Material guide: dipentene — its optical rotation field is empty, and that is correct

About a 4 minute read.

The short version

• Name: dipentene, whose head_synonym is simply limonene, CAS 138-86-3, 37 suppliers
• C10H16, molecular weight 136.24; odour type citrus: citrus, herbal, terpenic, camphoreous
• Its optical rotation field is empty, and that is correct. It is a racemate
• Substantivity 3 hours, among the shortest I have seen; store under nitrogen
• 9 formulas in the corpus at a median dose of 2.09%
• But the three limonene records disagree on vapour pressure: 1.550 / 1.541 / 0.198
• Its notes field carries an uncleaned ==(R)== markup, one of only two records that do

Sometimes an empty field is right

I have spent dozens of rounds chasing empty and broken fields. This material is the reverse: its optical rotation field is empty, and that is exactly what it should be.

Dipentene is racemic limonene, equal parts dextro and laevo. The two mirror-image molecules rotate polarised light equally in opposite directions, so they sum to zero. There is no number to write.

Beside its two single enantiomers:

	CAS	Suppliers	Optical rotation	Odour
dipentene (racemic)	138-86-3	37	(empty)	citrus, herbal, terpenic, camphoreous
dextro-limonene	58555-74-1	82	+87 ~ +102	citrus, orange, fresh, sweet, peely
laevo-limonene	5989-54-8	46	−118 ~ −108	terpenic, pine, herbal, peppery

Writing about optical rotation last round I noted that 24% of the 359 records carrying one span zero, and those are mostly materials that do not rotate. Dipentene goes further: it has no range at all.

The odour fields separate the three too. The dextro form is "orange", the laevo is "pine, peppery", and the racemate sits between them with a camphoreous edge. One molecular formula, three ways of smelling.

The three records disagree on vapour pressure

	Vapour pressure @25 °C	Substantivity
dipentene	1.550	3 h
laevo-limonene	1.541	4 h
dextro-limonene	0.198	4 h

Enantiomers have identical vapour pressures by definition. Mirror-image molecules experience the same intermolecular forces, boil at the same temperature and exert the same vapour pressure. A racemate can differ slightly (a different crystal lattice), but not by an order of magnitude as a liquid.

1.550 and 1.541 are all but identical, and 0.198 is 7.8-fold away. That record is the outlier.

Limonene boils at 176 °C, which puts it among the materials in this database reading around 1.5. The 0.198 does not fit its own boiling point.

And I used the wrong number

This part needs saying plainly.

Writing about the substantivity field in #109 I used d-limonene as a control:

2-Acetyl pyrazine's vapour pressure of 0.188 is nearly identical to dextro-limonene's 0.198, and one reads 400 hours while the other reads 4.

That comparison rests on the 0.198, and the 0.198 looks wrong. If limonene's real value is 1.55 it is eight times more volatile than 2-acetyl pyrazine, and "similar volatility, hundredfold difference in substantivity" gets much weaker.

That article's main conclusions (400 hours is a bucket label, every bucket is wide, only the bucket medians are clean) do not depend on the example, but the example itself is withdrawn.

Markup left in the notes field

The notes field contains this:

Found in over 300 essential oils, the ==(R)==-form is the most widespread, followed by the racemate and then the (S)-form.

Those equals signs are leftover typographic markup: some source used == for emphasis or superscript and it was never stripped during collection.

I swept the database: only two records carry this residue in their notes. The other is linalool with 135 suppliers, whose note reads "two stereoisomers: licareol is ==(R)==-…-linalool", with a mojibake character in the same sentence.

Both are materials that need two enantiomers explained. That explanatory text presumably comes from one source whose markup was never cleaned.

Three hours, and stored under nitrogen

Field	Value
Substantivity	3 hours @100%
Vapour pressure	1.550 mmHg
Storage	cool, dry, sealed, protected from light, store under nitrogen
Shelf life	24 months

Three hours is among the shortest substantivities in this database. It is a pure terpene: no oxygenated functional group, nothing to hold it down. That is also why folded citrus oils remove it.

The reason for the nitrogen is in the literature: oxidised limonene is a sensitiser. Karlberg and Dooms-Goossens reported contact allergy to oxidised d-limonene among dermatitis patients in Contact Dermatitis in 1997 (PMID 9165203), and in 2016 Bråred Christensson and colleagues compared concomitant sensitisation to oxidised limonene and oxidised linalool (PMID 26918793).

Fresh limonene is not the main sensitiser; aged limonene is. That is what the nitrogen is for.

In the corpus it trails the dextro form

	Formulas	Median dose	Quartiles
dipentene	9	2.09%	2.00–5.00%
d-limonene (spellings merged)	35	18.75%	4.28–35.00%

The dextro form goes in nine times heavier. That follows: it is a volume material for citrus accords, ranking second when I measured the dose distribution at a median of 15%, while dipentene's camphoreous edge makes it unsuitable in bulk.

Dipentene's most frequent company: benzyl acetate, citral, hedione, linalool, and three aldehyde C-number materials.

What this doesn't establish

• I measured no vapour pressures. "The 0.198 is the outlier" comes from comparing three records of one substance and from its disagreement with its own boiling point. I did not consult a literature value.
• Withdrawing that example from #109 does not change that article's conclusions, and I have not recomputed its other numbers.
• "A racemate's rotation is zero" is chemistry, not something this database states. The database simply left the field blank.
• I did not find the original source of the ==(R)== text. "Leftover markup" is my reading.
• Nine formulas will not support a robust median dose.
• The oxidised-limonene sensitisation cites the papers' titles and subject, with no dose or incidence figures quoted.