Material guide: hexanal (aldehyde C-6) — 107 natural sources, and almost nobody writes it into a formula

Short version: everywhere in nature, almost invisible in formulas, and not because it is useless. Substantivity 4 hours, strength rank 3. It is a material that covers the first thirty minutes.

The fields

Field	Value
CAS	66-25-1
Other names	Hexanal, n-hexanal, caproic aldehyde, aldehyde C-6
Formula	C6H12O, MW 100.16
Boiling point	130–131 °C @ 760 mmHg
Melting point	−56 °C
Flash point	32.22 °C
Specific gravity	0.812–0.822 @ 25 °C
logP	1.80 (est)
Water solubility	5,640 mg/L @ 30 °C (exp)
Odour type	Green
Odour	Fresh, green, fatty, aldehydic, grassy, leafy, fruity, sweaty, vegetable, clean, woody
Strength	High; evaluation recommended at 1% or less (rank 3)
Substantivity	4 hours at 100%
Natural occurrence	107 sources
Suppliers	70
Regulatory	JECFA 92, FEMA GRAS 2557, CoE 96, FLAVIS 05.008
Vapour pressure	10.888 mmHg @ 25 °C (est)

Two numbers read together

Substantivity 4 hours, strength rank 3.

That combination is unique among the 86 materials this series has covered. The other strength rank 3 entries have been indole, dimethyl sulfide, guaiacol and isobutyl quinoline, with substantivities from tens of hours to 168. Hexanal is the only one that is very loud and gone in four hours.

The vapour pressure field explains it: 10.888 mmHg @ 25 °C. Most materials covered here sit between 0.001 and 0.5 mmHg; hexanal is one to four orders of magnitude above them. It leaves fast.

Its place in a formula is therefore unambiguous: top note, and the first half of the top note. It is there in the first minute after spraying and receding at half an hour.

Why formulas barely contain it

This round's beginner article measured this. Against the forum's claim that "C6 is a very important aldehyde which smells very fatty but it's there almost on all fruits", the corpus says:

	Containing hexanal
Fruity formulas (122)	1.6%
All formulas (954)	1.4%
	1.20×

In formulas written for fruit, it is as rare as in any other formula.

And yet its natural occurrence field carries 107 sources. An extract shows where "almost all fruits" comes from:

apple fruit; apple juice; banana fruit; basil shoot; bay laurel leaf; bergamot oil turkey @ 0.02%; blueberry fruit juice; white bread; buckwheat seed; butter; cabbage leaf; camphor oil…

Apple, banana, blueberry, peach (it is among the 160 "peach fruit" materials this round's beginner article pulled). It genuinely is everywhere.

But ubiquity is not a reason to be written into a formula. Four hours of substantivity with rank 3 strength means it is prominent in the opening, then gone, and nothing is missing from the formula after it goes. For a perfume meant to stay on skin for eight hours, that is a poor return.

In flavour work it is a different material entirely. The notes field says "Used in fruit flavours and in perfumery", and the taste description field reads: "An extremely versatile material. In dilution it is used in many fruit and vegetable flavors as well as butter and rum."

You finish eating in seconds, so four hours is more than enough. This material's home ground is food, not perfume. Four food regulatory numbers — FEMA, JECFA, CoE, FLAVIS — against a single cosmetic use (fragrance) is evidence of the same thing.

Where it comes from

One line in the notes field is worth pausing on:

"Constit. of many foodstuffs. A prod. of aerobic enzymatic transformations of plant constits."

Hexanal is not an aroma molecule a plant manufactures; it is produced after plant tissue is damaged. When cells rupture, lipoxygenase (LOX) acts on linoleic and linolenic acid, and via hydroperoxide lyase (HPL) a six-carbon fragment is cut out. That whole family of products is called the green leaf volatiles (GLVs), and hexanal is one of them.

This is why cut grass smells, and why you have to crush a leaf before you can smell it.

Fitzky and colleagues, 2021, in Frontiers in Plant Science, supply sideways evidence. Measuring constitutive VOC emissions from unstressed seedlings of four broad-leaved tree species (Quercus robur, Fagus sylvatica, Betula pendula, Carpinus betulus), they identified 22 VOCs in total, and a principal component analysis revealed co-release of multiple compounds. Among the links they highlight:

"correlated emission rates of methanol, sesquiterpenes (mevalonate pathway), and green leaf volatiles (hexanal, hexenyl acetate, and hexenal; lipoxygenase pathway)."

Hexanal, hexenyl acetate and hexenal are grouped together and labelled as products of one pathway.

There is a practical implication for perfumery. When you want "green", what nature supplies is not a molecule but a set. cis-3-hexenol (leaf alcohol), cis-3-hexenyl acetate, hexanal, (E)-2-hexenal — these appear together in plants because they come from the same enzymatic cut. Putting one into a formula gives you one slice of that set.

And Liu and colleagues in 2022 connect C6 to fruit. Studying controlled atmosphere storage of cold-stored peaches, they measured 14 volatiles positively correlated with consumer acceptability, mainly three C6 compounds, three esters and four lactones, all derived from the fatty acid LOX pathway.

So the "green" in a peach and the "fruit lactones" in a peach are two ends of one pathway. This round's beginner article covers what that means for formulation.

That flash point

32.22 °C.

The lowest of any material this series has covered. For comparison: guaiacol 82 °C, TEC 118 °C, isobutyl quinoline > 100 °C, lavender oil 71 °C.

A flash point of 32 °C means its vapour can ignite in an environment at thirty-two degrees. A room in summer reaches that.

In practice:

• Do not open the bottle near a flame or heat source. This wants more care than the average fragrance material.
• Air freight may be restricted. A low flash point is one of the criteria for dangerous goods classification, and ordering internationally may put it in the flammable liquids category.
• Keep it sealed. It evaporates quickly (vapour pressure 10.888 mmHg), so a loose cap means two things at once: less material and a whole room that smells of it.

Its oral toxicity is low (rat LD50 4,890 mg/kg), dermal is not determined, and the inhalation TCLo is 2,000 ppm/4H. Its risk is physical — flammability and volatility — not toxicological.

Whether to use it

• For a cut-fruit, just-mown-grass or crushed-leaf effect, it is one of the core molecules of that direction, and 70 suppliers make it easy to get.
• But expect it gone in four hours. If the green has to stay into the heart and base, switch to cis-3-hexenol (longer) or the hexenyl esters.
• Start below 0.1%. Rank 3, evaluation at 1% or less, the same class as indole.
• It has a sweaty side. The odour field says "sweaty", which is a source of realism at low concentrations and a problem at high ones.
• Store it away from heat. Flash point 32 °C.
• If you work in flavour, this material's standing is entirely different. Four food regulatory numbers are not a coincidence.

What this doesn't establish

• I cannot fully verify "almost all fruits". Hexanal has 107 natural sources, of which I classified 22 (21%) as fruit. That classification is keyword-based, and a different keyword set moves the number.
• The natural occurrence field is not quantitative. Only a few of the 107 sources carry a concentration.
• The corpus figure of 1.6% against a 1.4% baseline rests on very few formulas (2 of 122). I read the ratio as "no enrichment", not as a measurement.
• Neither cited paper is about perfumery. Fitzky 2021 measured VOC emission from four broad-leaved tree seedlings; Liu 2022 measured peach storage and consumer acceptability. What is cited is the metabolic pathway relationship they establish, not any conclusion about perfume.
• Fitzky 2021 measured tree seedlings, not fruit. The co-release of green leaf volatiles holds in that system, and treating it as a general description of the LOX pathway is my extrapolation.
• The 4-hour substantivity and strength rank 3 are the database's values. This series has already established that the database does not document how either field was measured.