diene

A diene is an organic compound that contains two carbon–carbon double bonds. In acyclic hydrocarbons, a diene has the general composition CnH2n−2, reflecting two degrees of unsaturation relative to an all-alkane chain. The two double bonds can be arranged in three principal ways: cumulated (allenes), conjugated (the double bonds are separated by a single bond), or isolated (the double bonds are separated by two or more single bonds).

Cumulated dienes, or allenes, feature consecutive double bonds sharing a central carbon atom, as in propadiene (CH2=C=CH2). Conjugated dienes have alternating single and double bonds, such as 1,3-butadiene (CH2=CH−CH=CH2). Isolated dienes have their double bonds separated by at least one saturated carbon, for example 1,4-pentadiene (CH2=CH−CH2−CH=CH2).

Nomenclature for dienes follows standard hydrocarbon naming, with locants indicating the positions of the double bonds, e.g., 1,3-butadiene or 2,4-hexadiene. Each double bond can exhibit E or Z stereochemistry, and conjugation between the bonds can stabilize the system and influence reactivity.

Reactivity and applications are dominated by the conjugated class. Conjugated dienes participate in cycloaddition reactions, notably the Diels–Alder reaction with dienophiles to form six-membered rings. Allene-containing dienes exhibit unique geometries and reactivity due to cumulated bonds. Dienes are important industrial monomers, with butadiene and isoprene serving as major building blocks for synthetic elastomers and other polymers, as well as precursors in various organic syntheses.