758
CompTox Dashboard (EPA)	DTXSID30894858
InChI InChI=1S/2ClH.Cr.2O/h2*1H;;;/q;;+2;;/p-2 Y Key: AHXGRMIPHCAXFP-UHFFFAOYSA-L Y InChI=1S/2ClH.Cr.2O/h2*1H;;;/q;;+2;;/p-2 Key: AHXGRMIPHCAXFP-UHFFFAOYSA-L InChI=1/2ClH.Cr.2O/h2*1H;;;/q;;+2;;/p-2/rCl2CrO2/c1-3(2,4)5 Key: AHXGRMIPHCAXFP-GRYJOLFGAD
SMILES Cl[Cr](Cl)(=O)=O
Properties
Chemical formula	CrO2Cl2
Molar mass	154.9008 g/mol
Appearance	Blood-red fuming liquid, similar to bromine
Odor	Musty, burning, acrid[1]
Density	1.911 g/mL, liquid
Melting point	−96.5 °C (−141.7 °F; 176.7 K)
Boiling point	118.5 °C (245.3 °F; 391.6 K)
Solubility in water	Reacts with water
Vapor pressure	20 mmHg (20 °C)[1]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Toxic, oxidizer, carcinogenic, mutagenic, reacts violently with water[1]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H271, H314, H317, H340, H350, H410
Precautionary statements	P201, P210, P280, P303+P361+P353, P305+P351+P338+P310, P308+P313[2]
NFPA 704 (fire diamond)	3 0 2 W OX
Flash point	noncombustible[1]
NIOSH (US health exposure limits):
PEL (Permissible)	none[1]
REL (Recommended)	Ca TWA 0.001 mg Cr(VI)/m3[1]
IDLH (Immediate danger)	N.D.[1]
Safety data sheet (SDS)	Sigma Aldrich - Chromyl Chloride
Related compounds
Related compounds	Sulfuryl chloride Vanadium oxytrichloride Molybdenum dichloride dioxide Tungsten dichloride dioxide Chromyl fluoride Uranyl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Preparation

Chromyl chloride can be prepared by the reaction of potassium chromate or potassium dichromate with hydrogen chloride in the presence of concentrated sulfuric acid, followed by distillation.^[4][5]

K₂Cr₂O₇ + 6 HCl → 2 CrO₂Cl₂ + 2 KCl + 3 H₂O

The sulfuric acid serves as a dehydration agent. This reaction, the chloride test, gives a strikingly red product. It was once used as a test for chloride. No similar compound is formed in the presence of fluoride, bromide, iodide, or cyanide, making this test specific to chlorides.

It can also be prepared directly by exposing chromium trioxide to anhydrous hydrogen chloride gas.

CrO₃ + 2 HCl ⇌ CrO₂Cl₂ + H₂O

When trityl chloride is used in place of HCl, the products are CrO₂Cl₂ and the alkoxides CrO₂Cl(OCPh₃) and CrO₂(OCPh₃)₂ (Ph = C₆H₅).^[6]

Reactions

Chromyl chloride is a Lewis acid, and a highly aggressive oxidant. It reacts with cyclohexane to give chlorocyclohexane, chlorocyclohexanone, and other species.^[7]

Chromyl chloride oxidizes internal alkenes to alpha-chloroketones or related derivatives.^[8] It will also attack benzylic methyl groups to give aldehydes via the Étard reaction. Dichloromethane is a suitable solvent for these reactions.^[9]

When treated with conventional Lewis bases, chromyl chloride forms adducts, such as CrO₂Cl₂(2,2'-bipy):^[10]

CrO₂Cl₂ + 2 L → CrO₂Cl₂L₂

Safety considerations

Main article: Hexavalent chromium

Chromyl chloride is often stored in sealed glass ampoules to prevent vapours leaking out of the container

Chromyl chloride is severely corrosive and easily burns the skin and eyes. It is a probable human carcinogen.^[11]