# Structure of Dienes

Dienes are classified as cumulated, isolated, or conjugated; conjugated dienes are more stable than isolated dienes because of electron delocalization over the $\pi$ 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 $4\pi$-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 $\pi$ 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 $\pi$-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 $\pi$ bonds to overlap and electrons from the double bonds to delocalize over the entire conjugated $\pi$ 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.