Oxonium (also: Oxonium ion , outdated but common: Hydroxonium and Hydronium , according to IUPAC strictly systematic, but uncommon: Oxidanium ) is the name for a protonated water molecule (H 3 O + ). As a rule, hydrated ions such as H 9 O 4 + (H 3 O + · 3 H 2 O) are also referred to as oxonium or hydroxonium ions.
The charge carrier is essentially the oxygen atom and it is a cation , which can be derived from the word Oxonium , since Ox (o) ... stands for oxygen atom and ... onium for a cation. Oxonium is formed by protolytic reactions in water or in aqueous solutions.
In a broader sense, oxonium ions are a collective term for organic derivatives of the ion H 3 O + in the form [R − OH 2 ] + [R 2 OH] + [R 3 O] + , where R stands for organic radicals . Oxygen atoms also carry the charge in these cations.
If hydronium (also: hydronium ion ) is present as a hydrated H + particle, the terms H + , H + aq or H + · H 2 O are often chosen out of habit or convenience . What is meant, however, is usually H 3 O + particles. The designation hydronium and the particle H + are not very suitable for the formulation of chemical reactions, since a hydron (the H + particle, also called a proton ) does not exist in free form in solutions or in compounds, but immediately mixes with the next best molecule combines and therefore alone neither reactant nor product of a reaction.
Oxonium ions are created by autoprotolysis of water , whereby a proton (H + ) is transferred from one water molecule to another. In equilibrium in neutral water, at a temperature of 25 ° C, there is a molar concentration of oxonium ions (as well as hydroxide ions ) of 10 −7 mol / l, which defines the pH value 7.
- As a result of the autoprotolysis of the water, oxonium and hydroxide ions are constantly generated even in neutral water, which react back to water molecules.
The addition of acids increases this equilibrium concentration through the transition of the protons from the acid to water molecules, and the pH value is lowered. In alkaline solutions , the pH is increased as the concentration of oxonium ions decreases.
- Reaction of hydrochloric acid with water to form the oxonium ion and chloride anion.
The lifespan of the oxonium ion is very short (around 10 −13 seconds ), as the attached proton is very easily passed on to another water molecule:
- Transfer of a proton from one water molecule to another.
In solution there is a continuous transition between differently hydrated protons. During the transfer, a proton is always passed on from one oxygen atom to the next. The positions of the individual atoms change only minimally. This defect migration, which is also known as the Grotthuss mechanism , is the cause of the high equivalent conductivity of protons of 315 S · cm 3 · mol −1 compared to other ions .
The Zundel ion can formally be thought of as a proton that is hydrated by two water molecules:
- Zundel Ion (+1)
In contrast, the Eigen ion (formally [H 9 O 4 ] + or Tetraoxidanium) is considered to be an oxonium hydrated by three water molecules.
- Self-ion (+1)
Investigations by means of infrared spectroscopy showed a hydrated H 13 O 6 + ion, with the oxygen atoms of two water molecules immediately adjacent to the H + in the center being at a greater distance from one another than in the Zundel complex. Four more water molecules still belong to this ion, which is surrounded by eight hydrate molecules.
Since all such ion formation and hydration in aqueous solution do not play a role in stoichiometric calculations, the notation H 3 O + (the actual oxonium ion) or even just H + ( hydron ) is usually used. However, free protons are practically non-existent in aqueous solutions.
Stable oxonium salts are only used by the very strongest acids, e.g. B. perchloric acid , formed:
- Water is protonated by perchloric acid. This creates an oxonium ion and the perchlorate anion . Both together form the oxonium salt.
Oxonium ions in organic chemistry
Oxonium ions in organic compounds usually occur as intermediate stages of additions , substitutions , dehydration (of alcohols) and the pinacol rearrangement . They occur as protonated alcohols (alkoxonium ions), ethers (dialkoxonium ions), or more rarely as protonated carboxylic acids or phenols (phenolxonium ions). Most organic oxonium ions dissociate very quickly by splitting off water / alcohol or by deprotonation . Since water is a good leaving group , oxonium ions are an important intermediate in many substitution and elimination reactions.
Di- / alkoxonium ions in S N 2 reactions
Since OH - is a bad leaving group, the formation of di- or alkoxonium ions is usually necessary as the first step for a nucleophilic substitution of an OH group . The protonation of an OH or OR group is carried out by acids. A nucleophile can then easily attack the carbon atom that carries the OH 2 + group. To illustrate, the nucleophilic substitution of methanol (CH 3 OH) with hydrochloric acid (HCl) is shown below:
- 1st step: Protonation of the OH group and thus formation of the alkoxonium ion.
- 2nd step: Nucleophilic attack of the chloride anion and thus formation of chloromethane .
The reaction takes place analogously with ethers. Instead of the alkoxonium ion, a dialkoxonium ion would be obtained as an intermediate.
Di- / alkoxonium ions in acid-catalyzed addition to alkenes
Water and alcohols do not normally add to alkenes. They are too weak acids, i.e. their hydrogen atoms can not be removed from the double bond of the alkene . It is different with oxonium ions. The double bond can remove a hydrogen of an oxonium ion. The resulting carbenium ion is attacked nucleophilically by water or alcohol. The reaction is therefore only acid-catalyzed. The mechanism of the acid-catalyzed addition of water to ethene (C 2 H 4 ) is shown below as an example:
- 1st step: An oxonium ion releases an H + to ethene. A carbenium ion is formed .
- 2nd step: The carbenium ion is attacked nucleophilically by the water. The product of this intermediate step is an oxonium ion bound to an organic residue. More precisely, an alkoxonium (protonated alcohol).
- 3rd step: The ethanoxonium ion (protonated ethanol) transfers H + to a water molecule. H 3 O + and ethanol are formed. The catalyst was thus regenerated again.
The reaction proceeds analogously with alcohols. The addition of an alcohol would result in an ether. No reaction takes place without acid.
Alkoxonium ions as an intermediate stage in solvolysis
Alkoxonium ions can also be observed as an intermediate step in a nucleophilic substitution (according to the S N 1 mechanism) with water. A unimolecular nucleophilic substitution of tert-butyl chloride to tert-butanol is shown below:
- 1st step: Dissociation to a carbenium ion and chloride ion.
- Step 2: Nucleophilic attack by water. In this step, the product is an alkoxonium ion.
- 3rd step: Alkoxonium ions are very acidic and deprotonate quickly.
Protonated carboxylic acids
Carboxylic acids show typical pK s values 1 to 5 Protonation can be done at very weakly basic oxygen atom of the carbonyl group with very strong mineral acids. Protonated carboxylic acids have a pK s of about -6. These intermediate stages are formed in all acid-catalyzed esterifications.
Phenol is a weak acid (p K S 9.99) and a weak base at the same time. For protonation, very strong acids such as B. Hydrogen iodide. The p K S of a phenoloxonium ion is −6.7. Phenyl alkyl ethers can only be split under the catalysis of hydrogen iodide (reversal of the Williamson ether synthesis ). The phenoloxonium intermediate formed as an intermediate breaks down into phenol and alkyl iodide.
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