Rate and Regioselectivity in the Nitration of Toluene

Because substitution in (trifluoromethyl)benzene occurs primarily at positions meta to the substituent, a trifluoromethyl group is a meta director. 

The regioselectivity of substitution, like the rate, is strongly affected by the substituent. In the following several sections we will examine the relationship between the structure of the substituent and its effect on rate and regioselectivity of electrophilic aromatic substitution. 

Why is there such a marked difference between methyl and trifluoromethyl substituents in their influence on electrophilic aromatic substitution Methyl is activating and ortho, para-directing trifluoromethyl is deactivating and meta-directing. The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed. If we can explain why 

One way to assess the relative stabilities of these carbocations is to examine electron delocalization in them using a resonance description. The cyclohexadienyl cations leading to o- and p-nitrotoluene have tertiary carbocation character. Each has a contributing structure in which the positive charge resides on the carbon that bears the methyl group. 

Chapter 12 Reactions of Arenes Electrophilic and Nucleophilic Aromatic Substitution Para nitration 

The three resonance forms of the intermediate leading to meta substitution are all secondary carbocations. 

FIGURE 12.8 Comparative energy diagrams for nitronium ion attack on (a) benzene and at the (6) ortho, (c) meta, and (d) para positions of toluene. Egct (benzene) fact (meta) fact (ortho) fact (para). 

CHAPTER TWELVE Reactions of Arenes Electrophilic Aromatic Substitution 

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