6-Nonenal is an organic compound with the formula C2H5CH=CH(CH2)4CHO. Other isomeric nonenals are also known to exist naturally, e.g. 2-nonenal. The cis-isomer of 6-nonenal is often listed as the principal component in the aromas of muskmelon fruits. The trans-isomer is listed as an off-flavor aroma of milk foams, and thought to be a possible polypropylene odorant.
6-Nonenal is thought to be synthesized in nature from γ-lineolenic acid through a lipoxygenase system. The lipoxygenase system cleaves carbon-carbon double bonds into hydroperoxides, which can be cleaved by hydroperoide lysase into the corresponding cis-aldehydes. The rationale behind this mechanism is due to the observed odor properties of muskmelons based on air exposure. In the ripe, unmodified muskmelon there are small concentrations of cis-6-nonenal. A large increase in the concentration of 6-nonenal is noticed when the cells are lysed and exposed to air. This is attributed to enzymatic activity of hydroperoxide lysases when exposed to a high oxygen environment not present in the cells.
Either geometric isomer of this compound may be prepared by preparing by brominating 5-octene-1-ol, then preparing the appropriate Grignard reagent. Triethyl orthoformate is treated with this Grignard reagent, then hydrolyzed to give 6-nonenal.