Chromyl chloride

Chromyl chloride
Preferred IUPAC name Chromium(VI) dichloride dioxide
Systematic name Dichlorodioxochromium
Other names Chromic acid chloride Chromium oxychloride Etard Reagent
Identifiers
CAS number	14977-61-8
PubChem	22150757 Y
ChemSpider	21106426 Y
EC number	239-056-8
ChEBI	CHEBI:33038 Y
RTECS number	GB5775000
Jmol-3D images	Image 1
SMILES Cl[Cr](Cl)(=O)=O
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
Properties
Molecular formula	CrO2Cl2
Molar mass	154.9008 g/mol
Appearance	deep red fuming liquid
Density	1.911 g/mL, liquid
Melting point	-96.5 °C
Boiling point	117 °C
Solubility in water	Decomposes
Hazards
R-phrases	R49 R46 R8 R35 R43 R50/53
S-phrases	S53 S45 S60 S61
Related compounds
Related compounds	SO2Cl2; VOCl3; MoO2Cl2; WO2Cl2; CrO2F2
Y chloride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Chromyl chloride is a chemical compound with the formula CrO₂Cl₂. This compound is an opaque dark blood-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. However, they differ in both oxidizing power and in physical properties: one is a liquid and the other forms salts.

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, or the reaction with K₂CrO₄ with concentrated HCl, followed by 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, or by addition of concentrated sulphuric acid to an intimate mixture of sodium chloride and potassium dichromate, followed by gentle distillation.^[2]

K₂Cr₂O₇ + 4NaCl + 6H₂SO₄ → 2CrO₂Cl₂ + 2KHSO₄ + 4NaHSO₄ +3H₂O

Chemical properties

CrO₂Cl₂ is highly electrophilic and an aggressive oxidizing agent, e.g. causing spontaneous combustion when dripped onto amorphous sulfur; can also oxidize toluene into benzaldehyde^[3]. 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 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.^[4] It will also attack benzylic methyl groups to give aldehydes via the Étard reaction. Apart from this it can also be used for testing the absence of nitrate ions

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, chromyl chloride attacks most greases.

Safety considerations

CrO₂Cl₂ reacts with water to release hydrochloric acid (HCl) and hexavalent chromium (Cr^VI)

Acute: HCl can be acutely lethal. Exposure to chromyl chloride vapour irritates the respiratory system and severely irritates the eyes, and the liquid burns the skin and eyes. Ingestion would cause severe internal damage.^[5]

Chronic: Cr^VI can produce chromosomal aberrations and is a human carcinogen via inhalation.^[6] Frequent exposure of the skin to chromyl chloride may result in ulceration.^[5]

Thus, CrO₂Cl₂ should be carefully handled in a well ventilated area. CrO₂Cl₂ is so aggressive that its storage can be problematic as it attacks rubber and 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. ^ Moody, B.J. (1965). "22". Comparative Inorganic Chemistry (1 ed.). London: Edward Arnold. p. 381. ISBN 0713136790. 
3. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
4. ^ F. Freeman, R. H. DuBois, T. G. McLaughlin, "Aldehydes by Oxidation of Terminal Olefins with Chromyl Chloride: 2,4,4-Trimethylpentanal", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv6p1028 ; Coll. Vol. 6: 1028 
5. ^ ^{a b} Prof CH Gray, ed (1966). "IV". Laboratory Handbook of Toxic Agents (2 ed.). London: Royal Institute of Chemistry. p. 79. 
6. ^ IARC (1999-11-05) [1990] (PDF). Volume 49: Chromium, Nickel, and Welding. pp. 21–23. ISBN 92-832-1249-5. http://monographs.iarc.fr/ENG/Monographs/vol49/volume49.pdf. Retrieved 2008-03-26. 

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