Chromic acid is a chemical compound with the chemical formula . More generally, it is the name for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide.
The term "chromic acid" is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, of which the trioxide is the acidic oxide. Chromic acid features chromium in an oxidation state of +6 (and a valence of VI or 6). It is a strong and corrosive oxidizing agent and a moderate carcinogen.
Molecular chromic acid
Molecular chromic acid, , in principle, resembles
sulfuric acid, . It would ionize accordingly:
The pKa for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6.[ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands] The structure of the mono anion has been determined by X-ray crystallography. In this tetrahedral oxyanion, three Cr-O bond lengths are 156 picometer and the Cr-OH bond is 201 pm
condenses to form dichromate:
- , log KD = 2.05.
Furthermore, the dichromate can be protonated:
- , p Ka = 1.8
Loss of the second proton occurs in the pH range 4–8, making the ion a
weak acid.
Molecular chromic acid could in principle be made by adding chromium trioxide to water ( cf. contact process).
In practice, the reverse reaction occurs: molecular chromic acid dehydrates. Some insights can be gleaned from observations on the reaction of dichromate solutions with sulfuric acid. The first colour change from orange to red signals the conversion of dichromate to chromic acid. Under these conditions deep red crystals of chromium trioxide precipitate from the mixture, without further colour change.
Chromium trioxide is the acidic oxide of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as pyridine in a non-aqueous medium such as dichloromethane (Collins reagent).
Higher chromic acids with the formula are probable components of concentrated solutions of chromic acid.
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 or
chromosulfuric acid) is a powerful
oxidizing agent, it can be used to
tube cleaning laboratory glassware, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns.
Furthermore, the acid leaves trace amounts of
paramagnetic chromic ions () that can interfere with certain applications, such as
NMR spectroscopy. This is especially the case for
NMR tubes.
can be used for the same task, without leaving metallic residues behind.
Chromic acid was widely used in the musical 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.
It was used in hair dye in the 1940s, under the name Melereon.
It is used as a bleach in processing black and white photographic reversal film.
Reactions
Chromic acid is capable of oxidizing many kinds of
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 and respectively, while rarely affecting unsaturated bonds.
[Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, ]
-
Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding (R–CHO).
-
Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations.
-
Chromyl chloride, is a well-defined molecular compound that is generated from chromic acid.
Illustrative transformations
-
Oxidation of to .
-
Oxidative scission of indene to homophthalic acid.
-
Oxidation of secondary alcohol to ketone (cyclooctanone)
Use in qualitative organic analysis
In organic chemistry, dilute solutions of chromic acid can be used to oxidize primary or secondary alcohols to the corresponding
and
. Similarly, it can also be used to oxidize an aldehyde to its corresponding
carboxylic acid.
and ketones are unaffected. However, in acidic conditions, some tertiary alcohols may isomerize to an oxidizable alcohol, though such an isomerization is usually not of concern. Because the
oxidation is signaled by a color change from orange to brownish green (indicating chromium being reduced from oxidation state +6 to +3), chromic acid is commonly used as a lab reagent in high school or undergraduate college chemistry as a qualitative analytical test for the presence of primary or secondary alcohols, or aldehydes.
Alternative reagents
In oxidations of alcohols or
into
, chromic acid is one of several reagents, including several that are catalytic. For example, nickel(II) salts catalyze oxidations by bleach (hypochlorite).
Aldehydes are relatively easily oxidized to carboxylic acids, and mild
oxidant are sufficient. Silver(I) compounds have been used for this purpose. Each oxidant offers advantages and disadvantages. Instead of using chemical oxidants, electrochemical oxidation is often possible.
Safety
Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are
toxic and
carcinogenic. Chromium trioxide and chromic acids are strong oxidizers and may react violently if mixed with easily oxidizable organic substances.
Chromic acid chemical burn are treated with a dilute sodium thiosulfate solution.
Notes
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