Compounds · Oxides · Terpenoid pathway
1,8-Cineole
Also known as Eucalyptol · Cineol · 1,8-Epoxy-p-menthane — an analysis may use any of these names
Clean, cold in the nasal cavity, edged. One of very few molecules an untrained nose can name at once.
- CAS
- 470-82-6
- Formula
- C10H18O
- Molar mass
- 154.25
- Odour threshold
- 1 ppbin water
- Boiling point
- ≈ 176 °C at atmospheric pressure
For this compound the two axes sit close together. Volatility tells you whether it leaves a surface early or late; the threshold tells you how much of it the nose needs before it notices.
Where it comes from
Biosynthetic pathway: Terpenoid pathway — plants build them from identical five-carbon blocks.
About the threshold figure
Trade literature gives roughly 1 to 64 parts per billion. We use the low end for consistency with how other components are recorded.
That range spans a factor of sixty-four. A study in red wine found a recognition threshold of about three parts per billion, near the low end — so the figure of one part per billion is not groundless, but don’t read it as a precise measurement either.
Published thresholds differ by several orders of magnitude, because of differences in measuring method, sample purity and panel. The figure here is an order of magnitude for relative comparison, not a measurement. We give it only for compounds where the literature broadly agrees — 52 of the 106 compounds in the library. The rest are left blank rather than guessed.
Why it sits where it does
This is the clearest example in this library of the position of a functional group mattering more than the elemental composition.
Three compounds have the same formula, C₁₀H₁₈O, and the same molar mass of 154.25: 1,8-cineole boils at 176 degrees, terpinen-4-ol at 209, α-terpineol at 219. The same number of atoms, the same mass, forty-three degrees apart.
The reason lies in where the oxygen atom is placed. Cineole hides its oxygen in an ether bridge inside a two-ring skeleton, so that oxygen has no hydrogen to offer and cannot form hydrogen bonds with neighbouring molecules. The other two carry an exposed hydroxyl group, form ordinary hydrogen bonds, and are held back much longer.
The result is that cineole boils at exactly the same 176 degrees as limonene, even though it is eighteen mass units heavier and has an extra oxygen atom. An oxygen atom locked away does not count.
Where else you meet it
Eucalyptus, cajuput, galangal, bay leaf, and most inhalant products.