In
chemistry, an oxonium ion is any
cation containing an
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
atom that has three
bonds and 1+
formal charge. The simplest oxonium ion is the
hydronium
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid i ...
ion ().
Alkyloxonium
Hydronium is one of a series of oxonium ions with the formula R
''n''H
3−''n''O
+.
Oxygen is usually pyramidal with an sp
3 hybridization. Those with ''n'' = 1 are called primary oxonium ions, an example being protonated alcohol (e.g.
methanol). In acidic media, the oxonium
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
produced by protonating an alcohol can be a
leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited ...
in the
E2 elimination reaction. The product is an
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
. Extreme acidity, heat, and dehydrating conditions are usually required. Other hydrocarbon oxonium ions are formed by
protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
or
alkylation
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecti ...
of
alcohols or
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...
s (R−C−−R
1R
2).
Secondary oxonium ions have the formula R
2OH
+, an example being protonated ethers.
Tertiary oxonium ions have the formula R
3O
+, an example being
trimethyloxonium. Tertiary
alkyloxonium
salts
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
are useful
alkylating agents. For example,
triethyloxonium tetrafluoroborate
Triethyloxonium tetrafluoroborate is the organic oxonium compound with the formula CH3CH2)3OF4. It is often called Meerwein's reagent or Meerwein's salt after its discoverer Hans Meerwein. Also well known and commercially available is the relat ...
()(), a white crystalline solid, can be used, for example, to produce ethyl esters when the conditions of traditional Fischer
esterification
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
are unsuitable. It is also used for preparation of enol ethers and related functional groups.
Oxatriquinane and
oxatriquinacene are unusually stable oxonium ions, first described in 2008. Oxatriquinane does not react with boiling water or with
alcohols,
thiol
In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...
s,
halide ions, or
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
s, although it does react with stronger
nucleophiles such as
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...
,
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
, and
azide.
Oxocarbenium ions
Another class of oxonium ions encountered in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
is the
oxocarbenium ions, obtained by
protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
or
alkylation
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecti ...
of a
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
group e.g. R−C=−R′ which forms a
resonance structure with the fully-fledged
carbocation R−−O−R′ and is therefore especially stable:
Gold-stabilized species
An unusually stable oxonium species is the gold complex tris
riphenylphosphinegold(I)xonium tetrafluoroborate,
3PAu)3O">Ph3PAu)3OBF
4], where the intramolecular
aurophilicity, aurophilic interactions between the gold atoms are believed responsible for the stabilisation of the cation. This complex is prepared by treatment of Ph
3PAuCl with
Ag2O in the presence of
NaBF4:
:3 Ph
3PAuCl + Ag
2O + NaBF
4 →
3PAu)3O">Ph3PAu)3Osup>+
4">F4sup>− + 2 AgCl + NaCl
It has been used as a catalyst for the propargyl
Claisen rearrangement
The Claisen rearrangement is a powerful carbon–carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl vinyl ether will initiate a ,3sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl, ...
.
Relevance to natural product chemistry
Complex bicyclic and tricyclic oxonium ions have been proposed as key intermediates in the biosynthesis of a series of
natural products by the red algae of the genus ''
Laurencia''.
Several members of these elusive species have been prepared explicitly by total synthesis, demonstrating the possibility of their existence.
The key to their successful generation was the use of a
weakly coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
(Krossing's anion,
4">l(pftb)4sup>−, pftb = perfluoro-''tert''-butoxy) as the counteranion. As shown in the example below, this was executed by a transannular halide abstraction strategy through the reaction of the oxonium ion precursor (an
organic halide) with the silver salt of the Krossing's anion Ag
4">l(pftb)4��CH
2Cl
2, generating the desired oxonium ion with simultaneous precipitation of inorganic
silver halides. The resulting oxonium ions were characterized comprehensively by
nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fie ...
at low temperature (−78 °C) with support from
density functional theory
Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...
computation.
These oxonium ions were also demonstrated to directly give rise to multiple related
natural products by reacting with various
nucleophiles, such as water, bromide, chloride, and acetate.
See also
*
Acylium ion, a type of oxonium ion with the structure R–C≡O
+
*
Onium ion
In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake ...
, a +1 cation derived by protonation of a hydride (includes oxonium ions)
*
Pyrylium, a subtype of oxonium ion
*
Sulfonium, a sulfur analog that can be chiral
References
{{Reflist
Oxycations