In organic chemistry, a diene (/ˈdaɪiːn/ DY-een); also diolefin, /daɪˈoʊləfɪn/ dy-OH-lə-fin) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms.^[1] 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.

Classification

Dienes can be divided into three classes, depending on the relative location of the double bonds:^[1]

1. Cumulated dienes have the double bonds sharing a common atom. The result is more specifically called an allene.
2. Conjugated dienes have conjugated double bonds separated by one single bond. Conjugated dienes are more stable than other dienes because of resonance.
3. 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.^[1]

Compounds that contain more than two double bonds are called polyenes. Polyenes and dienes share many properties.

Synthesis of dienes

On an industrial scale, butadiene is prepared by thermal cracking of butanes. In a similarly non-selective process, dicyclopentadiene is obtained from coal tars.

α,ω-Dienes have the formula (CH₂)_n(CH=CH₂)₂. They are prepared industrially by ethenolysis of cyclic dienes. For example, 1,5-hexadiene and 1,9-decadiene, useful crosslinking agents and synthetic intermediates, are produced from 1,5-cyclooctadiene and cyclooctene, respectively. The catalyst is derived from Re₂O₇ on alumina.^[2]

In the laboratory, more directed and delicate processes are employed, such as dehydrohalogenations and condensations. Conjugated dienes can be formed by the addition of two good leaving groups across a double bond, followed by base workup.^{[3][better source needed]} Dienes can also be produced from the semihydrogenation of diynes. Pd catalysts tend to degrade homoconjugate dienes, but Ni works well.^[4] Myriad name reactions have been developed, including the Whiting reaction.

Families of nonconjugated dienes are derived from the oligomerization and dimerization of conjugated dienes. For example, 1,5-cyclooctadiene and 4-vinylcyclohexene are produced by dimerization of 1,3-butadiene.

Diene-containing fatty acids are biosynthesized from acetyl CoA.

Reactivity and uses

Acidity

The position adjacent to a double bond is acidic because the resulting allyl anion is stabilized by resonance. This effect becomes more pronounced as more alkenes are involved to create greater stability. For example, deprotonation at position 3 of a 1,4-diene or position 5 of a 1,3-diene give a pentadienyl anion. An even greater effect is seen if the anion is aromatic, for example, deprotonation of cyclopentadiene to give the cyclopentadienyl anion.

References