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Chromic acid



  Chromic acid refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. 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.

Additional recommended knowledge

Contents

Uses

Chromic acid is an intermediate in chromium plating, and is also used in ceramic glazes, and colored glass. Because a solution of chromic acid in sulfuric acid (also known as a sulfochromic mixture) is a powerful oxidizing agent, it can be used to clean laboratory 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:

Illustrative transformations

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.[6] Each oxidant offers advantages and disadvantages.

Safety

Chromium(VI) compounds are toxic and carcinogenic. For this reason, chromic acid oxidation is not used on an industrial scale.

References

  1. ^ Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, doi:10.1002/047084289X.rc164
  2. ^ Kamm O.; Matthews, A. O. (1941). "p-Nitrobenzoic Acid". Org. Synth.; Coll. Vol. 1: 392. 
  3. ^ Grummitt, O.; Egan, R.; Buck, A.. "Homophthalic Acid and Anhydride". Org. Synth.; Coll. Vol. 3: 449 (1955. 
  4. ^ Eisenbraun, E. J.. "Cyclooctanone". Org. Synth.; Coll. Vol. 5: 310 (1973. 
  5. ^ Meinwald, J.; Crandall, J.; Hymans W. E.. "Nortricyclanone". Org. Synth.; Coll. Vol. 5: 866. 
  6. ^ J. M. Grill, J. W. Ogle, S. A. Miller (2006). "An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids". J. Org. Chem. 71 (25): 9291-9296. doi:10.1021/jo0612574.
  1. Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds (PDF)
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chromic_acid". A list of authors is available in Wikipedia.
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