Flavylium
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| Names | |||
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IUPAC name
Pyrylium
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| Other names
Pyranium
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| Identifiers | |||
| 3D model (Jmol) | Interactive image | ||
| ChEBI |
CHEBI:36120 
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| ChemSpider |
7827742
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| PubChem | 9548819 | ||
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| Properties | |||
| C5H5O+ | |||
| Molar mass | 81.09 g/mol | ||
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |||
The pyrylium cation is a six-membered, unsaturated, mono-cyclic compound. Consisting of five carbon atoms and one positively charged oxygen atom, it is a heterocyclic compound, resembling benzene with one substitution in the ring. Like benzene, it is aromatic, and a conjugated system. Having three bonds on its positively charged oxygen atom, pyrylium is an oxonium ion.
Derivatives of the pyrylium cation are also called pyrylium.
A pyrylium salt is a salt containing a pyrylium cation or a derivative cation.
Like other oxonium ions, pyrylium is unstable in neutral water. However, pyrylium is much less reactive than ordinary oxonium ions because of aromatic stabilization. Pyrylium cations react with nucleophiles in the 2, 4, and 6 positions, which can induce ring-opening reactions. The high electronegativity of the oxygen results in the strongest single perturbation by one heteroatom in a six-membered ring.
Being aromatic, pyrylium salts are easily produced from simple starting materials.
Pyrylium salts with aromatic substituents, such 2,4,6-triphenylpyrylium tetrafluoroborate, can be obtained from two moles of acetophenone and one mole of benzaldehyde in the presence of tetrafluoroboric acid and an oxidizing agent (Dilthey synthesis). For pyrylium salts with alkyl substituents, such as 2,4,6-trimethylpyrylium salts, the best method uses the Balaban-Nenitzescu-Praill synthesis from tertiary butanol and acetic anhydride in the presence of tetrafluoroboric, perchloric, or trifluoromethanesulfonic acids. 2,4,6-Triphenylpyrylium salts are converted by bases into stable a 1,5-enedione (pseudobase), but 2,4,6-trimethylpyrylium salts on treatment with hot alkali hydroxides afford an unstable pseudobase that undergoes an intramolecular condensation yielding 3,5-dimethylphenol. In warm deuterium oxide, 2,4,6-trimethylpyrylium salts undergo isotopic exchange of 4-methyl hydrogens faster than for the 2- and 6-methyl groups, allowing the synthesis of regioselectively deuterated compounds.
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