Aldehydes: identification and storage

In another forum I was asked what aliphatic aldehydes were, as distinct from other kinds, in the context of a discussion about ageing and keeping qualities of various oils. It struck me that the answer I gave contains information that isn't easy to find, isn't intuitive and makes quite a lot of difference to what you do with aldehydes if you use them: so I'm repeating it here.

What are Aliphatic Aldehydes?

Aliphatic aldehydes (unsaturated) are the ones most people mean when they talk about aldehydes:
Octyl aldehyde (AKA Aldehyde C8 or octanal)
Nonyl aldehyde (AKA Aldehyde C9 or nonanal)
Decyl aldehyde (AKA Aldehyde C10 or decanal)
Undecenyl aldehyde (AKA Aldehyde C11 undecylenic;Aldehyde C11 enic; hendecenal or Aldehyde C111)
Undecylic aldehyde (AKA Aldehyde C11 undecylic or undecanal)
Lauric aldehyde (AKA Aldehyde C12 lauric or dodenanal)
Methyl nonyl acetaldehyde (AKA Aldehyde C12 MNA or 2-methyl undecanal)

But notice that the so-called Aldehyde C14 (AKA Peach Aldehyde or gamma-undecalactone), Aldehyde C16 (Strawberry glycidate or lots of alternate names) and Aldehyde C18 (Coconut Aldehyde or gamma-Nonalactone) are not on the list because they are not in fact aldehydes at all.


There are lots of other aldehydes but they don't exhibit the same special characteristics for storage purposes as the unsaturated aliphatic aldehydes - the ones listed above are the main ones used in perfumery, though there are a few others used by some manufacturers as captives, they are not of concern to the DIY community.

How do you store aliphatic aldehydes?

All the unsaturated aliphatic aldehydes are best diluted in a primary alcohol* as soon as you get them - in alcohols they form hemi-acetals which smell like the aldehyde they are formed from but are much more stable. Personally I like to keep my aldehydes at 1% for blending purposes but for storage 10% is more practical as it keeps the volume you have to store more manageable.

If you keep them neat they tend to either oxidise into the corresponding acids, which smell nasty; or polymerise into trimers, which have no smell at all. The presence of any acid (including oxidation products) will accelerate the trimer production significantly. The really counterintuitive point is that trimers continue to form at very low temperatures and seem to form faster, so you should keep your aliphatic aldehydes at room temperature.

You'll know if your aldehydes have trimerised because, besides smelling less strong than they should, they will have got thicker & will eventually be solid at room temperature as the trimer has a much higher melting point.

Adding BHT or other anti-oxidant can help with both problems, though won't eliminate them completely. There is also some evidence that they keep better in aluminium than in glass.

*the obvious primary alcohol to use is ethanol, since you are probably going to use them in an alcoholic fragrance in the end anyway, but phenyl ethyl alcohol or benzyl alcohol work too and might be preferable in some applications.

A more detailed version of this information with links to assist understanding of some of the less familiar terms appears on my blog under the same title.

In light of the fact that someone appears to be systematically running through Basenotes deleting threads I've started and editing out vital details from my posts, I've now captured all the information in the OP on my blog, where I've taken the opportunity to add further details, links to Wiki articles on some of the terms that might be unfamiliar some footnotes and cited sources.

If you'd like those additional details you'll have to look at the blog though, because I'm certainly not going to repeat all that work here.

I only noticed for the first time yesterday. I have of course reported it, but I've noticed more of the same nonsense today so decided not to risk the new information disappearing before I'd captured it elsewhere. It was Basenotes practicing to be a yoyo that caused me to build a DIY section on my blog in the first place. This is a rather more saddening development however.


In the absence of acids trimers should form only very slowly - months certainly - however if there is any oxidation then there will by definition be acid present and that can accelerate it such that you'll see material damage in weeks.

BHT: It should - I normally add 2g per litre of 1% solution to finished fragrances - I don't add any to the aldehyde dilutions unless I'm sure there isn't any in the material already.


It can be, but oxidised BHT isn't a good thing to have a lot of in your fragrance so better not to add more than is necessary. If you buy by the Kg you can will generally know as the material is labeled, for example, Iso E Super BHT or Gamma Methyl Ionone BHT and so on. Plus of course the MSDS will tell you.

Buying in small amounts though it's often harder to tell and most naturals don't have any added.