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Chromyl fluoride
Chemical compound From Wikipedia, the free encyclopedia
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Chromyl fluoride is an inorganic compound with the formula CrO2F2. It is a violet-red colored crystalline solid that melts to an orange-red liquid.[2]
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Structure
The liquid and gaseous CrO2F2 have a tetrahedral geometry with C2v symmetry, much like chromyl chloride.[3] Chromyl fluoride dimerizes via fluoride bridges (as O2Cr(μ-F)4CrO2) in the solid state, crystallizing in the P21/c space group with Z = 4. The Cr=O bond lengths are about 157 pm, and the Cr–F bond lengths are 181.7, 186.7, and 209.4 pm. Chromium resides in a distorted octahedral position with a coordination number of 6.[4]
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History and preparation
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Pure chromyl fluoride was first isolated in 1952 as reported by Alfred Engelbrecht and Aristid von Grosse.[5] It was first observed as red vapor in the early 19th century upon heating a mixture of fluorspar (CaF2), chromates, and sulfuric acid. These red vapors were initially thought to be CrF6, although some chemists assumed a CrO2F2 structure analogous to CrO2Cl2.[5] The first moderately successful synthesis of chromyl fluoride was reported by Fredenhagen who examined the reaction of hydrogen fluoride with alkali chromates. A later attempt saw von Wartenberg prepare impure CrO2F2 by treating chromyl chloride with elemental fluorine.[6] Another attempt was made by Wiechert, who treated HF with dichromate, yielding impure liquid CrO2F2 at −40 °C.
Engelbrecht and von Grosse's synthesis of CrO2F2, and most successive syntheses, involve treating chromium trioxide with a fluorinating agent:[5]
- CrO3 + 2 HF → CrO2F2 + H2O
The reaction is reversible, as water will readily hydrolyze CrO2F2 back to CrO3.
The approach published by Georg Brauer in the Handbook of Preparative Inorganic Chemistry[1] drew on von Wartenberg's approach[6] of direct fluoridation:
- CrO2Cl2 + F2 → CrO2F2 + Cl2
Other methods include treatment with chlorine fluoride, carbonyl fluoride, or some metal hexafluorides:
- CrO3 + 2 ClF → CrO2F2 + Cl2 + O2
- CrO3 + COF2 → CrO2F2 + CO2
- CrO3 + MF6 → CrO2F2 + MOF4 (M = Mo, W)
The last method involving the fluorides of tungsten and molybdenum are reported by Green and Gard to be very simple and effective routes to large quantities of pure CrO2F2.[2] They reported 100% yield when the reactions were conducted at 120 °C. As expected from the relative reactivities of MoF6 and WF6, the molybdenum reaction proceeded more readily than did the tungsten.[7]
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Reactions
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Chromyl fluoride is a strong oxidizing agent capable of converting hydrocarbons to ketones and carboxylic acids. It can also be used as a reagent in the preparation of other chromyl compounds.[2] Like some other fluoride compounds, CrO2F2 reacts with glass and quartz, so silicon-free plastics or metal containers are required for handling the compound. Its oxidizing power in inorganic systems has also been explored.[8] Chromyl fluoride can exchange fluorine atoms with metal oxides.
- CrO2F2 + MO → MF2 + CrO3
where M is a metal. Chromyl fluoride also converts the oxides of boron and silicon to their fluorides.[8]
Chromyl fluoride reacts with alkali and alkaline earth metal fluorides in perfluoroheptane (solvent) to produce orange-colored tetrafluorodioxochromates(VI):[8]
- CrO2F2 + 2 MF → (M+)2[CrO2F4]2−
Chromyl fluoride also reacts with Lewis acids, drawing carboxylate ligands from organic acid anhydrides and producing an acyl fluoride byproduct:[8]
- CrO2F2 + 2 (CF3CO)2O → (CF3COO)2CrO2 + 2 CF3COF
Chromyl fluoride forms adducts with weak Lewis bases NO, NO2, and SO2.
Chromium oxytetrafluoride is prepared by fluorination of chromyl fluoride with krypton difluoride:[9]
References
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