Dienes

(diolefins), unsaturated hydrocarbons having two double bonds. Depending on the mutual positions of the double bonds in the molecule, three types of dienes are distinguished: (1) dienes with isolated bonds, for example, pentadiene-1,4

and other dienes of the type

with chemical properties resembling those of olefins; (2)

dienes with adjacent double bonds forming

the class of allenes; (3) dienes with conjugated (alternating) double bonds, for example, 1,3-butadiene (divinyl)

2-methyl-butadiene-1,3 (isoprene)

CH₂=C(CH₃)—CHCH₂

2-chloro-butadiene-1,3 (chloroprene)

CH₂=CCl—CH—CH₂

and cyclopentadiene

Ordinarily the term “diene” implies compounds with conjugated double bonds. In such compounds, the π bonds are conjugated and a single π-electron cloud is formed, giving rise to an increase in the energy of formation of the molecule, some averaging of the distances between the carbon atoms, the high polarizability of the molecule, and the ability of the molecule to undergo not only ordinary addition reactions at double bonds (1,2-addition), but also addition to the ends of the conjugated system (1,4-addition). Thus a bromine molecule adds to butadiene-1,3 to give 3,4-dibromobutene-l and l,4-dibromobutene-2:

Diene synthesis of the 1,4-addition type. Conjugated dienes (for example, butadiene, isoprene, and chloroprene) readily polymerize and copolymerize forming highly elastic polymers and copolymers. Stereospecific polymerization of isoprene yields a rubber whose structure is identical to that of natural rubber.