Ceric ammonium nitrate

Ceric ammonium nitrate, or in lab jargon "CAN", is the chemical compound with the formula (NH₄)₂Ce(NO₃)₆. This orange-red, water-soluble salt is widely used as an oxidising agent in organic synthesis. This compound is used as a standard oxidant in quantitative analysis,

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Properties and structure

Two components comprise this salt, the anion [Ce(NO₃)₆]^2- and a pair of NH₄⁺ counter ions, which are not involved in the reactions of CAN. In the anion each nitrato group is chelated to the cerium atom in a bidentate manner as shown below:

Although the N-O bonds (on the metal side of the nitrato group) are unsymmetrical. The anion [Ce(NO₃)₆]^2- has idealized T_h molecular symmetry. The CeO₁₂ core defines an icosahedron.

Preparation

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

Key reactions

(NH₄)₂Ce(NO₃)₆ is a stronger oxidizing agent (E° ~ 0.96 V vs. N.H.E.) than even Cl₂. 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, some of which are listed below.

• Oxidation of C-H bonds:
  • Alkenes produces dinitroxylation, although the outcome is solvent-dependent.
  • Methylarenes undergo benzylic oxidation.
• Oxidation of alcohols, phenols, and ethers
  • Benzylic alcohols are converted into carbonyl compounds.
  • Quinones are produced from catechols and hydroquinones.
• Oxidation of nitroalkanes
  • An alternative to the Nef reaction, e.g. for ketomacrolide synthesis where complicating side reactions usually encountered using other reagents are avoided using CAN.

Oxidative halogenation can be promoted by CAN as an in situ oxidant, for benzylic bromination, the iodination of ketones and uracil derivatives.

In synthetic organic chemistry the use of protecting groups is basically ubiquitous. Two related protecting groups used to protect alcohols are the para-methoxybenzyl and 3,4-dimethoxybenzyl ethers. They are added to alcohols either as para-methoxybenzyl chloride in the presence of NaH, Ba(OH)₂, Ag₂O or a stannylene acetal^[1] with DMF or DMSO as solvent,^[2] or as para-methoxybenzyl trichloroacetimidate with ether and 0.3 mol% triflic acid.^[1][2] 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 camphor sulfonic acid (CSA) in methanol^[1][2] 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.^[2] 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.^[2] 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 + H₃CO-para-C₆H₄-CH₂-O-R + H₂O → 4NH₄⁺ + 2Ce(III) + 12NO₃^- + 2H⁺ + H₃CO-para-C₆H₄-CHO + H-O-R

Applications

It has been shown that catalytic amounts of aqueous CAN in tap water can be used to efficiently synthesize various quinoxaline derivates in excellent yields. Quinoxaline derivates are known for their applications in areas such as the following: dyes, organic semiconductors, and DNA cleaving agents. These derivatives are also important components in antibiotics such as Echinomycin and Actinomycin which are known to inhibit the growth of Gram-positive bacteria and can be used against transportable tumors. There are many methods for the synthesis of quinoxaline derivatives, however, most of these suffer from unsatisfactory product yields, expensive metal precursors, harsh reaction conditions for the use of those precursors, as well as other problems. CAN provides both an inexpensive and nontoxic solution to these problems.
CAN has many other synthetic applications, and it sometimes allows to carry out reactions that are not possible using other catalysts. For instance, 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.
CAN is also an important component of Chrome etchant, a material that is used in the production of LCDs.

References