Rhodizonic acid

Rhodizonic acid is a chemical compound with formula C
6H
2O
6 or (CO)
4(COH)
2. It can be seen as a twofold enol and fourfold ketone of cyclohexene, more precisely 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone.

Rhodizonic acid
Names
IUPAC name
5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone
Other names
dihydroxydiquinoyl
dioxydiquinone
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.003.888
EC Number
  • 204-276-5
MeSH C005690
Properties
C6H2O6
Melting point 130 to 132 °C (266 to 270 °F; 403 to 405 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Rhodizonic acid is usually obtained in the form of a dihydrate C
6H
2O
6·2H2O. The latter is actually 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, where two of the original ketone groups are replaced by two pairs of geminal diols. The orange to deep-red and highly hygroscopic anhydrous acid can be obtained by low-pressure sublimation of the dihydrate.[1][2]

Like many other enols, rhodizonic acid can lose the hydrogen cations H+ from the hydroxyls (pKa1 = 4.378±0.009, pKa2 = 4.652±0.014 at 25 °C),[3] yielding the hydrogenrhodizonate anion C
6HO−
6 and the rhodizonate anion C
6O2−
6. The latter is aromatic and symmetric, as the double bond and the negative charges are delocalized and evenly distributed over the six CO units. Rhodizonates tend to have various shades of red, from yellowish to purplish.

Rhodizonic acid has been used in chemical assays for barium, lead, and other metals.[4] In particular, the sodium rhodizonate test can be used to detect gunshot residue (which contains lead) in a subject's hands,[5] and to distinguish arrow wounds from gunshot wounds for hunting regulation enforcement.[6]

History

Rhodizonic acid was discovered by Austrian chemist Johann Heller in 1837, by analyzing the products of heating a mixture of potassium carbonate and charcoal.[7][8][9] The name comes from Greek ῥοδίζω (rhodizō, "to tinge red"),[10] on account of the color of its salts.

Chemistry

Salts

Rhodizonates tend to have various shades of red, from yellowish to purplish, in transmitted light, with a greenish luster in reflected light.

Potassium rhodizonate can be prepared with good yield and purity by oxidizing inositol with nitric acid and reacting the result with potassium acetate in the presence of oxygen. The rhodizonate crystallizes out of the solution due to its relative insolubility in water.[11]

Sodium rhodizonate is dark brown and stable when dry,[12] but the aqueous solution decomposes in a few days, even in the refrigerator.[4] Lead rhodizonate is dark violet.[12][13]

Oxidation and decomposition

Rhodizonic acid is a member of a chain of oxidation products: benzenehexol (COH)
6, tetrahydroxybenzoquinone (THBQ) (COH)
4(CO)
2, rhodizonic acid (COH)
2(CO)
4, and cyclohexanehexone (CO)
6.[4] Lithium rhodizonate, together with salts of THBQ and benzenehexol, has been considered for possible use in rechargeable electrical batteries.[14] The monovalent anion C
6O−
6 has been detected in mass spectrometry experiments.[15]

Rhodizonic acid and the rhodizonate anion can lose one of the CO units to yield croconic acid (CO)
3(COH)
2 and the croconate anion C
5O2−
5, respectively, by mechanisms that are still imperfectly known. In basic solutions (pH > 10), rhodizonic acid quickly converts to the THBQ anion (CO)4−
6 in the absence of oxygen, and to croconic acid in its presence. At pH 8.3 and exposure to light, solutions are stable for days in the absence of oxygen, and decompose to croconic acid and other products (possibly including cyclohexanehexone or dodecahydroxycyclohexane) in its presence.[16][17]

Structure

Acid

In solution, the acid and the hydrogenrhodizonate ion are mostly hydrated, with some of the carbonyl groups >C=O replaced by geminal hydroxyls, >C(OH)
2.[3]

Salts

In anhydrous rubidium rhodizonate 2Rb+
·C
6O2−
6, the rhodizonate anions are stacked in parallel columns, as are the rubidium ions. In the plane perpendicular to the columns, these are arranged as two interleaved hexagonal grids. The anions are planar.[2]

Anhydrous potassium rhodizonate 2K+
·C
6O2−
6 has a distinct but similar structure. The anions and cations are arranged in alternate planes. Within each plane, the anions are arranged in a hexagonal grid. Each potassium ion is arranged so that it connects symmetrically to eight oxygens of four anions, two from each adjacent plane. The anions are slightly twisted in the "boat" shape (with 0.108 Å of rms deviation from mean plane).[18] Sodium rhodizonate 2Na+
·C
6O2−
6 has the same structure, with slightly more distorted anions (0.113 Å rms)[19]

In solution, the rhodizonate anion is not hydrated.[3]

See also

References
  1. Patton, E.; West, R. (1970). "New aromatic anions. VIII. Acidity constants of rhodizonic acid". Journal of Physical Chemistry. 74 (12): 2512–2518. doi:10.1021/j100706a018.
  2. Braga, D.; Cojazzi, G.; Maini, L.; Grepioni, F. (2001). "Reversible solid-state interconversion of rhodizonic acid H2C6O6 into H6C6O8 and the solid-state structure of the rhodizonate dianion C
    6    O2−
    6     (aromatic or non-aromatic?)". New Journal of Chemistry. 25: 1221−1223. doi:10.1039/b107317f.
  3. Gelb, R. I.; Schwartz, L. M.; Laufer, D. A. (1978). "The structure of aqueous rhodizonic acid". Journal of Physical Chemistry. 82 (18): 1985–1988. doi:10.1021/j100507a006.
  4. Chalmers, R. A.; Telling, G. M. (1967). "A reassessment of rhodizonic acid as a qualitative reagent". Microchimica Acta. 55 (6): 1126–1135. doi:10.1007/BF01225955. S2CID 98540174.
  5. Di Maio, V. J. M. (1998). Gunshot Wounds: Practical aspects of firearms, ballistics, and forensic techniques (2nd ed.). CRC. p. 341. ISBN 0-8493-8163-0.
  6. Glover, R. L. (1981). "Detecting lead in "arrow" wounds in deer using rhodizonic acid". Wildlife Society Bulletin. 9 (3): 216–219. JSTOR 3781843.
  7. Heller, J. F. (1837). "Die Rhodizonsäure, eine aus den Produkten der Kaliumbereitung gewonnene neue Säure, und ihre chemischen Verhältnisse" [Rhodizonic acid, one of the new acids derived from potassium preparations, and its chemical composition]. Justus Liebigs Annalen der Pharmacie. 24 (1): 1–16. Retrieved 2009-07-08.
  8. Turner, E.; Gregory, W.; Parnell, E. A.; Liebig, J.; Rogers, J. B. (1846). Elements of Chemistry. Thomas, Cowperthwait & Co. Retrieved 2009-07-30. When potassium is heated in carbonic acid gas, combination takes place, and a dark olive powder is formed, composed of carbonic oxide and potassium, in the proportion C7O7+K3, or 7CO+3K. This substance is formed in large quantity in the preparation of potassium from carbonate of potash and charcoal, and is the source of great loss and inconvenience. No such compound is formed with sodium, for which reason that metal may be more cheaply prepared than potassium.
  9. Löwig, C. (1839). Chemie der organischen Verbindungen [Chemistry of Organic Compounds]. Zürich: F. Schultess.
  10. Hunter, R.; Morris, C., eds. (1900). "Universal Dictionary of the English language". New York: Collier. Retrieved 2009-08-07. Cite journal requires |journal= (help)
  11. Preisler, P. W.; Berger, L. (1942). "Preparation of Tetrahydroxyquinone and Rhodizonic Acid Salts from the Product of the Oxidation of Inositol with Nitric Acid". Journal of the American Chemical Society. 64 (1): 67–69. doi:10.1021/ja01253a016.
  12. Feigl, F.; Oesper, R. E. (1960). Spot Tests in Organic Analysis. Retrieved 2009-07-30.
  13. Gmelin, L.; Watts, H. (1856). Hand-book of Chemistry. Cavendish Society.
  14. Chen, H.; Armand, M.; Courty, M.; Jiang, M.; Grey, C. P.; Dolhem, F.; Tarascon, J.-M.; Poizot, P. (2009). "Lithium salt of tetrahydroxybenzoquinone: toward the development of a sustainable Li-ion battery". Journal of the American Chemical Society. 131 (25): 8984–8. doi:10.1021/ja9024897. PMID 19476355.
  15. Wyrwas, R. B.; Chick Jarrold, C. (2006). "Production of C
    6    O−
    6     from oligomerization of CO on molybdenum anions". Journal of the American Chemical Society. 128 (42): 13688–9. doi:10.1021/ja0643927. PMID 17044687.
  16. Iraci, G.; Back, M. H. (1988). "The photochemistry of the rhodizonate dianion in aqueous solution". Canadian Journal of Chemistry. 66 (5): 1293. doi:10.1139/v88-209.
  17. Zhao, B.; Back, M. H. (1991). "The photochemistry of the rhodizonate dianion in aqueous solution". Canadian Journal of Chemistry. 69 (3): 528. doi:10.1139/v91-079. Archived from the original on 2012-07-07. Retrieved 2009-08-07.
  18. Cowan, J. A.; Howard, J. A. K. (2004). "Dipotassium rhodizonate". Acta Crystallographica. E60 (4): m511–m513. doi:10.1107/S160053680400529X.
  19. Dinnebier, R. E.; Nuss, H.; Jansen, M. (2005). "Disodium rhodizonate: a powder diffraction study". Acta Crystallographica. E61 (10): m2148–m2150. doi:10.1107/S1600536805030552.
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