Diène

In organic chemistry, a diene (ˈdaɪiːn ); also diolefin, daɪˈoʊləfᵻn ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alkene units, with the standard prefix di of systematic nomenclature. As a subunit of more complex molecules, dienes occur in naturally occurring and synthetic chemicals and are used in organic synthesis. Conjugated dienes are widely used as monomers in the polymer industry. Polyunsaturated fats are of interest to nutrition. Dienes can be divided into three classes, depending on the relative location of the double bonds: Cumulated dienes have the double bonds sharing a common atom. The result is more specifically called an allene. Conjugated dienes have conjugated double bonds separated by one single bond. Conjugated dienes are more stable than other dienes because of resonance. Unconjugated dienes have the double bonds separated by two or more single bonds. They are usually less stable than isomeric conjugated dienes. This can also be known as an isolated diene. According to the Gold Book definition, a "diene" could include one or more heteroatoms which replace unsaturated carbon atoms, giving structures that could more specifically be called heterodienes. Compounds that contain more than two double bonds are called polyenes. Polyenes and dienes share many properties. On an industrial scale, butadiene is prepared by thermal cracking of butanes. In a similarly non-selective process, dicyclopentadiene is obtained from coal tars. In the laboratory, more directed and more delicate processes are employed such as dehydrohalogenations and condensations. Myriad methods have been developed, such as the Whiting reaction. Families of nonconjugated dienes are derived from the oligomerization and dimerization of conjugated dienes. For example, 1,5-cyclooctadiene and vinylcyclohexene are produced by dimerization of 1,3-butadiene. Diene-containing fatty acids are biosynthesized from acetyl CoA. α,ω-Dienes have the formula (CH2)n(CH=CH2)2.