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.

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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:

• 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.^[1]
• 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.^[2]
• Oxidative scission of indene to homophthalic acid.^[3]
• Oxidation of secondary alcohol to ketone (cyclooctanone)^[4] and nortricyclanone.^[5]

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)