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In chemistry, an oxonium ion is any oxygen cation with three bonds.[1] The simplest oxonium ion is the hydronium ion H3O+.[2]


Hydronium is one of a series of oxonium ions with the formula R3−nHnO+. Oxygen is usually pyramidal with an sp3 hybridization. Those with n = 2 are called primary oxonium ions, an example being protonated methanol. Other hydrocarbon oxonium ions are formed by protonation or alkylation of alcohols or ethers (R−C−+O−R1R2). In acidic media, the oxonium functional group produced by protonating an alcohol can be a leaving group in the E2 elimination reaction. The product is an alkene. Extreme acidity, heat, and dehydrating conditions are usually required.

Secondary oxonium ions have the formula R2OH+, an example being protonated ethers.

Tertiary oxonium ions have the formula R3O+, an example being trimethyloxonium.[3] Tertiary alkyloxonium salts are useful alkylating agents. For example, triethyloxonium tetrafluoroborate (Et
), a white crystalline solid, can be used, for example, to produce ethyl esters when the conditions of traditional Fischer esterification are unsuitable.[4] It is also used for preparation of enol ethers and related functional groups.[5][6]

general pyramidal
oxonium ion
skeletal formula of the
trimethyloxonium cation
ball-and-stick model
of trimethyloxonium
space-filling model
of trimethyloxonium

Oxatriquinane and oxatriquinacene are unusually stable oxonium ions, first described in 2008. Oxatriquinane does not react with boiling water or with alcohols, thiols, halide ions, or amines, although it does react with stronger nucleophiles such as hydroxide, cyanide, and azide.

Oxocarbenium ionsEdit

Another class of oxonium ions encountered in organic chemistry is the oxocarbenium ions, obtained by protonation or alkylation of a carbonyl group e.g. R−C=+O−R′ which forms a resonance structure with the fully-fledged carbocation R−+C−O−R′ and is therefore especially stable:


Gold-stabilized speciesEdit

An unusually stable oxonium species is the gold complex tris[triphenylphosphinegold(I)]oxonium tetrafluoroborate, [(Ph3PAu)3O]+[BF4]. This complex is prepared by treatment of Ph3PAuCl with Ag2O in the presence of NaBF4:[7]

3Ph3PAuCl + Ag2O + NaBF4 → [(Ph3PAu)3O]+[BF4] + 2AgCl + NaCl

The stability of this complex is attributed to aurophilic interactions between gold atoms.

It has been used as a precatalyst in cationic gold catalysis.[8]

See alsoEdit

  • Onium ion, a +1 cation derived by protonation of a hydride (includes oxonium ions)
  • Pyrylium, one of the oxonium ions


  1. ^ {{citation |author= March, Jerry |authorlink= Jerry March |year= 2007 |title= Advanced Organic Chemistry: Reactions, Mechanisms, and Structure |edition= 4th |location= New York |publisher= Wiley |isbn= |page= 497
  2. ^ George A. Olah (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.
  3. ^ Olah, George A. (1993). "Superelectrophiles". Angew. Chem. Int. Ed. Engl. 32 (6): 767–88. doi:10.1002/anie.199307673.
  4. ^ Douglas J. Raber, Patrick Gariano, Jr, Albert O. Brod, Anne L. Gariano, And Wayne C. Guida "Esterification Of Carboxylic Acids With Trialkyloxonium Salts: Ethyl And Methyl 4-acetoxybenzoates" Org. Synth. 1977, 56, 59.doi:10.15227/orgsyn.056.0059
  5. ^ Justin R. Struble And Jeffrey W. Bode "Synthesis Of A N-mesityl Substituted Aminoindanol-derived Triazolium Salt" Org. Synth. 2010, 87, 362. doi:10.15227/orgsyn.087.0362
  6. ^ Lous S. Hegedus, Michael A. Mcguire, And Lisa M. Schultze "1,3-Dimethyl-3-methoxy-4-phenylazetidinone" Org. Synth. 1987, 65, 140.doi:10.15227/orgsyn.065.0140
  7. ^ Bruce, M. I.; Nicholson, B. K.; Shawkataly, O. Bin; Shapley, J. R.; Henly, T. (2007-01-05), Kaesz, Herbert D. (ed.), "Synthesis of Gold-Containing Mixed-Metal Cluster Complexes", Inorganic Syntheses, John Wiley & Sons, Inc., pp. 324–328, doi:10.1002/9780470132579.ch59, ISBN 9780470132579
  8. ^ Sherry, Benjamin D.; Toste, F. Dean (2004-12-01). "Gold(I)-Catalyzed Propargyl Claisen Rearrangement" (PDF). Journal of the American Chemical Society. 126 (49): 15978–15979. doi:10.1021/ja044602k. ISSN 0002-7863.