Dienes

Dienes are hydrocarbons with two double bonds. Conjugated double bonds are separated by one single bond. Isolated double bonds are separated by more than one single bond. Cumulated double bonds are adjacent to each other.  

A conjugated diene is more stable than an isolated diene, which is more stable than a cumulated diene. The least stable alkene has the greatest -DH value.

A diene can have as many as four configurational isomers:

• E-E
• Z-Z
• E-Z
• Z-E

An isolated diene, like an alkene, undergoes only 1,2 addition. If there is only enough electrophilic reagent to add to one of the double bonds, it will add preferentially to the more reactive bond.

A conjugated diene reacts with a limited amount of electrophilic reagent to form a 1,2-addition product and a 1,4-addition product. The first step is addition of the electrophile to one of the carbons at the end of the conjugated system.

When a reaction produces more than one product, the product formed most rapidly is the kinetic product; the most stable product is the thermodynamic product. If the reaction is carried out under mild conditions so that it is irreversible, the major product will be the kinetic product; if the reaction is carried out under vigorous conditions so that it is reversible, the major product will be the thermodynamic product.

When a reaction is under kinetic control, the relative amounts of the products depend on the rates at which they are formed; when a reaction is under thermodynamic control, the relative amounts of the products depend on their stabilities.

A common intermediate is an intermediate that both products have in common.