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EspañolCHROMIC ACID
| Chromic acid | |
|---|---|
 | |
| General | |
| Molecular formula | H2CrO4 (approximate) |
| Appearance | dark red solution |
| CAS number | 13530-68-2 |
'Chromic acid' refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving chromium trioxide in sulfuric acid.[1] Often the species are assigned the formulas H2CrO4 and H2Cr2O7. The anhydride of these "chromic acids" is chromium trioxide, also called chromium(VI) oxide; industrially, this compound is sometimes sold as "chromic acid."
Regardless of its exact formula, chromic acid features chromium is in oxidation state +6 (or VI), often referred to as hexavalent chromium. Chromium can exist in a number of oxidation states, hexavalent state is the highest. In its reactions chromic acid is reduced in redox reactions to the [Cr(H2O)6]3+ ion, which has a distinctive blue green colour.
| Contents |
| Uses |
| Reactions |
| Illustrative transformations |
| Use in qualitative organic analysis |
| Alternative reagents |
| Safety |
| References |
| External links |
Uses
Chromic acid is an intermediate in chromium plating, and is also used in ceramic glazes, and colored glass. Because chromic acid solutions are powerful oxidants they are used to clean glassware. This application has declined due to environmental concerns. Furthermore the acid leaves residues that can interfere with certain applications, such as NMR spectroscopy.
Reactions
Chromic acid is capable of oxidizing many kinds of organic compounds and many variations on this reagent have been developed:
★ Chromic acid in aqueous sulfuric acid and acetone is known as the Jones reagent, which will oxidize primary and secondary alcohols to carboxylic acids and ketones respectively, while rarely affecting unsaturated bonds.[2]
★ Pyridinium chlorochromate is generated from chromium trioxide and pyridinium hydrochloride. This reagent converts primary alcohols to the aldehyde group (-CHO).[1]
★ Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations.
★ Chromyl chloride, CrO2Cl2 is a well-defined molecular compound that is generated from chromic acid.
Illustrative transformations
★ Oxidation of methylbenzenes to benzoic acids.[3]
★ Oxidative scission of indene to homophthalic acid.[4]
★ Oxidation of secondary alcohol to ketone (cyclooctanone)[5] and nortricyclanone.[6]
Use in qualitative organic analysis
In organic chemistry, dilute solutions of hexavalent chromium can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Tertiary alcohol groups are unaffected. Because of the oxidation is signaled by a color change from orange to a blue-green, chromic acid is used as a qualitative analytical test for the presence of primary or secondary alcohols.[1]
Alternative reagents
In oxidations of alcohols or aldehydes into carboxylic acids, chromic acid is one of several reagents, including several that are catalytic. For example nickel(II) salts catalyze oxidations by bleach.[7] Each oxidant offers advantages and disadvantages.
Safety
Hexavalent chromium compounds are toxic and carcinogenic. For this reason, chromic acid oxidation is not used on an industrial scale.
References
1. Chromic acid is prepared by treating a solution of 140 g of sodium dichromate dihydrate in 100 mL of water with 2 liters of sulfuric acid.
2. Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, DOI: 10.1002/047084289X.rc164
3. Kamm O.; Matthews, A. O. "p-Nitrobenzoic Acid" Organic Syntheses, Collected Volume 1, p.392 (1941).http://www.orgsyn.org/orgsyn/pdfs/CV1P0392.pdf
4. Grummitt, O.; Egan, R.; Buck, A. "Homophthalic Acid and Anhydride" Organic Syntheses, Collected Volume 3, p.449 (1955). http://www.orgsyn.org/orgsyn/pdfs/CV3P0449.pdf
5. Eisenbraun, E. J. "Cycloöctanone" Organic Syntheses, Collected Volume 5, p.310 (1973).
http://www.orgsyn.org/orgsyn/pdfs/CV5P0310.pdf
6. Meinwald, J.; Crandall, J.; Hymans W. E. "Nortricyclanone" Organic Syntheses, Collected Volume 5, p.866 (1973).
7. J. M. Grill, J. W. Ogle, S. A. Miller "An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids" Journal of Organic Chemistry 2006; 71(25) pp. 9291-9296. DOI: 10.1021/jo0612574
# Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds (PDF)
# Index of Organic Synthesis procedures utilizing Chromic acid
External links
★
★ National Pollutant Inventory - Chromium (III) compounds fact sheet
★
★ IARC Monograph "Chromium and Chromium compounds"
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