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Sulfoxylic acid

Names
IUPAC name Sulfanediol
Other names hyposulfurous acid sulfur dihydroxide dihydroxidosulfur sulfanediol 2-Thiatrioxidane
Identifiers
CAS Number	20196-46-7
3D model (JSmol)	Interactive image
ChEBI	CHEBI:33536
ChemSpider	4574173
Gmelin Reference	1452
PubChem CID	5460696
CompTox Dashboard (EPA)	DTXSID201316909
InChI InChI=1S/H2O2S/c1-3-2/h1-2HKey: HRKQOINLCJTGBK-UHFFFAOYSA-N
SMILES OSO
Properties
Chemical formula	S(OH)2
Molar mass	66.07 g·mol
Conjugate base	Bisulfoxylate (chemical formula SO2H)
Related compounds
Related isoelectronic	trioxidane trisulfane
Related compounds	hydroxysulfonyl radical HOSO2 sulfinic acid sulfenic acid HSOH dihydroxydisulfane HOSSOH
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

Sulfoxylic acid (H₂SO₂) (also known as hyposulfurous acid or sulfur dihydroxide) is an unstable oxoacid of sulfur in an intermediate oxidation state between hydrogen sulfide and dithionous acid. It consists of two hydroxy groups attached to a sulfur atom. Sulfoxylic acid contains sulfur in an oxidation state of +2. Sulfur monoxide (SO) can be considered as a theoretical anhydride for sulfoxylic acid, but it is not actually known to react with water.

The complementary base is the sulfoxylate anion SO
₂ which is much more stable. In between these states is the HSO
₂ ion, also somewhat stable.

Sulfoxylate ions can be made by decomposing thiourea dioxide in an alkaline solution. To do this, thiourea dioxide first forms an amidine-sulfinic acid tautomer, H₂NC(=NH)SO₂H, which then breaks apart. Sulfoxylate reacts with formaldehyde to yield a hydroxymethanesulfinate called rongalite:

HSO
₂ + H₂CO → HOCH
₂SO
₂,

which is an important chemical for dyeing.

Formation

Sulfoxylic acid has been detected in the gas phase. It is likely to be formed as an intermediate when hydrogen sulfide is oxidised by living organisms, or in the atmosphere, or anywhere else in the natural environment. It may also exist in circumstellar disks. When H₂S is oxidised it starts from oxidation state −2, and should then pass through intermediate values of 0 and +2 before getting to well known sulfite at +4 and sulfate at +6. When sulfide in alkaline conditions is oxidised by air in the presence of nickel ions, sulfoxylate concentration first increases to around 5% and then decreases over several days. Polysulfide concentration also grows and then shrinks on a slower timescale reaching about 25% of the sulfide. The sulfur ends up forming thiosulfate.

Sulfoxylic acid has been made by ultraviolet irradiation of a mixture of solid H₂S and H₂O, followed by warming. This is a possible natural process in comets or circumstellar disks.

Fender et al. claimed to make "sulfinic acid" (an isomer of sulfoxylic acid) by ultraviolet irradiation on solid sulfur dioxide and hydrogen sulfide in a solid argon matrix, measuring the infrared vibrational spectrum. However the assignment of the lines in the spectrum is doubtful, so this may not be the substance produced.

Sulfoxylic acid can be made in the gas phase in an electric discharge through a neon, H₂, SO₂ mixture. This also yields some sulfhydryl hydroperoxide.

Properties

Sulfoxylic acid is an isomer of sulfinic acid, which has a hydrogen atom bonded to the sulfur, and the oxygen connected with a double bond (HS(O)OH). Other isomers are thiadioxirane (a ring of two oxygen atoms and a sulfur), dihydrogen sulfone (a sulfur atom linked to two hydrogen and two oxygen atoms), sulfhydryl hydroperoxide (HSOOH), and dihydrogen persulfoxide H₂SOO. Sulfoxylic acid has the lowest energy of any of these isomers.

The pK_a1 of sulfoxylic acid is 7.97. The pK_a2 of bisulfoxylate (HSO
₂) is 13.55.

Calculations of the molecule suggest there may be two alignments termed C₂, and C_s. The H−O distance is 96.22 (or 96.16) pm, S−O distance is 163.64 (or 163.67) pm, ∠HOS = 108.14° (108.59°), ∠OSO = 103.28° (103.64°) HOSO twist is 84.34° (+90.56 and −90.56) (C_s dimensions in parentheses).

The microwave spectrum has absorption lines at 10.419265, 12.2441259, 14.0223698, 16.3161779 GHz and many others for the C_s and 12.8910254, 19.4509030, 21.4709035, 24.7588445, 29.5065050, 29.5848250, 32.8772050 GHz for the C₂ form.

The sulfoxylate ion apparently has an X-ray absorption near edge structure at 2476.1 eV. With sulfur the X-ray absorption edge changes with oxidation state as per Kunzl's law. The edge corresponds to the energy needed to excite and inner S electron to a P orbital. Sulfoxylate has an infrared absorption peak at 918.2 cm.

Reactions

Sulfoxylic acid disproportionates into sulfur and hydrogensulfite HSO
₃. Some of this in turn reacts to form thiosulfate S
₂O
₃.

Sulfoxylates are sensitive to air, and will be oxidised by the oxygen in it.

Sulfoxylate is oxidised to sulfur dioxide radical anion and then to sulfur dioxide.

SO
₂ + O₂ → SO
₂ + O
₂

SO
₂ + O₂ → SO₂ + O
₂

2 SO
₂ ⇌ S
₂O
₄ (dithionite)

The known sulfoxylate salts include cobalt sulfoxylate CoSO₂·3H₂O. This can dissolve in an ammoniacal solution. However cobalt sulfide will precipitate if sulfide is formed during a reaction.

Sulfoxylate in solution reacts with thiosulfate to form sulfides and sulfites.

Sulfoxylate reduces nitrite to hydronitrite radical dianion NO
₂. This in turn reacts with water forming hydroxide ions and nitric oxide (NO). Nitric oxide and nitrous oxide N₂O in turn are further reduced by sulfoxylate.

When sulfoxylate reacts with hypochlorite, bromine or chlorine dioxide it forms hydrogen sulfite and sulfates.

Dithionite is unstable in a pH 4 solution, decomposing to sulfoxylic acid and hydrogen sulfite. This sulfoxylic acid reacts with more dithionite to yield more hydrogen sulfite, and some kind of sulfur, and a small amount of thiosulfate.

S
₂O
₄ + H → H₂SO₂ + HSO
₃

S
₂O
₄ + H₂SO₂ → 2 HSO
₃ + S

S
₂O
₄ + H₂SO₂ → HSO
₃ + S
₂O
₃ + H

By reducing sulfur dioxide, hydrogen sulfoxylate forms as an intermediate, and this is much more reactive. Hydrogen sulfoxylate reacts with organic compounds with a double bond (vinyls) to make an organic sulfinate. Hydrogen sulfoxylate reacts with divinyl sulfone to make 1,4-dithiane 1,1,4,4-tetroxide. Perfluorophenyl iodide is reduced to pentafluorobenzene.

The reaction of sulfoxylic acid with sulfite yields trithionate (S
₃O
₆) and with thiosulfate yields pentathionate (S
₅O
₆).

Salts

Salts of sulfoxylic acid that have been claimed to have been made include cobalt, thallium and zinc. Cobalt sulfoxylate is made from sodium hyposulfite, cobalt chloride and ammonia. Zinc sulfoxylate is produced by reacting zinc metal with sulfuryl chloride. Thallous sulfoxylate is made by allowing oxygen onto thallous sulfide. This is olive brown in colour. When heated to 250 °C it recrystallizes to another yellow form. Cuprous sufoxylate Cu₂SO₂ can be made as a solid or liquid by heating cuprous sulfide and copper sulfate. Cu₂SO₂ melts at 610 K (337 °C) and is stable as a liquid phase to over 680K. There are also solid phase transitions at 382 K (109 °C) and 423 K (150 °C).

Derivatives

Sulfoxylic acid forms a wide variety of organic esters of the form R–O–S–O–R′.

The nomenclature for these molecules is not entirely standardized, and a wide variety of IUPAC-acceptable names are possible. For substances with the −OSOH group, one can use suffixes ‑oxy­sulfanol (preferred), ‑hydrogen sulfoxylate, or ‑oxy­sulfenic acid; or prefixes hydroxy­sulfanyl­oxy- (preferred) or sulfeno­oxy-. The ionic group –OSO can use the preferred suffix ‑oxy­sulfanolate or ‑sulfoxylate; or preferred prefix oxy­sulfanolato- or sulfenato­oxy-. The R-OSO-R' can be suffixed with ‑dioxy­sulfane or ‑sulfoxylate; or prefixed with oxy­sulfanyl­oxy- or sulfeno­oxy-.

In general, sulfoxylate esters are synthesized from sulfur dichloride and a primary or secondary alcohol, to give a diorganyl sulfoxylate (ROSOR) compound. Because sulfur dichloride unavoidably forms some disulfur dichloride and chlorine in solution, this technique also forms di(organyloxy) disulfides and dialkyl sulfites. The yield of sulfoxylates is maximised at low temperatures (around -75 °C) and with dilute reactants. Successfully synthesized compounds include the methyl, ethyl, propyl, isopropyl, n-butyl, n-pentyl, and cholesterol esters, as well as the hemiester para‑(hydroxysulfanyl)­cresol (i.e., 1‑hydroxysulfanyloxy-4‑methylbenzene). With 1,2‑diols, SCl₂ forms polymeric sulfoxylates; for 1,3-diols, SCl₂ reaction creates cyclic oligomers (typically, a six-member 1,3,2-dioxathiane or a twelve-membered cyclic sulfoxylate dimer). Allylic, propargylic, or benzylic sulfoxylate esters are generally unstable, rearranging to give a sulfinate ester; although they are stable at −18 °C if chloride is promptly removed from the solution. The rearrangment appears to develop a temporary positive charge on the organyl group, as it is inhibited at the benzyl position of electron-poor arenes.

Diethylsulfoxylate can be made by reacting diethoxydisulfide with sodium ethoxide.

Sulfoxylic acid dimethyl ester (also called dimethylsulfoxylate or dimethoxysulfane) (S(OCH₃)₂) has been studied using electron diffraction, X-ray crystallography, Raman spectroscopy, and infrared spectroscopy. Studying this molecule is much easier than the unstable sulfoxylic acid. It is a liquid at standard conditions that boils at 74 °C and freezes at -67° In the gas state the molecular bond angle ∠OSO is 103°, with the oxygen-sulfur distance 1.625 Å. The oxygen-carbon distance is 1.426 Å and carbon-hydrogen distance 1.105 Å with ∠COS 115.9° and ∠COS 109°. The molecule as C₂ symmetry with one methyl group rotated above the plane determined by the OSO atoms, and the other below. The C−O−S−O dihedral angle is about 84°. The energy barrier to moving one methyl group to the other side of the plane is 37 kJ/mol and it would be in a 12 kJ/mol higher energy state in this C_s symmetry state.

References

1. ^ Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Academic Press. ISBN 9780123526519.
2. Koritsanszky, Tibor; Juergen Buschmann; Peter Luger; Heinar Schmidt; Ralf Steudel (1994). "Sulfur compounds. Part 173. Structure and Electron Density of Solid Dimethoxydisulfane, (CH₃O)₂S₂". The Journal of Physical Chemistry. 98 (21): 5416–5421. doi:10.1021/j100072a005. ISSN 0022-3654.
3. Eldridge, D.L.; Guo, W.; Farquhar, J. (December 2016). "Theoretical estimates of equilibrium sulfur isotope effects in aqueous sulfur systems: Highlighting the role of isomers in the sulfite and sulfoxylate systems". Geochimica et Cosmochimica Acta. 195: 171–200. Bibcode:2016GeCoA.195..171E. doi:10.1016/j.gca.2016.09.021. hdl:1912/8677.
4. ^ Makarov, S. V.; Salnikov, D. S.; Pogorelova, A. S. (9 March 2010). "Acid-base properties and stability of sulfoxylic acid in aqueous solutions". Russian Journal of Inorganic Chemistry. 55 (2): 301–304. doi:10.1134/S0036023610020269. S2CID 95780677.
5. Grady, B.J.; Dittmer, D.C. (November 1990). "Reaction of perfluoroaryl halides with reduced species of sulfur dioxide (HSO
   ₂, SO
   ₂, S
   ₂O
   ₄)". Journal of Fluorine Chemistry. 50 (2): 151–172. doi:10.1016/S0022-1139(00)80493-5.
6. ^ Crabtree, Kyle N.; Martinez, Oscar Jr.; Barreau, Lou; McCarthy, Michael C.; Thorwirth, Sven (2013). "Detection of the rotational spectrum of sulfoxylic acid (HOSOH)". hdl:1811/55161. {{cite journal}}: Cite journal requires |journal= (help)
7. ^ Vairavamurthy, Murthy A.; Zhou, Weiqing (1995). "15". Geochemical transformations of sedimentary sulfur: developed from a symposium sponsored by the Division of Geochemistry, Inc. at the 208th National Meeting of the American Chemical Society, Washington, DC, August 21 - 25, 1994. Washington, DC: American Chemical Society. pp. 280–292. doi:10.1021/bk-1995-0612.ch015. ISBN 9780841233287.
8. ^ Crabtree, Kyle N.; Martinez, Oscar; Barreau, Lou; Thorwirth, Sven; McCarthy, Michael C. (2 May 2013). "Microwave Detection of Sulfoxylic Acid (HOSOH)". The Journal of Physical Chemistry A. 117 (17): 3608–3613. Bibcode:2013JPCA..117.3608C. doi:10.1021/jp400742q. PMID 23534485.
9. Napolion, Brian; Huang, Ming-Ju; Watts, John D. (May 2008). "Coupled-Cluster Study of Isomers of H₂SO₂". The Journal of Physical Chemistry A. 112 (17): 4158–4164. Bibcode:2008JPCA..112.4158N. doi:10.1021/jp8009047. PMID 18399676.
10. ^ van der Heijde, Herman B. (2 September 2010). "Tracer studies in sulfoxylic acid chemistry: (Short communication)". Recueil des Travaux Chimiques des Pays-Bas. 73 (3): 193–196. doi:10.1002/recl.19540730304.
11. ^ Makarov, Sergei V.; Makarova, Anna S.; Silaghi-Dumitrescu, Radu (2014). "Sulfoxylic and thiosulfurous acids and their dialkoxy derivatives". Patai's Chemistry of Functional Groups. John Wiley & Sons. pp. 266–273. doi:10.1002/9780470682531.pat0829. ISBN 9780470682531.
12. Fentress, J.; Selwood, P. W. (February 1948). "Thallous Sulfoxylate Isomerism". Journal of the American Chemical Society. 70 (2): 711–716. doi:10.1021/ja01182a083.
13. Dębinski, H.; Walczak, J. (January 1987). "The Cu₂SO₂ phase". Journal of Thermal Analysis. 32 (1): 35–41. doi:10.1007/BF01914544. S2CID 102200739.
14. Huibers, Martijn (2009). Sulfation via sulfite and sulfate diesters and synthesis of biologically relevant organosulfates. Nijmegem: Radboud University. p. 114. hdl:2066/75818. ISBN 9789490122515.
15. ^ Thompson, Q. E. (August 1965). "Organic Esters of Bivalent Sulfur. 111. Sulfoxylates". The Journal of Organic Chemistry. 30: 27032707.
16. S. Braverman and T. Pechenick, Tetrahedron Lett., 43, 499 (2002). doi:10.1016/S0040-4039(01)02174-8
17. Tardif, Sylvie L.; Harpp, David N. (2000) . "Preparation of 4-substituted benzyl sulfoxylates and related chemistry". Journal of Organic Chemistry. 65 (16). American Chemical Society: 4791–4795. doi:10.1021/jo991360b.
18. Baumeister, Edgar; Oberhammer, Heinz; Schmidt, Heinar; Steudel, Ralf (December 1991). "Gas phase structure and vibrational spectra of dimethoxysulfane (CH₃O)₂S". Heteroatom Chemistry. 2 (6): 633–641. doi:10.1002/hc.520020605.

Extra reading

• Steiger, Thomas; Steudel, Ralf (May 1992). "Sulphur compounds Part 149. Structures, relative stabilities and vibrational spectra of several isomeric forms of sulphoxylic acid (H₂SO₂) and its anion (HSO
  ₂): an ab initio study". Journal of Molecular Structure: THEOCHEM. 257 (3–4): 313–323. doi:10.1016/0166-1280(92)85048-P.
• Tossell, J.A. (August 1997). "Theoretical studies on possible sulfur oxides with +2 oxidation states in aqueous solution". Chemical Geology. 141 (1–2): 93–103. Bibcode:1997ChGeo.141...93T. doi:10.1016/S0009-2541(97)00065-X.

• Sulfur(II) compounds
• Sulfur oxoacids