Chromic acid

Chromic acid
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Chromic acid generally 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 in an oxidation state of +6 (or VI), often referred to as hexavalent chromium. Chromium, like most transition metals, can exist in a number of oxidation states. The +6 oxidation state is the highest known; the +3 oxidation state is the most stable, and the +2 oxidation state is also well known, although it is a strong reducing agent. In its reactions chromic acid is reduced in redox reactions to the purple [Cr(H₂O)₆³⁺ ion, or other Cr(III) species, which usually have a green colour.

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1 Uses
2 Reactions
3 Safety
4 Notes
5 References
6 External links

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, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns.¹ Furthermore the acid leaves trace amounts of paramagnetic ions Cr(III) and Cr(V) that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes.²

Chromic acid has also been widely used in the band instrument repair industry, due to its ability to "brighten" raw brass. A chromic acid dip leaves behind a bright yellow patina on the brass. Due to growing health and environmental concerns, many have discontinued use of this chemical in their repair shops.

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.³
Pyridinium chlorochromate is generated from chromium trioxide and pyridinium hydrochloride. This reagent converts primary alcohols to the corresponding aldehydes (R-CHO).³
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.⁴
Oxidative scission of indene to homophthalic acid.⁵
Oxidation of secondary alcohol to ketone (cyclooctanone)⁶ and nortricyclanone.⁷

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.³

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.⁸ Aldehydes are relatively easily oxidised to carboxylic acids, and mild oxidising agents are sufficient. Silver(I) compounds have been used for this purpose. 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.

Notes

^ J. M. McCormick (2006-06-30). "Cleaning Glassware". Truman State University.
^ "NMR-010: Proper Cleaning Procedures for NMR Sample Tubes". Wilmad. Retrieved on 2008-06-27.
^ ^a ^b ^c Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, doi:10.1002/047084289X.rc164
^ Kamm O.; Matthews, A. O. (1941). "p-Nitrobenzoic Acid". Org. Synth.; Coll. Vol. 1: 392.
^ Grummitt, O.; Egan, R.; Buck, A.. "Homophthalic Acid and Anhydride". Org. Synth.; Coll. Vol. 3: 449 (1955.
^ Eisenbraun, E. J.. "Cyclooctanone". Org. Synth.; Coll. Vol. 5: 310 (1973.
^ Meinwald, J.; Crandall, J.; Hymans W. E.. "Nortricyclanone". Org. Synth.; Coll. Vol. 5: 866.
^ 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.