Rhodizonic acid

Rhodizonic acid
Names
Preferred 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 Edit this at Wikidata
EC Number
  • 204-276-5
MeSH C005690
  • InChI=1S/C6H2O6/c7-1-2(8)4(10)6(12)5(11)3(1)9/h7-8H ☒N
    Key: WCJLIWFWHPOTAC-UHFFFAOYSA-N ☒N
  • InChI=1/C6H2O6/c7-1-2(8)4(10)6(12)5(11)3(1)9/h7-8H
    Key: WCJLIWFWHPOTAC-UHFFFAOYAV
  • C1(=C(C(=O)C(=O)C(=O)C1=O)O)O
Properties
H2C6O6
Molar mass 170.076 g·mol−1
Appearance Orange to deep-red highly hygroscopic crystals
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).
☒N verify (what is checkY☒N ?)

Rhodizonic acid is a chemical compound with formula H2C6O6 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 is usually obtained in the form of a dihydrate H2C6O6·2H2O. 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 hydrogen rhodizonate anion HC6O−6 and the rhodizonate anion C6O2−6. 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]

  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.