The term farnesene refers to a set of six closely related chemical compounds which all are sesquiterpenes

Terpene

Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

. α-Farnesene and β-farnesene are isomer

Isomer

In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s, differing by the location of one double bond

Double bond

A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...

. α-Farnesene is 3,7,11-trimethyl-1,3,6,10-dodecatetraene and β-farnesene is 7,11-dimethyl-3-methylene-1,6,10-dodecatriene. The alpha form can exist as four stereoisomers

Stereoisomerism

Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms , but that differ only in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections...

 that differ about the geometry of two of its three internal double bonds (the stereoisomers of the third internal double bond are identical). The beta isomer exists as two stereoisomers about the geometry of its central double bond.

Two of the α-farnesene stereoisomers are reported to occur in nature. (E,E)-α-Farnesene is the most common isomer. It is found in the coating of apples, and other fruits, and it is responsible for the characteristic green apple odour. Its oxidation by air gives compounds that are damaging to the fruit. The oxidation products injure cell membranes which eventually causes cell death in the outermost cell layers of the fruit, resulting in a storage disorder known as scald. (Z,E)-α-Farnesene has been isolated from the oil of perilla

Perilla

Perilla is the common name of the herbs of the genus Perilla of the mint family, Lamiaceae. In mild climates, the plant reseeds itself. There are both green-leafed and purple-leafed varieties, which are generally recognized as separate species by botanists. The leaves resemble stinging nettle...

. Both isomers are also insect semiochemical

Semiochemical

A semiochemical is a generic term used for a chemical substance or mixture that carries a message. These chemicals acts as messengers within or between species...

s; they act as alarm pheremones in termite

Termite

Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...

s or food attractants for the apple tree pest, the codling moth

Codling moth

The codling moth is a member of the Lepidopteran family Tortricidae. They are known as an agricultural pest, their larva being the common apple worm or maggot. It is native to Europe and was introduced to North America, where it has become one of the regular pests of apple orchards. It is found...

. α-Farnesene is also the chief compound contributing to the scent of gardenia

Gardenia

Gardenia is a genus of 142 species of flowering plants in the coffee family, Rubiaceae, native to the tropical and subtropical regions of Africa, southern Asia, Australasia and Oceania....

, making up ~65% of the headspace constituents.

β-Farnesene has one naturally occurring isomer. The E isomer is a constituent of various essential oil

Essential oil

An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...

s. It is also released by aphid

Aphid

Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...

s as an alarm pheremone upon death to warn away other aphids. Several plants, including potato

Potato

The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...

 species, have been shown to synthesize this pheromone as a natural insect repellent

Insect repellent

An insect repellent is a substance applied to skin, clothing, or other surfaces which discourages insects from landing or climbing on that surface. There are also insect repellent products available based on sound production, particularly ultrasound...

.