Oxonium ion

Oxonium (also: oxonium ion, dated but widespread: hydroxonium and hydronium, strictly according to IUPAC systematic, but uncommon: Oxidanium ) is the name for a protonated water molecule ( H3O ). As a rule, hydrated ions such as H9O4 ( H3O · 3 H2O) are referred to as oxonium or hydroxonium ions.

Charge carriers is essentially the oxygen atom and is a cation, which can be derived from the word oxonium because Ox ( o) ... is oxygen atom and ... onium a cation. Oxonium formed by protolytic reactions in water or in aqueous solutions.

In a broader sense oxonium ion is a collective term for organic derivatives of the ion H3O in the form [ R- OH2 ] [ R 2 OH ] [ R 3 O ] , where R stands for organic radicals. Even with these cations carry oxygen atoms the charge.

Lying oxonium ( H3O ) or its derivatives as organic cation of a salt, one counts the connections to the oxonium salts.

If hydronium (also: hydronium ion ) as a hydrated H particles before, the designations H , H or H aq · H2O are often chosen out of habit or out of convenience. However, this refers are H3O particles in the rule. The term hydronium and the particles H are suitable for the formulation of the chemical reactions is little as a Hydron (the H - particles and proton called ) in free form, in solutions or in the compounds does not exist, but combines immediately with the next best molecule and therefore alone neither reactant is still the product of a reaction.

Properties

The pKa value of the oxonium ion is a fixed point of the proto -chemical series zero.

Oxonium ions resulting from the autoprotolysis of the water, a proton (H ) moves from one water molecule to another. In the balance is in neutral water, at a temperature of 25 ° C, a molar concentration of hydronium ions ( as the hydroxide ions ) of 10-7 mol / l, whereby the pH is 7 defined.

By the addition of acids, this equilibrium concentration increased by transfer of protons from the acid to the water molecules, the pH is lowered. In alkaline solutions, the pH is increased as the concentration of hydronium ions is less.

The oxonium ion is the strongest acid which may exist in aqueous solution. Stronger acids than the oxonium ion (eg H2SO4 or HCl) dissociate completely in water to H3O

Means Strong and weak acids (eg acetic acid ) transferred only account for a portion of their protons to the water.

The life of the oxonium ion is very short ( about 10-13 seconds ), since the annealed proton is very easily passed on to another water molecule:

In solution, a continuous transition between different hydrated protons takes place. When transferring a proton from one oxygen atom is always passed on to the next. Here, the positions of the individual atoms change only minimally. This defect migration, which is also referred to as Grotthuss mechanism is the cause of the in comparison to other high ion conductivity of equivalent protons of 315 S · cm3 · mol -1.

The Zundel ion and the self -Ion represent possible Hydratisierungsformen Represents the Zundel ion can be formally considered as a proton, which is hydrated by two water molecules:

In contrast, the self - ion ( formally [ H9O4 ] and Tetraoxidanium ) is regarded as one of the three water molecules hydrated oxonium.

Studies based on infrared spectroscopy revealed a hydrated H13O6 ion, the standing in the center of the H immediately adjacent oxygen atoms of two water molecules to each other at a greater distance than in Zundelkomplex. Four other water molecules still belong to this ion, which is surrounded by eight Hydratmolekülen.

Since all such ions training and hydrations play in aqueous solution at stoichiometric calculations not matter, by convention, the H3O (the actual oxonium ion ) or even H ( hydrogen ion ) will be used. However, free protons are in aqueous solutions practically inexistent.

Oxonium salts

Stable oxonium salts are formed only by the strongest of acids, eg perchloric acid:

Oxonium ions in the organic chemistry

Oxonium ions in organic compounds usually occur as intermediates of additions, substitutions, dehydration ( alcohols ) and the pinacol rearrangement on. They act as protonated alcohols ( Alkoxonium ions), ethers ( Dialkoxonium ions), or more rarely as protonated carboxylic acids or phenols ( Phenolxonium ions) on. Most organic oxonium ions dissociate very rapidly by Wasser-/Alkoholabspaltung, or deprotonation. Since water is a good leaving group, oxonium ions are an important intermediate stage for many substitution Us elimination reactions.

Di-/Alkoxonium-Ionen in SN2 reactions

Since OH a bad leaving group, is the formation of di-or Alkoxonium ions, as a first step for a nucleophilic substitution of an OH group, usually necessary. The protonation of an OH or OR group is carried out by acids. Subsequently, a nucleophile to attack the carbon atom easily carrying the OH2 group. For illustration, the nucleophilic substitution of methanol ( CH3OH ) with hydrochloric acid ( HCl) is shown below:

The reaction is analogous to ethers. Instead of Alkoxonium ion would be obtained as an intermediate, a Dialkoxonium ion.

Di-/Alkoxonium-Ionen in acid-catalyzed addition to alkenes

Water and alcohols usually do not add to alkenes. They are weak acids, that is, their hydrogen atoms can not be removed from the double bond of the alkene. Not so with oxonium ions. The double bond can remove a hydrogen of an oxonium ion. The resulting carbenium ion is nucleophilic attack by water or alcohol. Therefore, the reaction proceeds only under acid catalysis. By way of example, the mechanism of acid-catalyzed addition of water is shown to ethene ( C2H4) in the following:

The reaction is analogous with alcohols. In the addition of an alcohol, one would obtain an ether. Without acid no reaction takes place.

Alkoxonium ion as an intermediate solvolysis

Alkoxoniumionen can be observed with water as an intermediate step a nucleophilic substitution ( by SN1 mechanism). Below is a unimolecular nucleophilic substitution of tert- butyl chloride is shown to tert- butanol:

Protonated carboxylic acids

Oxonium ions can also be present in the form of the protonated carboxylic acids. Carboxylic acids are more difficult to protonate than alcohols. Alkoxoniumionen have a pKa of about -3. Protonated carboxylic acids have a pKa of about -6. The protonation would occur due to the resonance stabilization of the carbonyl carbon.

Phenoloxonium ion

Phenols and their ethers can be protonated. However, these split off water or alcohol to form a phenyl cation. In the presence of a nucleophile, however, the ether of a phenol can be cleaved to a phenol. The pK a Phenoloxonium - ion is -6.7.