Rate and Regioselectivity in the Nitration of Trifluoromethyl Benzene

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

The major influence of the methyl group is its electronic effect on carbocation stability. To a small extent, the methyl group sterically hinders the approach of the electrophile to the ortho positions, making substitution slightly slower at a single ortho carbon than at the para carbon. However, para substitution is at a statistical disadvantage because there are two equivalent ortho positions but only one para position. 

The partial rate factors for nitration of fe/t-buty I benzene are as shown. 

Sample Solution (a) Benzene has six equivalent sites at which nitration can occur. Summing the individual relative rates of nitration at each position in ferf-butylbenzene compared with benzene, we obtain 

All alkyl groups, not just methyl, are electron-releasing, activating substituents and ortho, para directors. This is because any alkyl group, be it methyl, ethyl, isopropyl, tert-h xiy, or any other, stabilizes a carbocation site to which it is directly attached. When R = alkyl. 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene, we consider the electronic properties of a trifluoromethyl group. Because of their high electronegativity the three fluorine atoms polarize the electron distribution in their ct bonds to carbon, so that carbon bears a partial positive charge. 

Recall from Section 4.10 that effects that are transmitted by the polarization of j bonds are called inductive effects. 

Unlike a methyl group, which is shghtly electron-releasing, a trifluoromethyl group is a powerful electron-withdrawing substituent. Consequently, a CF3 group destabilizes a car-bocation site to which it is attached. 

Methyl group releases electrons, stabilizes carbocation 

Trifluoromethyl group withdraws electrons, destabilizes carbocation 

When we examine the cyclohexadienyl cation intemediates involved in the nitration of (trifluoromethyl)benzene, we find that those leading to ortho and para substitution are strongly destabilized. 

---

Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 474 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 476 Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents 480 Substituent Effects in Electrophilic Aromatic Substitution Halogens 482 Multiple Substituent Effects 484 Retrosynthetic Analysis and the Synthesis of Substituted Benzenes 486 Substitution in Naphthalene 488 Substitution in Heterocyclic Aromatic Compounds 489... 

---