1Frequently with distinguishing Greek letter. Each of a number of fragrant unsaturated isomeric ketones which are products of the breakdown of carotenoids and occur naturally in some essential oils; specifically (more fully "β-ionone") a compound that arises as a product of ψ-ionone in the synthesis of vitamin A.
The α- and β-ionones are used in perfumery for their odour of violets and roses. They are cyclic ketones, (CH₃)₃C₆H₆CH=CH·CO·CH₃, differing only in the position of the double bond in the —C₆H₆— ring. γ-Ionone has the double bond outside the ring in a methylene group.
2In full "ψ-ionone". An acyclic compound isomeric with α- and β-ionone which can be cyclized to form those compounds and is used in the synthesis of vitamin A.
Systematic name: (3E,5E)-6,10-dimethylundeca-3,5,9-trien-2-one; C₁₃H₂₀O.
Late 19th century; earliest use found in Bulletin Pharmacy. From German Ionon from ancient Greek ἴον violet (probably, like classical Latin viola, a loan from a language of the Mediterranean) + German -on.