Potassium Dichromate

[7778-50-9]  · Cr2K2O7  · Potassium Dichromate  · (MW 294.20)

(reagent for converting alcohols to aldehydes, carboxylic acids, and ketones)

Physical Data: mp 398 °C; d 2.676 g cm-3.

Solubility: H2O (4.9 g/100 mL at 0 °C), DMSO, polyethylene glycol (PEG).

Form Supplied in: red crystals.

Handling, Storage, and Precautions: highly toxic cancer suspect agent. All chromium(VI) reagents must be handled with care. The mutagenicity of CrVI compounds is well documented.1 This reagent should be handled in a fume hood.

Oxidation of Alcohols to Carbonyl Compounds.

Primary alcohols in CH2Cl2 are oxidized to aldehydes and secondary alcohols are oxidized to ketones in high yields by potassium dichromate in 9 M sulfuric acid in the presence of tetrabutylammonium hydrogen sulfate as a phase-transfer catalyst. The reaction is complete in a few seconds at rt, the yields are lower with water-soluble alcohols, and double bonds may be cleaved.2 Adogen 464, a mixture of methyltrialkylammonium chlorides, facilitates the solubilization of potassium dichromate in several apolar organic solvents including benzene, CH2Cl2, CHCl3, and CCl4.3 The solution in benzene is a mild oxiding agent which converts alcohols to carbonyl compounds with some selectivity.

Allylic and benzylic alcohols are oxidized to carbonyl compounds (70-80%) by potassium dichromate in either Dimethyl Sulfoxide (DMSO) or polyethylene glycol (PEG).4 Oxidations with potassium dichromate in PEG are comparable to those in crown ethers or Hexamethylphosphoric Triamide.5 A novel one-pot procedure for the selective oxidation of 5-substituted 2-hydroxy-3-hydroxymethylbenzaldehydes (1), via the N,N-ethylene bis(5-substituted 3-hydroxymethylsalicylaldimino)copper complexes (2), to 5-substituted 2-hydroxy-1,3-benzenedicarbaldehydes (3) by potassium dichromate in DMSO at 100 °C is shown in eq 1.6 The complexes (2), which were prepared (90-95%) from (1), 1,2-Diaminoethane, and Copper(II) Acetate, are oxidized by Potassium Permanganate in aqueous Pyridine to 5-substituted 3-formyl-2-hydroxybenzoic acids (25-40%).6

Oxidation of Cyclobutanols and Cyclobutanones to g-Butyrolactones.

Cyclobutanols and cyclobutanones are oxidized by potassium dichromate in aqueous sulfuric acid to g-butyrolactones.7 Cyclobutanones tetrasubstituted at C-2, C-3, or at C-2, C-4 are oxidized to g-butyrolactones in good yields (eqs 2 and 3).8 The oxidation is not applicable to larger cyclic ketones.

Oxidation of a-Nitro Alcohols to a-Nitro Ketones.

a-Nitro alcohols are oxidized to a-nitro ketones by potassium dichromate under phase-transfer conditions (CH2Cl2, tetrabutylammonium sulfate, 30% sulfuric acid).9 Reaction of an aldehyde and a nitroalkane on alumina followed by in situ oxidation allows for a one-pot synthesis (eq 4).

Oxidation of Primary Alcohols to Acids.

Adding primary alcohols to an excess of potassium dichromate in sulfuric acid affords the corresponding carboxylic acids in good to excellent yields.10,11

Other Applications.

Potassium dichromate in benzene in the presence of Adogen 4643 oxidizes o-allylphenols to 3-chromenes (eq 5).12 o-Quinone methides are postulated intermediates. Although o-quinones are generally prepared from the corresponding catechol, potassium dichromate in sulfuric acid oxidizes 2-amino-4,5-dimethylphenol to 4,5-dimethyl-o-quinone (45%).13 4-Aminonaphthol is oxidized to 1,4-naphthoquinone (78-81%) by potassium dichromate in sulfuric acid.14 Furfural is oxidized to furoic acid (75%) by potassium dichromate in aqueous sulfuric acid.15 Potassium dichromate in aqueous acetic acid oxidizes N,N-bis-4-nitrobenzylhydroxylamine to the 4-nitrophenyl nitrone of 4-nitrobenzaldehyde (98%).16 Aqueous potassium dichromate selectively oxidizes 2-methylnaphthalene to 2-naphthoic acid without ring degradation or formation of a considerable amount of 2-methylnaphthoquinone.17,18

1. Cupo, D. Y.; Wetterhahn, K. E. Cancer Res. 1985, 45, 1146.
2. Pletcher, D.; Tait, S. J. D. TL 1978, 1601.
3. Hutchins, R. O.; Natale, N. R.; Cook, W. J.; Ohr, J. TL 1977, 4167.
4. Santaniello, E.; Ferraboschi, P.; Sozzani, P. S 1980, 646.
5. Santaniello, E.; Manzocchi, A.; Sozzani, P. TL 1979, 4581.
6. Hu, Y.; Hu, H. S 1991, 325.
7. Jeanne-Carlier, R.; Bourelle-Wargnier, F. TL 1975, 1841.
8. Jeanne-Carlier, R.; Bourelle-Wargnier. F. BSF 1976, 297.
9. Rosini, G.; Ballini, R.; Sorrenti, P.; Pertrini, M. S 1984, 607.
10. Marckwald, W. CB 1904, 37, 1038.
11. Gryszkiewicz-Trochimowski, E. RTC 1947, 66, 430.
12. Cardillo, G.; Orena, M.; Porzi, G.; Sundri, S. CC 1979, 836.
13. Willstatter, R.; Muller, F. CB 1911, 44, 2171.
14. Fieser, L. OSC 1941, 1, 383.
15. Hurd, C. D.; Garrett, J. W.; Osborne, E. N. JACS 1933, 55, 1082.
16. Behrend, R.; Koenig, E. LA 1891, 263, 339.
17. Friedman, L.; Fishel, D. L.; Shechter, H. JOC 1965, 30, 1453.
18. Fieser, L. F.; Campbell, W. P.; Fry, E. M.; Gates, Jr., M. D. JACS 1939, 61, 3216.

Fillmore Freeman

University of California, Irvine, CA, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.