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.