May 23, 2022
You smell me when you mow your lawn.
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Image of cis-3-Hexenal

cis-3-Hexenal is an unsaturated aliphatic aldehyde. Its discovery dates to 1962, when several articles appeared that described its natural occurrence and synthesis. That year, researchers identified cis-3-hexenal in the essential oils of Hypericum plants from Uzbekistan and the volatile constituents of raspberries and strawberries. And Swiss chemists M. Winter and F. Gautschi reported its synthesis and that of its trans isomer.

Since that time, cis-3-hexenal has also been found in Houttuynia cordata, a flowering plant native to southeast Asia; Perilla frutescens var. crispa, an herb that grows in the mountains of China, India, and elsewhere; and in the volatile components of tomatoes.

Despite its widespread occurrence, there is one specific circumstance in which you are most likely to have come across cis-3-hexenal: in the aroma of freshly cut grass. In the fourth edition of Plant Biochemistry (2011), German biochemists Hans-Walter Heldt at the University of Göttingen and Birgit Piechulla at the University of Rostock wrote

The characteristic smell of freshly cut grass is caused primarily by the release of hexenals and hexenols, indicating that the activity of lipoxygenase and hydroperoxide lyase is greatly increased by tissue wounding. This is part of a defense reaction, e.g., when leaves are damaged by feeding larvae, enemies of the herbivores are attracted by the emission of the volatiles.

Because of its intense aroma, cis-3-hexenal is an ingredient in commercial products such as insect repellents, flavoring agents, fragrances, and odor-masking agents. It is a close relative of cis-3-hexen-1-ol, the Molecule of the Week for October 20, 2017.

cis-3-Hexenal hazard information

Hazard class* GHS code and hazard statement
Flammable liquids, category 3 H226—Flammable liquid and vapor  Chemical Safety Warning
Acute toxicity, oral, category 4 H302—Harmful if swallowed  Chemical Safety Warning
Serious eye damage/eye irritation, category 2A H319—Causes serious eye irritation Chemical Safety Warning

*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.

Molecules from the journals

Brivaracetam1 is an anticonvulsant drug that was first described, along with dozens of other 2-oxo-1-pyrrolidine derivatives, by Edmond Differding and co-inventors at Union Chimique Belge (Brussels) in a 2001 WO2 patent. Used primarily to treat epilepsy patients, it was approved for use in 2016 by the US Food and Drug Administration and the European Commission. In January 2022, Jhillu Singh Yadav and colleagues at Indrashil University (Rajpur, India) reviewed several strategies for synthesizing brivaracetam.

Trifluoroacetonitrile3 (CF3CN) is a toxic, water-insoluble, colorless gas that is celebrating its centennial this year. A handy reagent for organic synthesis, it was first prepared by Frédéric Swarts4 at the University of Ghent (Belgium) in 1922. Swarts’s method and several others developed over the years require specialized apparatus, expensive reagents, elaborate purification, and high temperatures; but earlier this year, Zhiqiang Weng and co-workers at Minjiang and Fuzhou universities (both in Fuzhou, China) reported that CF3CN is easily prepared from newly available 2,2,2-trifluoroacetaldehyde O-(2,4-dinitrophenyl)oxime5.

1. CAS Reg. No. 357336-20-0.
2. WO refers to the World Intellectual Property Organization.
3. CAS Reg. No. 353-85-5.
4. Swarts, a fluorine chemistry pioneer, also synthesized the first chlorofluorocarbon, CF2Cl2, CAS Reg. No. 75-71-8, commonly known as Freon-12.
5. CAS Reg. No. 2643982-16-3.

Molecules from the journals

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cis-3-Hexenal fast facts

CAS Reg. No. 6789-80-6
3-Hexenal, (3Z)-
Empirical formula C6H10O
Molar mass 98.14 g/mol
Appearance Colorless liquid
Melting point 126 °C
Water solubility 4–5 g/L (est)
Chemical Abstract Service - a division of ACS

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