Compounds · Monoterpenes · Terpenoid pathway
Limonene
Also known as d-Limonene · (R)-(+)-Limonene · Carvene — an analysis may use any of these names
Citrus peel, bright, slightly bitter at the edges. This is the most common molecule in the whole fragrance trade, and also one everyone has smelled after it has gone off — the smell of old peel is oxidised limonene. It is not the terpene that spoils fastest; α-terpinene and α-phellandrene are quicker still. But because limonene is everywhere, its trace of decay is the one we know best.
- CAS
- 138-86-3
- Formula
- C10H16
- Molar mass
- 136.23
- Odour threshold
- 200 ppbin water
- Boiling point
- ≈ 176 °C at atmospheric pressure
The two axes are 53 points apart. Volatility tells you whether the compound leaves a surface early or late; the threshold tells you how much of it the nose needs before it notices. When the two sit this far apart, they explain things the percentages on an analysis cannot explain on their own.
Where it occurs
Each bar spans the lowest to the highest value measured across published lots. The dot is the mean. A wide range is not an error — it is the mark of a natural raw material.
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Cold-pressed lemon peel 1 lot65.2–65.2
mean 65.2
About the CAS number and optical isomers
The CAS number above is the racemic form — a mixture of the two optical isomers in equal proportions. Limonene in natural citrus peel is almost entirely the (R) form, d-limonene, which has its own CAS number.
We keep the racemic CAS because it is the number commonly used in the trade literature, and because routine gas chromatography cannot separate the two optical isomers — your analysis almost certainly carries this number too. To know which form a sample contains, you need to run a chiral column.
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: LIMONENE.
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
High compared with most compounds in this table. Limonene makes up as much as ninety per cent of citrus peel oils, and it is precisely because its threshold is high that it takes so much of it to be heard clearly.
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
A ten-carbon hydrocarbon with no functional group, so almost nothing holds it back. It is one of the components that leave a smelling strip earliest in any essential oil.
Where else you meet it
The peel of every kind of citrus, dill, and most orange-scented cleaning products. It is one of the most produced molecules in the aromatics industry.
Dictionary entries that mention this compound
Reading the table above correctly
Percentage is not odour strength. The human nose’s detection thresholds differ by up to millions of times from one compound to another.
A wide range is normal. A supplier who publishes identical figures year after year is something to question, not something to trust.