Chromyl chloride is an inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal compounds. It is the dichloride of chromic acid.
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.12
K2Cr2O7 + 6 HCl → 2 CrO2Cl2 + 2 KCl + 3 H2OThe sulfuric acid serves as a dehydration agent.
It can also be prepared directly by exposing chromium trioxide to anhydrous hydrogen chloride gas.
CrO3 + 2 HCl ⇌ CrO2Cl2 + H2OUsage
Test for the presence of chlorides
The chromyl chloride test involves heating a sample suspected to contain chlorides with potassium dichromate and concentrated sulfuric acid. If a chloride is present then chromyl chloride forms which is indicated by the evolution of red smoke. No similar compound is formed in the presence of fluoride, bromide, iodide, or cyanide, making this test specific to chlorides.
Reagent for oxidation of alkenes
Chromyl chloride oxidizes internal alkenes to alpha-chloroketones or related derivatives.3 It will also attack benzylic methyl groups to give aldehydes via the Étard reaction. Dichloromethane is a suitable solvent for these reactions.4
Others
CrO2Cl2 can also be used to test for the absence of nitrate ions.
Safety considerations
Main article: Hexavalent chromium
Chromyl chloride is severely corrosive and easily burns the skin and eyes. It is a probable human carcinogen.5
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References
Moody, B.J. (1965). "22". Comparative Inorganic Chemistry (1 ed.). London: Edward Arnold. p. 381. ISBN 0-7131-3679-0. 0-7131-3679-0 ↩
Sisler, Harry H. (1946). Chromyl Chloride [Chromium(VI) Dioxychloride]. Inorganic Syntheses. Vol. 2. pp. 205–207. doi:10.1002/9780470132333.ch63. ISBN 9780470132333. 9780470132333 ↩
Freeman, Fillmore; DuBois, Richard H.; McLaughlin, Thomas G. (1971). "Aldehydes by Oxidation of Terminal Olefins with Chromyl Chloride: 2,4,4-Trimethylpentanal". Org. Synth. 51: 4. doi:10.15227/orgsyn.051.0004. /wiki/Doi_(identifier) ↩
F. Freeman (2004). "Chromyl Chloride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc177. ISBN 0471936235. 0471936235 ↩
Prof CH Gray, ed. (1966). "IV". Laboratory Handbook of Toxic Agents (2 ed.). London: Royal Institute of Chemistry. p. 79. /wiki/Royal_Institute_of_Chemistry ↩