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In chemistry, chromic acid is a hypothetical chromium (Cr) compound, yet to be isolated, that would have the formula H2CrO4. There is a related acid, also yet to be isolated called dichromic acid with the formula H2Cr2O7. While these acids are not available, they are known through their divalent anions (chromate and dichromate respectively) and the compounds formed from them. Please note that the acid anhydride of chromic acid exists, chromium(VI) oxide or chromium trioxide (CrO3), and that industrially, this compound is sometimes sold as "chromic acid". This compound is an intensely-colored dark red/orange brown, water-soluble, granular solid which is stable by itself, but is a strong oxidant which will react when mixed with many things that can be oxidized. Ethanol, for example, will ignite on contact with it.
In all of the compounds discussed here, the element chromium is in oxidation state +6 (or VI). Chromium(VI) is often referred to as hexavalent chromium. Although chromium can exist in a number of oxidation states, hexavalent chromium is commonly reduced in redox reactions to a +3 oxidation state, where the Cr3+ ion is a distinctively different blue green color. Hexavalent chromium compounds are toxic and carcinogenic.
Chromic acid solutions are generally acidified aqueous solutions of chromate or dichromate salts. These solutions tend to be powerful oxidants, capable of destroying all organics in glassware and so often used for the cleaning of glass. Such solutions are often red-brown, orange, or yellow colored depending on concentration.
Reactions
In organic chemistry, more dilute hexavalent chromium solutions can be used to oxidize a hydroxyl group and the carbon atom it is bonded to in a primary or secondary alcohol to a carbonyl group. Chromic acid in the presence of aqueous sulfuric acid is known as the Jones reagent, and will oxidize primary and secondary alcohols to carboxylic acids and ketones, respectively. Chromium trioxide and pyridinium hydrochloride (pyridine neutralized by HCl) can be used to form a pyridinium chlorochromate reagent under anhydrous conditions. When a primary alcohol is treated with anhydrous pyridinium chlorochromate in methylene chloride, the oxidation of the -CH2OH group in primary alcohols will stop at the aldehyde group (-CHO). In contrast, other oxidizing solutions, such as aqueous potassium permanganate, will have a tendency to oxidize the -CH2OH group all the way to a carboxyl group (-COOH). Dilute hexavalent chromium solutions will not oxidize a tertiary alcohol hydroxyl group. Because of the color change from a variation of orange to a blue-green color, a hexavalent chromium reagent can be formulated as a qualitative analytical test for the presence of primary or secondary alcohols. For example, chromium trioxide dissolved in aqueous sulfuric acid solution has been used as such a test reagent.[1 Potassium dichromate, when mixed with concentrated sulfuric acid and a sample containing inorganic chloride and heated strongly, produces red fumes that condense to form a red liquid - chromyl chloride, CrO2Cl2. This compound hydrolyses in water to form aqueous chromic and hydrochloric acids: CrO2Cl2(l) + 2H2O(l) → H2CrO4(aq) + 2HCl(aq) As bromides and iodides do not give respective analogues, the test is specific for chlorides.
References
- Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds (PDF)
- Index of Organic Synthesis procedures utilizing Chromic acid
External links
- International Chemical Safety Card 1194
- National Pollutant Inventory - Chromium (III) compounds fact sheet
- NIOSH Pocket Guide to Chemical Hazards
- IARC Monograph "Chromium and Chromium compounds"
"Chromic acid"
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