Inorganic nonaqueous solvent
An inorganic nonaqueous solvent is a solvent other than water, that is not an organic compound. Common examples are liquid ammonia, liquid sulfur dioxide, sulfuryl chloride and sulfuryl chloride fluoride, phosphoryl chloride, dinitrogen tetroxide, antimony trichloride, bromine pentafluoride, hydrogen fluoride, sulfuric acid and other inorganic acids. These solvents are used in chemical research and industry for reactions that cannot occur in aqueous solutions or require a special environment.
Applications in research
Inorganic solvents are often employed to study highly electrophilic or highly oxidizing compounds or ions. The generation of [IS7]+ and [BrS7]+ are illustrative. They are prepared in SO2 solution, and precipitated with SO2FCl.[1] The preparation of SBr3]+ salts also calls for SO2 and SO2FCl.[2]
The combination of HF and SbF6 is the basis of a superacid solution. Using this mixture, hydrogen sulfide can be isolated as its conjugate acid:
- H2S + HF SbF5 → [H3S]SbF6
The chemistry of xenon compounds is often conducted in hydrogen fluoride or bromine pentafluoride, which dissolve readily both xenon difluorides and its multiple derivatives,[3] Sulfuryl chloride fluoride is also useful for strong oxidants.[4]
Sulfuryl chloride fluoride is the solvent of choice for handling extreme oxidants. For example, it can be used to generate and study free carbocations[5] and arenium ions.[6]
Nonaqueous acid-base chemistry
Autoionization
Many inorganic solvents participate in autoionization reactions. In the solvent system definition of acids and bases, autoionization of solvents affords the equivalent to acids and bases. Relevant autoionizations:
- 2BrF3 BrF2+ + BrF4-
- N2O4 ⇌ NO+ (nitrosonium) + NO3− (nitrate)
- 2SbCl3 ⇌ SbCl2+ (dichloroantimonium) + SbCl4− (tetrachloroantimonate)
- 2POCl3 ⇌ POCl2+ + POCl4−
According to the solvent-system definition, acids are the compounds that increase the concentration of the solvonium (positive) ions, and bases are the compounds that result in the increase of the solvate (negative) ions, where solvonium and solvate are the ions found in the pure solvent in equilibrium with its neutral molecules:
The solvent SO2 is a relatively uncomplicated, it does not autoionize.
Limiting acids and limiting base
The limiting acid in a given solvent is the solvonium ion, such as H3O+ (hydronium) ion in water. An acid which has more of a tendency to donate a hydrogen ion than the limiting acid will be a strong acid in the solvent considered, and will exist mostly or entirely in its dissociated form. Likewise, the limiting base in a given solvent is the solvate ion, such as OH− (hydroxide) ion, in water. A base which has more affinity for protons than the limiting base cannot exist in solution, as it will react with the solvent.
For example, the limiting acid in liquid ammonia is the ammonium ion, which has a pKa value in water of 9.25. The limiting base is the amide ion, NH2−. NH2− is a stronger base than the hydroxide ion and so cannot exist in aqueous solution. The pKa value of ammonia is estimated to be approximately 34 (c.f. water, 14[7][8]).
References
- ^ . doi:10.1002/9780470132586.ch67.
{{cite journal}}: Cite journal requires|journal=(help); Missing or empty|title=(help) - ^ . doi:10.1002/9780470132555.ch23.
{{cite journal}}: Cite journal requires|journal=(help); Missing or empty|title=(help) - ^ Pointner BE, Suontamo RJ, Schrobilgen GJ. Syntheses and X-ray crystal structures of alpha- and beta-[XeO
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2F][AsF
6], [FO
2XeFXeO
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6], and [XeF
5][SbF
6)]·XeOF
4 and computational studies of the XeO
2F+
and FO
2XeFXeO
2F+
cations and related species. Inorg Chem. 2006 Feb 20;45(4):1517-34. - ^ Mercier HP, Moran MD, Sanders JC, Schrobilgen GJ, Suontamo RJ. "Synthesis, structural characterization, and computational study of the strong oxidant salt [XeOTeF
5][Sb(OTeF
5)
6]·SO
2ClF." Inorg Chem. 2005 Jan 10;44(1):49-60. - ^ Mercier HP, Moran MD, Schrobilgen GJ, Steinberg C, Suontamo RJ. The syntheses of carbocations by use of the noble-gas oxidant, [XeOTeF
5][Sb(OTeF
5)
6]: the syntheses and characterization of the CX+
3 (X = Cl, Br, OTeF
5) and CBr(OTeF
5)+
2 cations and theoretical studies of CX+
3 and BX
3 (X = F, Cl, Br, I, OTeF
5). J Am Chem Soc. 2004 May 5;126(17):5533-48. - ^ V D Shteingarts, Polyfluorinated Arenonium Ions, Russ. Chem Rev 1981;50(8):735-748.
- ^ Meister, Erich C.; Willeke, Martin; Angst, Werner; Togni, Antonio; Walde, Peter (2014). "Confusing Quantitative Descriptions of Brønsted-Lowry Acid-Base Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators". Helvetica Chimica Acta. 97 (1): 1–31. doi:10.1002/hlca.201300321. ISSN 1522-2675.
- ^ Silverstein, Todd P.; Heller, Stephen T. (2017-06-13). "pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water?". Journal of Chemical Education. 94 (6): 690–695. doi:10.1021/acs.jchemed.6b00623. ISSN 0021-9584.