Material guide: rhodinol — one name over two isomers, two botanical sources, and an optical rotation nobody can smell

About 5 minutes.

Short version

• Name: rhodinol, CAS 141-25-3, C10H20O, MW 156.27
• head_synonym: 3,7-dimethyl-6 & 7-octen-1-ol
• FEMA 2980, CoE 76, FCC listed Yes, structure class I
• Odour type: floral; the odour field carries fifteen words — floral, red rose, spicy, waxy, powdery, geranium, sweet, rose, fatty, citronella, fresh, citrus, mandarin, lemon, lime
• Strength: medium; substantivity 236 hours; vapour pressure 0.018 mmHg
• Assay: 70.00 to 100.00 — one of the widest ranges of any fragrance material here
• Optical rotation: −5.00 to −10.00
• Occurrence: citronella oils; geranium oils
• Suppliers: 41

The name says it is a mixture

The head_synonym field:

3,7-dimethyl-6 & 7-octen-1-ol

"6 & 7" is not a typo. Those are two positional isomers — the double bond at position 6 or at position 7 — sharing one trade name.

The 6-ene is citronellol; the 7-ene is rhodinol in the strict sense. What is sold as "rhodinol" is a mixture of the two, in no fixed ratio.

That is where the 70-to-100 assay range comes from — not impurity, but the span the name covers. (The three causes of a wide assay range are in How wide is the assay window; rhodinol is the most extreme case of "the name refers to a mixture".)

(Citronellol has its own record, and geraniol at 198 suppliers is the third material on this line.)

Two botanical sources, one name

Among the supplier entries carried in the solubility field, the same house (Argeville) lists both:

RHODINOL EX-CITRONNELLA

RHODINOL EX-GERANIUM

And the occurrence field reads exactly that: citronella oils; geranium oils.

So the rhodinol you receive varies along two axes: the isomer ratio, and which plant it came from.

Among those fifteen odour words, "citronella" and "geranium" both appear — not a contradiction, but the traces of two sources preserved in one field.

The two describers, this time a hundredfold apart

The odour description field carries the same two names again:

Luebke (1987): at 100.00%. floral red rose spicy waxy powdery geranium.

Mosciano (2000): at 1.00%. Sweet, rosy floral, waxy/fatty, citronellal-like with fresh citrus mandarin, lemon and lime nuances.

100% against 1% — a hundredfold.

Since writing Who wrote the odour field, this is the largest concentration gap I have found between the two. One smelled the neat material; the other a one-percent solution.

Those are not two opinions; they are two entirely different experiments. And the odour field merges both sets of words into a single list of fifteen, where a reader cannot tell which came from where.

The optical rotation is a range too, and it may not matter

The optical_rotation field reads −5.00 to −10.00.

That is a range, meaning enantiomeric composition differs between lots. (S)-rhodinol is a separate record in the database (CAS 6812-78-8, 15 suppliers), with the same 70-to-100 assay.

The question is whether anyone can smell it.

Laska and Teubner ran a direct test in Chemical Senses in 1999. Twenty subjects, a forced-choice triangular procedure, ten enantiomeric odour pairs presented repeatedly, with the task of identifying the odd bottle.

The result comes in four layers:

1. As a group, subjects could significantly discriminate only α-pinene, carvone and limonene.
2. They failed to distinguish the (+) and (−) forms of menthol, fenchone, rose oxide, camphor, α-terpineol, β-citronellol and 2-butanol.
3. Individual differences were marked — from subjects who significantly discriminated 6 of the 10 pairs to subjects who failed on 9 of 10.
4. For none of the ten pairs were the antipodes reported to differ significantly in subjective intensity at equal concentrations, and error rates were stable across sessions, with no learning or training effect observed.

β-Citronellol is in the second group — the ones they could not tell apart.

(I have used this study before, in the rose oxide articles, where the relevant pair was rose oxide — also among the seven they could not distinguish. Here the relevant pair is β-citronellol. Same study, different row.)

So: rhodinol's optical rotation range is measurable variation that may not be smellable variation.

(That paper also puts α-terpineol in the cannot-discriminate group, and α-terpineol happens to be the material with the most grades — five of them. Measurable difference and audible difference are separate things, and this database keeps producing the distinction.)

Do not over-extend it. That study compared pure enantiomers pairwise, not commercial samples of differing rotation. When optical rotation varies, isomer ratio and botanical source are varying too, and the paper addresses neither.

There is marketing copy in the toxicity field

This record carries a small data-quality problem worth noting. The toxicity_oral field reads:

"oral-rat LD50 5540 mg/kg This natural extract of rhodinol from citronella has a bright, petal fresh, wonderfully well-balanced and rounded rose fragrance…"

Supplier marketing copy runs straight on from the toxicity datum.

Field contamination is not rare, and this series has logged several kinds: Veramoss's notes field says the molecule is not found in nature, which is wrong, and ambrein's notes field calls the sperm whale a "sperm blue whale".

Read the whole cell before quoting any field.

How to use it

• A backbone material for rose accords, with 236 hours of substantivity. Red rose, geranium and waxy are its main line in the odour field.
• Ask three things when buying: purity (where in 70 to 100), isomer ratio, and whether it is ex-citronella or ex-geranium.
• Re-evaluate when you change supplier. This material varies along more axes than most.
• Do not pay for optical rotation. Laska's subjects could not distinguish β-citronellol's enantiomers — though that is not exactly rhodinol's case.
• It is not a synonym for citronellol, despite the overlap. If you need precision, buy citronellol.

What this doesn't establish

• I have not smelled it, and have not compared an ex-citronella against an ex-geranium product.
• The "6-ene and 7-ene" identification is read off the head_synonym field. I did not check the official scope of CAS 141-25-3.
• Laska's study compared pure enantiomers pairwise, not commercial samples. Extending it to "the rotation range does not matter" is my inference, with its limits stated above.
• That study had 20 subjects, with large individual differences — itself one of its findings.
• The two describers are a hundredfold apart in concentration, so their passages are not comparable. The odour field merges them anyway.
• I have no data on how optical rotation, isomer ratio and botanical source relate. They may be correlated or independent.
• I confirmed the marketing-copy contamination on this record only; I did not count how many records database-wide are affected.