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. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. Chromates and dichromates are Salts of Chromic acid and dichromic acid respectively Chromates and dichromates are Salts of Chromic acid and dichromic acid respectively The acid Anhydride of Chromic acid, CrO3 is chromium trioxide or chromium(VI oxide; industrially this compound is sometimes sold as " Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. Often the species are assigned the formulas H2CrO4 and H2Cr2O7. A chemical formula is a way of expressing information about the Atoms that constitute a particular Chemical compound, and how the relationship between those atoms changes The anhydride of these "chromic acids" is chromium trioxide, also called chromium(VI) oxide; industrially, this compound is sometimes sold as "chromic acid. In Chemistry, an anhydride is a compound that can be considered as derived from another compound by subtracting the molecules of water. The acid Anhydride of Chromic acid, CrO3 is chromium trioxide or chromium(VI oxide; industrially this compound is sometimes sold as " "
Regardless of its exact formula, chromic acid features chromium in an oxidation state of +6 (or VI), often referred to as hexavalent chromium. In Chemistry, the oxidation state is an indicator of the degree of Oxidation of an Atom in a Chemical compound. Hexavalent chromium ( Cr(VI) compounds are those which contain the element Chromium in the +6 Oxidation state. 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 purple [Cr(H2O)6]3+ ion, or other Cr(III) species, which usually have a green colour. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state
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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. An oxidizing agent or oxidising agent (also called an oxidant, oxidizer or oxidiser) can be defined as either a Chemical compound Laboratory glassware refers to a variety of equipment traditionally made of Glass, used for Scientific experiments and other work in Science, especially 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 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. An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. The Jones oxidation is a Chemical reaction described as the Chromic acid oxidation of primary and secondary Alcohols to Carboxylic acids Carboxylic acids are Organic acids characterized by the presence of a Carboxyl group, which has the formula -C(=OOH usually written -COOH or -CO2H A ketone (pronounced as key tone) is either the Functional group characterized by a Carbonyl group (O=C linked to two other Carbon atoms or [1]
- Pyridinium chlorochromate is generated from chromium trioxide and pyridinium hydrochloride. Pyridinium chlorochromate is a reddish orange solid reagent used to Oxidize primary Alcohols to Aldehydes and secondary Alcohols to Ketones This reagent converts primary alcohols to the corresponding aldehydes (R-CHO). An aldehyde is an organic compound containing a terminal Carbonyl group. [1]
- Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations. Collins reagent is the complex of chromium(VI oxide with Pyridine in Dichloromethane.
- Chromyl chloride, CrO2Cl2 is a well-defined molecular compound that is generated from chromic acid. Chromyl chloride is a chemical compound with the formula CrO2Cl2
Illustrative transformations
- Oxidation of methylbenzenes to benzoic acids. Toluene, also known as methylbenzene or phenylmethane, is a clear water -insoluble liquid with the typical smell of Paint thinners redolent of Benzoic acid, C7H6O2 (or C6H5COOH is a colorless crystalline solid and the simplest Aromatic Carboxylic acid [2]
- Oxidative scission of indene to homophthalic acid. Indene is a Flammable polycyclic Hydrocarbon with Chemical formula C9H8 [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. Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon An aldehyde is an organic compound containing a terminal Carbonyl group. A ketone (pronounced as key tone) is either the Functional group characterized by a Carbonyl group (O=C linked to two other Carbon atoms or 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. Sodium hypochlorite is a Chemical compound with the formula NaClO [6] Each oxidant offers advantages and disadvantages.
Safety
Chromium(VI) compounds are toxic and carcinogenic. Hexavalent chromium ( Cr(VI) compounds are those which contain the element Chromium in the +6 Oxidation state. Toxicity is the degree to which a substance is able to damage an exposed organism The term carcinogen refers to any substance Radionuclide or radiation that is an agent directly involved in the promotion of Cancer or in the fatation of its propagation For this reason, chromic acid oxidation is not used on an industrial scale.
Notes
- ^ Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, doi:10.1002/047084289X.rc164
- ^ Kamm O. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. ; Matthews, A. O. (1941). "p-Nitrobenzoic Acid". Org. Synth.; Coll. Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures Vol. 1: 392.
- ^ Grummitt, O. ; Egan, R. ; Buck, A. . "Homophthalic Acid and Anhydride". Org. Synth.; Coll. Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures Vol. 3: 449 (1955.
- ^ Eisenbraun, E. J. . "Cyclooctanone". Org. Synth.; Coll. Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures Vol. 5: 310 (1973.
- ^ Meinwald, J. ; Crandall, J. ; Hymans W. E. . "Nortricyclanone". Org. Synth.; Coll. Organic Syntheses is a Scientific journal that since 1921 has provided the Chemistry community with annual collections of detailed and checked procedures 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. The Journal of Organic Chemistry (abbreviated as J Org Chem or JOC) is a Scientific journal for original contributions of fundamental doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
References
External links
- International Chemical Safety Card 1194
- National Pollutant Inventory - Chromium (III) compounds fact sheet(This reference really shouldn't be here, since this page is about hexavalent chromium. )
- NIOSH Pocket Guide to Chemical Hazards 0138
- IARC Monograph "Chromium and Chromium compounds"
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