Chromic acid
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[[Image:ChromicAcid.png|thumb|right|300px]]
'''Chromic acid''' generally refers to a collection of [[chemical compound|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 [[chemical formula|formulas]] H<sub>2</sub>CrO<sub>4</sub> and H<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>. 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 can exist in a number of oxidation states, hexavalent state is the highest. In its reactions chromic acid is reduced in [[redox reaction]]s to the purple [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> ion, or other Cr(III) species, which usually have a green colour.
==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.
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 compound]]s 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 acid]]s and [[ketone]]s respectively, while rarely affecting unsaturated bonds.<ref>Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, {{doi|10.1002/047084289X.rc164}}</ref>
* [[Pyridinium chlorochromate]] is generated from chromium trioxide and pyridinium hydrochloride. This reagent converts primary alcohols to the corresponding [[aldehyde]]s (R-CHO).<sup>[1]</sup>
* [[Collins reagent]] is an adduct of chromium trioxide and pyridine used for diverse oxidations.
* [[Chromyl chloride]], CrO<sub>2</sub>Cl<sub>2</sub> is a well-defined molecular compound that is generated from chromic acid.
====Illustrative transformations====
* Oxidation of [[methylbenzene]]s to [[benzoic acid]]s.<ref>{{OrgSynth | author = Kamm O.; Matthews, A. O. | title = p-Nitrobenzoic Acid | collvol = 1 | collvolpages = 392 | year = 1941 | prep = CV1P0392}}</ref>
* Oxidative scission of [[indene]] to [[homophthalic acid]].<ref>{{OrgSynth | author = Grummitt, O.; Egan, R.; Buck, A. | title = Homophthalic Acid and Anhydride | collvol = 3 | collvolpages = 449 (1955| prep = CV3P0449}}</ref>
* Oxidation of secondary alcohol to ketone (cyclooctanone)<ref>{{OrgSynth | author = Eisenbraun, E. J. | title = Cyclooctanone | collvol = 5 | collvolpages = 310 (1973 | prep = CV5P0310}}</ref> and nortricyclanone.<ref>{{OrgSynth | author = Meinwald, J.; Crandall, J.; Hymans W. E. | title = Nortricyclanone | collvol = 5 | collvolpages = 866 | 1973 | prep = cv5p0866}}</ref>
====Use in qualitative organic analysis====
In [[organic chemistry]], dilute [[solution]]s of hexavalent chromium can be used to oxidize primary or secondary [[alcohol]]s to the corresponding [[aldehyde]]s and [[ketone]]s. 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.<sup>[1]</sup>
====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 [[Sodium hypochlorite|bleach]].<ref>{{cite journal | author = J. M. Grill, J. W. Ogle, S. A. Miller | title = An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids | journal = [[J. Org. Chem.]] | year = 2006 | volume = 71 | issue = 25 | pages = 9291–9296 | doi = 10.1021/jo0612574}}</ref> 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.
==Notes==
{{reflist}}
==References==
* [http://www.hamline.edu/~rmajerle/Chem3460/Feb2105.pdf Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds] ([[PDF]])
==External links==
* {{ICSC|1194|11}}
* [http://www.npi.gov.au/database/substance-info/profiles/24.html National Pollutant Inventory - Chromium (III) compounds fact sheet](This reference really shouldn't be here, since this page is about hexavalent chromium.)
* {{PGCH|0138}}
* [http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
[[Category:Chromates]]
[[Category:Acids]]
[[Category:Oxidizing agents]]
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