Electron-rich aromatic substances

A review on the oxidative coupling of electron-rich aromatic substances (Scholl reaction) and the synthesis of various biaryl substances by double C-H activation suggests that some of these reactions proceed only in the presence of non-oxidizing Lewis acids and some only in the presence of certain oxidants because there are two possible mechanisms that depend on the electronic structure of the substrates and the nature of the catalyst . One of the mechanisms involves the intermediacy of a radical cation while in the other the formation of a sigma complex between the acid and the substrate is suggested. ... 

Color reactions are more or less clearly defined reactions of the substances with suitable reagents. Substance-specific reactions are not usually available, e. g. many compounds with aromatic skeletons give both a positive Vitali-Morin reaction [1 -4] and a positive Marquis reaction [4]. Again, numerous aldehydes react with electron-rich compounds in acidic medium to yield colored substances (cf. Chapters 2 and 3.1). 

Schmidt64 was the first to use wave mechanical methods to study the mechanism of the action of carcinogenic substances. He concluded that in order for an aromatic hydrocarbon to be carcinogenic, it was necessary that there exist in the molecule a region rich in n electrons the density of which exceeded a certain threshold value. 

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