Be8ae,atoniugatedorcictnenesunusuaW rn人人 b-a 14-8 A Special Trans mation of Conjugated Dienes The cycload ielsnAieiCeogiienesgves g Diel-Alder HC. CH AnexmpefDiels-Aldercyco t g一- e8esga38aee 0 66 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 π 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