Diene

In organic chemistry, a diene ( ) or diolefin ( ) is a hydrocarbon that contains two carbon pi bonds. 
Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers. Dienes occur occasionally in nature but are widely used in the polymer industry.
Classes.
Dienes can be divided into three classes, depending on the relative location of the double bonds: 
Compounds that contain more than two double bonds are called polyenes. Polyenes and dienes, share many of their properties.
Cycloadditions.
An important reaction for conjugated dienes is the Diels–Alder reaction. Many specialized dienes have been developed to exploit this reactivity for the synthesis of natural products, e.g. Danishefsky’s diene.