- Ceric ammonium nitrate
Ammonium cerium(IV) nitrate Identifiers CAS number PubChem ChemSpider EC number Jmol-3D images Image 1 Properties Molecular formula H8N8CeO18 Molar mass 548.26 g/mol Appearance orange-red crystals Melting point
Solubility in water 141 g/100 mL (25 °C)
227 g/100 mL (80 °C)
Structure Crystal structure Monoclinic Coordination
Icosahedral Related compounds Related compounds Ammonium nitrate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references
Ceric ammonium nitrate (CAN) is the inorganic compound with the formula (NH4)2Ce(NO3)6. This orange-red, water-soluble cerium salt is widely used as an oxidising agent in organic synthesis and as a standard oxidant in quantitative analysis.
Preparation, properties, and structure
This salt consists of the anion [Ce(NO3)6]2− and a pair of NH4+ counter ions. The ammonium ions are not involved in the oxidising reactions of this salt. In the anion each nitrato group is chelated to the cerium atom in a bidentate manner as shown below:
Applications in organic chemistry
Ce4+ is a stronger oxidizing agent (E° ~ 1.61 V vs. N.H.E.) than even Cl2 (E° ~ 1.36 V). Few shelf-stable reagents are stronger oxidants. In the redox process Ce(IV) is converted to Ce(III), a one-electron change, signaled by the fading of the solution color from orange to a pale yellow (providing that the substrate and product are not strongly colored). CAN is useful as an oxidant for many functional groups (alcohols, phenols, and ethers) as well as C–H bonds, especially those that are benzylic. Alkenes undergo dinitroxylation, although the outcome is solvent-dependent. Quinones are produced from catechols and hydroquinones and even nitroalkanes are oxidized.
CAN provides an alternative to the Nef reaction; for example, for ketomacrolide synthesis where complicating side reactions usually encountered using other reagents. Oxidative halogenation can be promoted by CAN as an in situ oxidant for benzylic bromination, and the iodination of ketones and uracil derivatives.
For the synthesis of heterocycles
Catalytic amounts of aqueous CAN allow the efficient synthesis of quinoxaline derivatives. Quinoxalines are known for their applications as dyes, organic semiconductors, and DNA cleaving agents. These derivatives are also components in antibiotics such as echinomycin and actinomycin. The CAN-catalyzed three-component reaction between anilines and alkyl vinyl ethers provides an efficient entry into 2-methyl-1,2,3,4-tetrahydroquinolines and the corresponding quinolines obtained by their aromatization.
As a deprotection reagent
In synthetic organic chemistry the use of protecting groups is ubiquitous. Two groups used to protect alcohols are the para-methoxybenzyl and 3,4-dimethoxybenzyl ethers, the former are generated by treatment of the para-methoxybenzyl chloride in the presence of NaH, Ba(OH)2, Ag2O or a stannylene acetal. with DMF or DMSO as solvent, or as para-methoxybenzyl trichloroacetimidate with ether and 0.3 mol% triflic acid. 3,4-Dimethoxybenzyl ethers are produced in the same ways. When no longer needed the para-methoxybenzyl ether can be cleaved either by aqueous mineral acids in methanol or camphorsulfonic acid (CSA) in methanol or they can be cleaved oxidatively with either 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in dichloromethane/water or with ceric ammonium nitrate (CAN) in acetonitrile/water.
The reaction mechanism is probably similar for DDQ and CAN. DDQ accepts two electrons from the para-methoxybenzyl ether, one at a time. The DDQ becomes 2,3-dichloro-5,6-dicyano-1,4-hydroquinone and the para-methoxybenzyl ether (minus two electrons) gains a water molecule on the benzylic carbon. The alcohol is remade and the para-methoxybenzyl ether becomes para-methoxybenzaldehyde.
CAN probably works the same way. Since Ce(IV) gains one electron to become Ce(III), two Ce(IV) ions each accept one electron from the para-methoxybenzyl ether to become two Ce(III). Two electrons in total are taken from the para-methoxybenzyl ether. The para-methoxybenzyl ether (minus two electrons) gains a water molecule on the benzylic carbon. The alcohol is remade and the para-methoxybenzyl ether becomes para-methoxybenzaldehyde. The balanced equation is as follows:
- 2 (NH4)2Ce(NO3)6 + H3COC6H4CH2OR + H2O → 4 NH4+ + 2 Ce3+ + 12 NO3− + 2 H+ + H3COC6H4CHO + HOR
- ^ Thomas A. Beineke; J. Delgaudio (1968). "Crystal structure of ceric ammonium nitrate". Inorg. Chem. 7 (4): 715–721. doi:10.1021/ic50062a020.
- ^ a b c Boons, Geert-Jan.; Hale, Karl J. (2000). Organic Synthesis with Carbohydrates (1st ed.) Sheffield, England: Sheffield Academic Press. pp.33
- ^ a b c d e Kocienski, Phillip J. (1994). Protecting Groups Stuttgart, New York Georg Thieme Verlag. pp 8-9, 52-54
- ^ Walker, Perrin; William H. Tarn (1991). CRC Handbook of Metal Etchants. pp. 287–291. ISBN 0-8949-3623-6.
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