Alkenals

Acrolein (systematic name: propenal) is the simplest unsaturated aldehyde. It is a colorless liquid with a foul and acrid aroma. The smell of burnt fat (as when cooking oil is heated to its smoke point) is caused by glycerol in the burning fat breaking down into acrolein. It is produced industrially from propene and mainly used as a biocide and a building block to other chemical compounds, such as the amino acid methionine.

Citronellal or rhodinal (C10H18O) is a monoterpenoid aldehyde, the main component in the mixture of terpenoid chemical compounds that give citronella oil its distinctive lemon scent.

Crotonaldehyde is a chemical compound with the formula CH3CH=CHCHO. The compound is usually sold as a mixture of the E- and Z-isomers, which differ with respect to the relative position of the methyl and formyl groups. The E-isomer is more common. This lachrymatory liquid is moderately soluble in water and miscible in organic solvents. As an unsaturated aldehyde, crotonaldehyde is a versatile intermediate in organic synthesis. It occurs in a variety of foodstuffs, e.g. soybean oils.

Methacrolein, or methacrylaldehyde, is an unsaturated aldehyde. It is a clear, colorless, flammable liquid.

2-Nonenal is an unsaturated aldehyde. The colorless liquid is an important aroma component of aged beer and buckwheat, and is insoluble in water.

'''cis-3-Hexenal, also known as (Z)-3-hexenal and leaf aldehyde''', is an organic compound with the formula CH3CH2CH=CHCH2CHO. It is classified as an unsaturated aldehyde. It is a colorless liquid and an aroma compound with an intense odor of freshly cut grass and leaves.

'''trans-2-Methyl-2-butenal' is an organic compound with the formula CH3CH=C(CH3)CHO. This colorless liquid is a building block in organic synthesis. It is an α,β-unsaturated aldehyde related to the better-known crotonaldehyde. The European rabbit, Oryctolagus cuniculus, uses 2-methyl-2-butenal as a pheromone. The rabbit pheromone, trans''-2-methyl-2-butenal, was reported to be involved in the communication between species, defined under the class of "interomone."

6-Nonenal is an organic compound with the formula C2H5CH=CH(CH2)4CHO. Other isomeric nonenal compounds 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.

'''trans-2-Hexenal' is an organic unsaturated aldehyde with a six-carbon chain. This clear, pale yellow liquid has a green, leafy, herbal fruit smell. It occurs naturally in a wide variety of plants, fruits, vegetables, and spices, and appears to be an important mediating and signalling chemical in plant-fungus and plant-insect interactions, such as the symbiosis between acacia ants and Acacias''.