Reactions of Dienes

In addition to being more stable than unconjugated dienes, conjugated dienes react differently with electrophilic reagents. Dienes, like all alkenes, can undergo electrophilic addition. Electrophilic addition is a reaction of a double (or triple) bond with an electrophile to give a carbocation (or another cationic intermediate) that then reacts with a nucleophile. The reaction breaks a $\pi$ bond and a $\sigma$ bond in the nucleophile while simultaneously creating two $\sigma$ bonds. In electrophilic addition reactions, however, conjugated dienes undergo either 1,2 additions or 1,4 additions. The predominant product obtained from these electrophilic addition reactions is determined by the temperature at which the reaction is conducted.

If the reaction is run at very cold temperatures (−80 °C), then mostly the 1,2 addition product will form. At this temperature, the reaction is under kinetic control. Kinetic control is a situation in which the reaction to form products is irreversible and the product ratio is determined by the reaction rate. If the reaction mixture is allowed to warm or if the original reaction is conducted at a warmer temperature (40 °C), the reaction mixture will consist primarily of the 1,4-addition product. At this temperature, the reaction is under thermodynamic control. Thermodynamic control is a situation in which the reaction to form products is reversible and the product ratio is determined by relative thermodynamic stabilities. The 1,2 addition product is the kinetic product and forms at cold temperatures, while the 1,4-addition product is the thermodynamic product and forms at higher temperatures.

The formation of both addition products starts with the generation of an allylic carbocation. This carbocation is generated when one of the double bonds donates its electrons to an electrophile. The electrophile will add to one side of the double bond, and the positive charge will either stay on the other side of the double bond (as a 2° carbocation) or move through the $\pi$ system to the far end of the other double bond (as a 1° carbocation) in the diene system. This charge movement will move the two $\pi$ electrons from the double bond to the single bond that had been separating the two original double bonds. If the reaction forms the 1,2-addition product, a nucleophile will add to the 2° carbocation; if the reaction forms the 1,4-addition product, a nucleophile will add to the 1° carbocation. The 2° carbocation is the more stable carbocation, which is why the 1,2-addition transition state is lower in activation energy than the 1,4-addition transition state and why the 1,2-addition product forms faster than the 1,4-addition product. However, the 1,4-addition product is the most substituted alkene and thus the more stable product. Therefore, at very low temperatures when there is not enough energy for equilibrium to be established, the product with the lowest activation energy will predominate. At warmer temperatures, the reaction becomes reversible, and any 1,2-addition products formed can revert to the transition state and then to the allylic cation before re-forming the more stable 1,4-addition product.