Substituent effects in electrophilic aromatic substitution

In acylation the starting compound is the corresponding acyl-chloride  

Reaction of nitric acid with sulfuric acid yields the nitronium ion (N02 ), which as an electrophile reacts with benzene and forms nitrobenzene. 

The chemistry of substituted benzenes belongs to the field of organic synthesis which was developed during the nineteenth century for the needs of the dye industry. Aromatic compounds which have several substituents on the benzene ring have 

Conversely, a deactivating group such as -NO2 in nitrobenzene induces a positive charge in the ortho and para positions, so the electrophilic attack will not take place at these positions. Instead, the electrophile will attack the meta position. 

These initial ideas were inadequate to explain the orientation effect of all groups. The problem appeared with the methyl group in toluene, which is activating, but for which no resonance structures could be constructed. The proper explanation of the regioselectivity (the preference for certain substituent positions) only came with the development of the theory of reaction mechanisms by Robinson and Ingold. As we have seen at the beginning of this chapter, the electrophilic attack causes the formation of a reaction intermediate, the carbocation. Since we know that the rate of chemical reaction depends mostly on the stability of the reaction intermediate, let us discuss the structure of the activated and deactivated cations. 

---

SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION ACTIVATING SUBSTITUENTS... 

Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents... 

These relative rate data per position are experimentally detennined and are known as partial rate factors. They offer a convenient way to express substituent effects in electrophilic aromatic substitution reactions. 

Forsyth, D.A. (1973). Semiempirical Models for Substituent Effects in Electrophilic Aromatic Substitution and Side-Chain Reactions. J.Am.Chem.Soc., 95,3594-3603. 

TABLE 16.2 Substituent Effect] in Electrophilic Aromatic Substitution... 

Forsyth, D.A. (1973) Semiempirical models for substituent effects in electrophilic aromatic substitution and side-chain reactions. J. Am. Chem. Soc., 95, 3594-3603. 

---