Ceric ammonium nitrate

Ammonium cerium(IV) nitrate
IUPAC name Diammonium cerium(IV) nitrate
Other names Ceric ammonium nitrate (CAN)
Identifiers
CAS number	16774-21-3
PubChem	16211559
ChemSpider	17339538
EC number	240-827-6
Jmol-3D images	Image 1
SMILES [Ce+4].O=[N+]([O-])[O-].[O-][N+]([O-])=O.[O-][N+]([O-])=O.[O-][N+](=O)O.[O-][N+](=O)O.[O-][N+]([O-])=O.N.N
InChI InChI=InChI=1S/Ce.2HNO3.4NO3.2H3N/c;6*2-1(3)4;;/h;2*(H,2,3,4);;;;;2*1H3/q+4;;;4*-1;;
Properties
Molecular formula	H8N8CeO18
Molar mass	548.26 g/mol
Appearance	orange-red crystals
Melting point	107-108 °C
Solubility in water	141 g/100 mL (25 °C) 227 g/100 mL (80 °C)
Structure
Crystal structure	Monoclinic
Coordination geometry	Icosahedral
Related compounds
Related compounds	Ammonium nitrate Cerium(IV) oxide
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

The anion [Ce(NO₃)₆]²⁻ is generated by dissolving Ce₂O₃ in hot concentrated HNO₃.

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:

The anion [Ce(NO₃)₆]²⁻ has idealized O_h molecular symmetry. The CeO₁₂ core defines an icosahedron.^[1]

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.^[2] with DMF or DMSO as solvent,^[3] or as para-methoxybenzyl trichloroacetimidate with ether and 0.3 mol% triflic acid.^[2][3] 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^[2][3] 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.^[3]

The reaction mechanism is probably^{[citation needed]} 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.^[3]

CAN probably^{[citation needed]} 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

Other applications

CAN is also an important component of chrome etchant,^[4] a material that is used in the production of photomasks and liquid crystal displays.

References

1. ^ Thomas A. Beineke; J. Delgaudio (1968). "Crystal structure of ceric ammonium nitrate". Inorg. Chem. 7 (4): 715–721. doi:10.1021/ic50062a020.  edit
2. ^ ^{a b c} Boons, Geert-Jan.; Hale, Karl J. (2000). Organic Synthesis with Carbohydrates (1st ed.) Sheffield, England: Sheffield Academic Press. pp.33
3. ^ ^{a b c d e} Kocienski, Phillip J. (1994). Protecting Groups Stuttgart, New York Georg Thieme Verlag. pp 8-9, 52-54
4. ^ Walker, Perrin; William H. Tarn (1991). CRC Handbook of Metal Etchants. pp. 287–291. ISBN 0-8949-3623-6. 

External links

• Oxidizing Agents: Cerium Ammonium Nitrate