ABSTRACT
Dienes are molecules that contain two carbon-carbon double bonds. Dienes can be isolated, conjugated, and cumulated. Isolated, or non-conjugated, dienes are where there is at least one tetrahedral carbon atom between alkenes (C=C–C–C=C). Conjugated dienes are where there are no tetrahedral carbon atoms in between (C=C–C=C). Cumulated dienes, also known as allenes, are where one carbon atom is part of each double bond (C=C=C). Isolated dienes tend to react as though they are two separate alkenes. Cumulated dienes are less stable than two alkenes alone and so they are more reactive than isolated alkenes. Finally, conjugated alkenes are stabilized by delocalization of electrons, and they have unique modes of reactivity. In this chapter, readers will be introduced to alkene addition reactions for conjugated dienes, though 1,2- and 1,4-addition pathways are possible depending on conditions. Conjugated dienes also have unique reactivity: Diels–Alder reaction (where conjugated dienes combine together) and sigmatropic rearrangements (where a σ bond migrates along a π system).
