Nitration trifluoromethyl benzene

To illustrate substituent effects on rate consider the nitration of benzene toluene and (trifluoromethyl)benzene... 

Nitration of (trifluoromethyl)benzene on the other hand yields almost exclusively m nitro(trifluoromethyl)benzene (91%) The ortho and para substituted isomers are minor components of the reaction mixture... 

All the ring positions of (trifluoromethyl)benzene are deactivated compared with benzene The meta position is simply deactivated less than the ortho and para positions The partial rate factors for nitration of (trifluoromethyl)benzene are... 

Figure 12 11 compares the energy profile for nitration of benzene with those for attack at the ortho meta and para positions of (trifluoromethyl)benzene The presence of the electron withdrawing trifluoromethyl group raises the activation energy for attack at all the ring positions but the increase is least for attack at the meta position... 

When we exanine the cyclohexadienyl cation intermediates involved in the nitration of (trifluoromethyl)benzene, we find that those leading to ortho and para substitution ar e strongly destabilized. 

In a plant for the continuous nitration of chlorobenzene, maloperation during startup caused the addition of substantial amounts of reactants into the reactor before effective agitation and mixing had been established. The normal reaction temperature of 60°C was rapidly exceeded by at least 60° and an explosion occurred. Subsequent investigation showed that at 80° C an explosive atmosphere was formed above the reaction mixture, and that the adiabatic vapour-phase nitration would attain a temperature of 700° C and ignite the explosive atmosphere in the reactor. See l,3-Bis(trifluoromethyl)benzene, above... 

Trifluoromethyl)benzene (benzotrifluoride, 15) was the first organic fluoride to incorporate a trifluoromethyl group. By a standard nitration process, it formed l-nitro-3-(trifluoromethyl)-benzene (16) which was reduced to the 1-amino derivative, 17. This we a-directive influence on electrophilic aromatic substitution contrasted with that for fluorobenzene, which gave 4-and 2-nitro products. 

In the chapter, we established that electron-withdrawing groups direct meta. Among such reactions is the nitration of trifluoromethyl benzene. Draw out the detailed mechanism for this reaction and also for a reaction that does not happen—the nitration of the same compound in the para position. Draw all the delocalized structures of the intermediates and convince yourself that the intermediate for para substitution is destabilized by the CF3 group while that for meta substitution is not. 

Treatment of 2,2,2-trifluoro-] -phenylethanone (6) and (trifluoromethyl)benzene (8) with ozone or ozonized air in chlorinated hydrocarbons, and in the presence of excess nitrogen dioxide and a catalytic amount of an iron(Hl) salt at - 10 to O C, leads to the respective nitro compounds 7 and 9 in good to excellent yields. The reaction is clean and rapid, and little or no hydrolysis of the trifluoromethyl moiety is observed during the nitration. ... 

When the nitration of (trifluoromethyl)benzene was carried out with nitronium tetrafluoroborate at -80 °C, Olah was able to isolate the Wlieland intermediate 1 (Scheme 7.2). Subsequent warming resulted in the formation of l-nitro-3-(trifluoromethyl)benzene. 

Toluene undergoes nitration some 20-25 times faster than benzene. Because toluene is more reactive than benzene, we say that a methyl group activates the ring toward electrophilic aromatic substitution. (Trifluoromethyl)benzene, on the other hand, undergoes nitration about 40,000 times more slowly than benzene. We say that a triflu-oromethyl group deactivates the ring toward electrophilic aromatic substitution. 

---