Material guide: trans-2-hexenal — the database holds two records under this name, one with 91 suppliers and one with 2

About a 4 minute read.

Too long, didn't read

• Name: (E)-2-hexenal (trans-2-hexenal), CAS 6728-26-3, 91 suppliers
• Odor family green; descriptors: green, banana, aldehydic, fatty, cheesy, sharp, fresh, leafy, clean, fruity, herbal, spicy
• Strength field: high, evaluate at 1.00% or less. Leaf alcohol and its acetate both say 10%
• Longevity 8 hours, vapour pressure 4.624 mmHg, flash point 38.33 °C, shelf life 12 months
• Listed in all four regulatory indexes: JECFA, FEMA GRAS, CoE, FLAVIS
• The database holds two records under this name. The other has 2 suppliers

One name, two records

Query the database for "(E)-2-hexenal" and two answers come back:

id	CAS	Suppliers	Odor field
19207	6728-26-3	91	green, banana, aldehydic, fatty, cheesy, sharp, fresh, leafy, clean, fruity, herbal, spicy (12 words)
19194	85761-70-2	2	green, leafy (2 words)

Same name, same formula C6H10O, same molecular weight of 98.14, and both proper-name fields read trans-2-hexenal. A 45-fold difference in suppliers and a six-fold difference in how much the odor field says.

Buying or citing from the wrong one gets you a nearly empty record. Same CAS, different odor family measured the problem in the other direction; this is the same name under different CAS numbers. Identity is settled by the CAS, and the name hands you two candidates.

The notes field warns about a second naming trap

This record's notes field carries a line:

do not confuse with the hexabarbital synonym, hexenal

Do not confuse it with hexenal, a synonym for hexobarbital, a barbiturate anaesthetic. The word "hexenal" is at once a short name for a fragrance material and an alternative name for a drug.

Warnings like that are rare in a catalogue, and this one is in the right place, on a name collision that could genuinely hurt someone. The same notes field also carries "product of lipid peroxidation in the rat liver", which is another instance of the seams the juniper piece described.

Of the cut-grass set, this is the difficult one

The site has covered the other two green materials. All three side by side:

	Leaf alcohol	cis-3-Hexenyl acetate	trans-2-Hexenal
CAS	928-96-1	3681-71-8	6728-26-3
Suppliers	120	103	91
Odor family	green	green	green
Recommended dilution	high, 10% or less	high, 10% or less	high, 1% or less
Longevity	4 hours	4 hours	8 hours
Vapour pressure, mmHg	1.039	1.219	4.624
Flash point	61.11 °C	57.00 °C	38.33 °C
Shelf life	24 months	18 months	12 months

All three carry the odor family green, and their physical data sit in different classes. This one has four times the vapour pressure of the other two, a flash point 20 degrees lower, half the shelf life of leaf alcohol, and a recommendation to dilute ten times further before smelling it.

An aldehyde being livelier than an alcohol or an ester is chemically unsurprising. What the fields add is the size of it: under one odor family, the handling conditions differ by an order of magnitude. "All green" does not mean "all used the same way."

The tail of the odor field is worth reading too: banana, cheesy. Those two words look out of place on a material filed as green, and they are exactly where an aldehyde goes at higher concentrations. Another reason for the 1% recommendation.

The plant makes it when it is injured

In 2006 Matsui and colleagues at Yamaguchi University studied cucumber's fatty acid 9/13-hydroperoxide lyase (9/13-HPL) in Phytochemistry (PMID 16497344).

The mechanism: this enzyme cleaves either the 9- or the 13-hydroperoxides of polyunsaturated fatty acids to form C9- or C6-aldehydes respectively. They found its expression developmentally regulated and high in hypocotyls, female flowers and mature fruits, but in mature leaves its transcript and its activity were induced only by mechanical wounding.

The paper also records that, as with C6-aldehydes in most plant species, C9-aldehydes were formed rapidly after disruption of the tissues.

The fungicidal part needs stating as written: it was the C9-aldehydes that had fungicidal activity, against the pathogens Botrytis cinerea and Fusarium oxysporum, at a concentration needed for the toxic effect that was almost equivalent to that found in disrupted tissues. The authors' reason for saying C9-aldehydes could help sterilise the wound is this: they are less volatile in comparison to C6-aldehydes.

So the division of labour is: C6 is the one that leaves and gets smelled, C9 is the one that stays on the wound and works. The smell of cut grass is the half that escaped when the plant was injured.

Its vapour protects pears

The same year, Neri and colleagues at the University of Bologna did the applied side (PMID 16696666): treating 'Conference' pears with trans-2-hexenal vapour against blue mould caused by Penicillium expansum.

The conditions: fruits wounded and inoculated, then exposed to 12.5 µl/l of the vapour at 20 °C.

The result turns on timing. A greater reduction of decay came from applying the treatment 24 or 48 hours after inoculation, while application 2 hours after inoculation was ineffective. Storing the fruit at −1 °C afterwards did not affect the antifungal activity.

A second pair of numbers separates two goals: a 2-hour exposure was effective at reducing blue mould, but at least 8 hours was required to reduce the fruit's patulin content. The treatments did not affect the fruit's physical-chemical characteristics.

The best line is the residue one: after 6 days at 20 °C following exposure, the trans-2-hexenal residue in treated fruits was less than the natural content of the compound in unripe fruits.

The authors' conclusion is measured: it could be a natural alternative to fungicides against P. expansum, with further work needed on conditions that avoid persistence.

Buying and storing

91 suppliers, easy to source. But it handles more strictly than the other two greens:

• Order by CAS 6728-26-3. Two records share the name, and a name alone may not match.
• Evaluation is recommended at 1% or less. Build a two-step dilution first; do not smell it neat. The routine in what strength should I smell it at is required here.
• Flash point 38.33 °C is below many summer room temperatures. Ventilate and keep away from ignition sources. That field puts it in the same bracket as the neighbours in the ethyl acetate flash point table.
• Shelf life 12 months, the shortest of the three greens. Aldehydes oxidise to acids. Decant, date, refrigerate.
• Longevity 8 hours, vapour pressure 4.624 mmHg. A top note, and a fast-moving one.

→ trans-2-Hexenal in the database: full physical data, 91 suppliers and suggested blends.

What this article does not establish

• I have not determined which of the two same-name records has the correct CAS. The point is that a name search returns two answers with a 45-fold difference in suppliers.
• The cucumber paper demonstrated fungicidal activity for C9-aldehydes, not for the C6 subject of this article. I use it for the formation mechanism and the C6/C9 division of labour, not to transfer the fungicidal conclusion to C6.
• The pear paper is postharvest application research, not a safety assessment of perfumery use levels. A vapour concentration of 12.5 µl/l and a percentage in a formula are different coordinates.
• "Banana and cheesy are where an aldehyde goes at higher concentration" is a common working claim; the database does not record the concentration at which those descriptors were written.
• The three-way physical comparison covers only the records that filled those fields, and actual values vary by lot.