Dienes are classified as cumulated, isolated, or conjugated; conjugated dienes are more stable than isolated dienes because of electron delocalization over the π system.
Compounds with double bonds are known as alkenes, and those that contain more than one double bond are known as polyalkenes. The simplest type of polyalkene is a diene, a compound containing two double bonds (or a 4π-electron system). These double bonds may be arranged in a variety of ways and are classified based on their arrangement in the overall molecule.
An isolated diene is a diene with two double bonds that are separated by two or more single bonds.
A conjugated diene is a diene with two double bonds that are separated by one single bond.
A cumulated diene is a diene with two successive double bonds without an intervening single bond.
Structures of Dienes
The separation between an isolated diene's double bonds causes the bonds to react in the same manner as a simple alkene. Cumulated bonds are linear, like the triple bond of an alkyne, and have a similar stability. The bonds are also shorter than those of a typical alkene. Conjugated dienes are the most stable of the polyalkenes because of the delocalization of electrons over the π system. This is because the single bonds between the double bonds are shorter than the single bonds found in an alkane molecule. The shortening of the bond is caused by π-bonding overlap and a partial double-bond character for the single bond. The length of the single bond may be affected by the hybridized carbons surrounding the single bond. A single bond between two sp3 carbons is longer than a single bond between two sp2 carbons. Additionally, the planar conformation of the molecule allows the π bonds to overlap and electrons from the double bonds to delocalize over the entire conjugated π system.
Conjugated dienes may exist in two planar conformations known as s-cis and s-trans. These planar conformations are analogues to cis- and trans- configurations. The dienes may rotate between the two conformations very quickly, so they are not fixed. Placing an s in front of the cis- or trans- represents the single bond about which the double bonds rotate, and clarifies whether the double bonds are cis- or trans- to the single bond. Conjugated dienes tend to exist predominantly in the s-trans conformation if they are free to rotate, as it is lower in energy than the s-cis conformation. This is because the hydrogens on the double bonds are closer together in the s-cis conformation and thus experience steric repulsion.