Olfactory Chemistry: Ionones, Irones, and Methyl Ionones- Purple Rain

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Two of my favorite notes in perfumery are violet and iris. The molecules which provide the smell of both of these notes are from a family of closely related molecules called ionones and irones. As you can see below the structures are amazingly similar:

alpha2

By just adding the CH3 group (in red), what we call a methyl group, to the ring of ionone you get alpha-Irone. If you add the methyl group to the end of the string of atoms to the right of Ionone you get alpha-methyl ionone.

The ionones, irones, and methyl ionones are arguably the second set of synthetic molecules to mark the beginning of modern perfumery. The use of synthetic coumarin in Fougere Royale in 1882 is the acknowledged beginning of modern perfumery. The three molecules above would be isolated in 1893 and have formed the building blocks of many of the synthetic aromamolecules in the 121 years since their discovery.

The olfactory differences in these three molecules are dramatic considering the tiny change in their structure. Alpha-Ionone gives off a woody violet quality with a bit of raspberry to it. Alpha-Irone is the smell of iris. Alpha-Methyl Ionone is softer and imparts the powdery quality to iris. If it was just these three molecules that would be more than enough but the reality is far richer as you can see below by just moving the double bond in the lower half of the molecule you create a new set of molecules called beta or gamma-Ionones and the analogous irones and methyl ionones.

alpha

Beta-Ionone has a fruitier and woodier quality compared to the alpha-Ionone. Gamma-ionone is the standard violet you find as an aromachemical.

This all leads to a discussion I had on one of my Facebook perfume groups. One of the members asked about the difference between orris and iris when listed in the perfume notes. The difference is orris is a natural source of irones. Orris is the dried root of iris and contains six of the Irone variations described above. The idea is that the combination of all six naturally occurring Irones present in the root together forms that complex iris effect so prized by perfumers. Because of the need to dry the root for five years after harvest makes orris one of the costliest ingredients in all of perfume. This is why you will usually only see it on the more expensive niche brands note lists. It provides a singular effect but at a high cost. As a result when a perfumer wants a bit of iris character in their fragrance without breaking the budget they will turn to the isolated synthetic molecules like alpha-irone and use that as it is much less costly than orris concrete or orris butter.

In the past thirty years these molecules have been the starting point for the discovery of numerous synthetic molecules which have seen widespread use like Norlimbanol and Iso E Super.

So when you smell a bit of purple in your fragrance it is likely due to one of the molecules related to the ionones, irones, or methyl ionones.

Mark Behnke

4 thoughts on “Olfactory Chemistry: Ionones, Irones, and Methyl Ionones- Purple Rain

  1. Fascinating post!  Why oh why is it so very impossible for mere mortals – well, non professionals – to get their hands on alpha irone?  Every time I read about ionones and play with the alpha, beta and methyl ionones I do have I can't help but feel the sting: oh how wonderful it would be to toy with the irone queen.

    Thanks for your fantastic blog!

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