Delocalization Among More than Two π Bonds: Extended Conjugation and Benzene 14-7 Extended π systems are thermodynamically stable but kinetically reactive. When more than two double bonds are in conjugation, the molecule is called an extended  system. 1,3,5-hexatriene is quite reactive, polymerizes readily in the presence of electrophiles, but is also relatively stable thermodynamically. Electrophilic additions proceed through a highly delocalized carbocation.

Benzene, a conjugated cyclic triene, is unusually stable. Benzene and its derivatives are unusually stable, both thermodynamically and kinetically. Benzene’s stability is related to its two equally contributing resonance forms:

Because of its stability towards catalytic hydrogenation, hydration, halogenation and oxidation, benzene can be used as a solvent in organic reactions.

A Special Transformation of Conjugated Dienes: Diels-Alder Cycloaddition 14-8 The cycloaddition of dienes to alkenes gives cyclohexenes. The simplest example of a cycloaddition reaction between  systems is the addition of ethene to 1,3-butadiene in the gas phase. This reaction is known as a Diels-Alder reaction or a [4+2]cycloaddition.

The yield of the reaction between 1,3-butadiene and ethene is low. Better yields result when an electron-poor alkene and an electron-rich diene are used.

Other electron-poor alkenes result when the alkene substituents interact with the double bond by resonance, placing a positive charge on an alkene carbon atom:

An example of an efficient Diels-Alder cycloaddition: 1,3-Butadiene, without activating substituents, is sufficiently electron-rich to undergo cycloadditions with electron-poor alkenes.

The Diels-Alder reaction is concerted. Both new C-C single bonds and the new  bond form simultaneously, just as the three  bonds in the starting materials break. This type of one-step reaction is called concerted.

The Diels-Alder reaction is stereospecific. The stereochemistry at the original double bond of the dienophile is retained in the product.

The stereochemistry of the diene is also retained. Note that only one enantiomer is shown in each of the above reactions. The other enantiomer is equally probable and a racemic mixture is formed in each case.

Diels-Alder cycloadditions follow the endo rule. In the reaction of 1,3-cyclopentadiene with dimethyl cis-2-butenedioate, two products are conceivable: an exo adduct and an endo adduct:

The Diels-Alder reaction usually proceeds with endo selectivity. This is a kinetic effect known as the endo rule. As an aid to determining the positioning of the substituents in an adduct, use the following diagram: