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I have been studying a bit about odors and plant essential oils. Several sources claim that the fragrant terpenes found in these oils, such as menthol, eugenol, limonene, linalool, etc. are volatile compounds.

I understand that all molecules are technically volatile, but since these molecules all have boiling points of over $170~\mathrm{^\circ C}$, and their vapor pressures are not particularly high, I don't see why they are considered 'volatile compounds'.

I have two guesses:

a) There is some fundamental aspect of chemistry that I am ignoring, and these compounds do have a set of properties that makes them especially volatile. This to me sounds very interesting, and I would really like to learn what is that makes these compounds especially well-made for being vaporized.

b) The compounds are not especially volatile. They are synthesized in large amounts, concentrated to high concentrations in the plant, and our nose is very sensitive to them. They are called 'volatile' compounds because we happen to notice their smell when they are in the air.

For my question I've chosen to focus on menthol. Is there anything about menthol that makes it especially volatile?

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    $\begingroup$ It's especially curious to see menthol described as being 'volatile' as its crystalline in its pure state, but I think in the context of flavour/fragrance 'science' the word volatile has simply been (mis) appropriated. Many of these terpenes have incredibly low odour-thresholds so we can smell them even at concentrations lower than parts per billion (the human nose is pretty incredible, lookup the threshold for dimethyl sulfide), so its entirely possibly that its vapour pressure is enough that even a tiny tiny amount of menthol given off is sufficient. $\endgroup$
    – NotEvans.
    Nov 22, 2015 at 17:17
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    $\begingroup$ For a solid, it is relatively volatile. If you keep a small phial in your pocket, for example, it is sufficiently volatile that it will sublime and recrystallise in the cooler parts of the vessel. $\endgroup$
    – matt_black
    Nov 22, 2015 at 19:04

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The term "volatile oils" is a traditional term.

From the 1916 book A TREATISE ON PHARMACY FOR STUDENTS AND PHARMACISTS 5th edition at page 217:

Although the boiling points of volatile oils are considerably above that of water, the oils pass over rapidly with the vapor of boiling water, and in the leading establishments in this country [USA] and Europe volatile oils are now distilled by passing steam under pressure into stills which contain the material on a series of perforated trays...

So it means volatile to the degree that it could be steam distilled.

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