Compounds · Aldehydes · Terpenoid pathway
Geranial
Sharp lemon, edged, slightly metallic. This is the part that gives citral the harshness many people find a little coarse when smelled strongly.
- CAS
- 141-27-5
- Formula
- C10H16O
- Molar mass
- 152.23
- Odour threshold
- 32 ppbin water
- Boiling point
- ≈ 229 °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.
Must be declared on the label
This compound is on the list of allergens that must be declared individually on cosmetic labels in the European Union, as one of the 24 substances listed before Regulation 2023/1545. Name to use on the label: CITRAL — geranial and neral are declared together under one name.
The declaration threshold is 0.001% in leave-on products and 0.01% in rinse-off products. This is an EU requirement — see the regulations page for why it matters beyond the EU. This is not legal advice.
Where it comes from
Biosynthetic pathway: Terpenoid pathway — plants build them from identical five-carbon blocks.
About the threshold figure
Trade literature gives the detection threshold of geranial — also called alpha-citral — as between 32 and 460 parts per billion. We use the low end for consistency with how citral is recorded.
That range spans a factor of fourteen, while the range for mixed citral spans only a factor of four. In other words, measurements for each isomer on its own are less certain than for the mixture — read any odour value calculated from it with exactly that degree of confidence.
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
The (E) isomer of citral — the methyl group and the carbonyl group lie on opposite sides of the double bond. Neral is the (Z) isomer, with both on the same side.
That geometric difference is small but enough to change how the molecules pack together, and geranial boils about two degrees higher than neral. On a smelling strip neral leaves first, so the soft sweet part arrives before the sharp part.
In most natural sources geranial dominates, usually at around three to two relative to neral. That is why the boiling point given for mixed citral — 229 degrees — is in fact the boiling point of geranial.
Where else you meet it
It accompanies neral in every natural source of citral: lemongrass, litsea, lemon peel, ginger. An analysis that reports the two on separate lines tells you more than one that combines them as citral.