Chromic acid
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In chemistry, chromic acid (or 'Jones reagent') is a 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.
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 Cr+3 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. 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).[1] 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 primary or secondary alcohols. For example, chromium trioxide dissolved in aqueous sulfuric acid solution has been used as such a test reagent.[1]
Reference External Link
- Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds (http://www.hamline.edu/~rmajerle/Chem3460/Feb2105.pdf)