Uppingham School, Rutland, UK Also available: HTML, Chime
Enhanced and Jmol versions. cis-3-hexenal is the key aroma
substance emitted when grass is cut and other types of
vegetation are damaged. It has a very low odour threshold of
0.25 parts per billion. The Odour Threshold is the lowest
concentration of a vapour in air which can be detected by smell.
Unfortunately, cis-3-hexenal is unstable and tends to rearrange
to the more stable trans-2-hexenal ("leaf aldehyde"), which has
an odour threshold of 17 ppb. The related cis-3-hexen-1-ol
("leaf alcohol") has an odour threshold of 70 ppb - it's used in
perfumes. When grass is cut, breakdown of fats and phospholipids
leads to α-linolenic acid (and lineoleic acid). This is oxidised
to 12-hydroperoxylinoleinic acid by a lipoxygenase enzyme. In
turn, the C18 chain in this molecules is split by another
enzyme, hydroperoxide lyase , usually into C6 and C12 fragments.
The initial C6 product is cis-hex-3-enal (3E-hexenal). One may
be in protecting the plant from bacter