# Alkenes: Introduction

## Alkenes: Occurence and Importance in Nature

In nature, alkenes most frequently are part of large ring systems. Terpenes which are the primary constituents of essential oils presumably constitute the most important compound class among these ring systems.

Fig.1
α-Pinene, a major constituent of turpentine oil
Fig.2
1-Menthene, peppermint fragrance

Conjugated polyenes are, for example, A (retinal) and β-carotene (provitamin A).

Fig.3
Vitamin A (retinal) is produced in the body from β-carotene
Fig.4
β-Carotene (provitamin A) is the red pigment in carrots

In this context it is of interest to mention that the vision process is based on a light induced cis-trans isomerization of protein-bound retinol.

Fig.5

Natural rubber is a polyene produced from the milky sap of the rubber tree. Formally, this unsaturated hydrocarbon is derived by of isoprene (2-methyl-1,3-butadiene).

Fig.6

Ethene, the most simple alkene, can also be found in nature. As a plant hormone, it effects the ripening of fruits. Fruits, e.g. kiwis, which either have to be transported over long distances or have to be stored for longer periods of time are harvested early and stored at low temperatures and high $CO2$ concentration. Subsequently, ripening is initiated by treating the fruits with gaseous ethene.

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