Electron-withdrawing substituents give meta products

Electron-withdrawing substituents give meta products 

The same thing happens with the CF3 group. The three very electronegative fluorine atoms polarize the C-F bonds so much that the Ar-C bond is polarized too. Nitration of trifluoro-methylbenzene gives a nearly quantitative yield of meta nitro compound. 

Draw the mechanism for this reaction and you see the reason for the switch to meta selectivity. 

The intermediate cation is again delocalized over three carbons, but importantly none of these carbons is the one next to the CF3 group. 

on the other hand, the electrophile were to attack the ortho or para position (the hypothetical reaction para to CF3 is shown below) then the carbon next to CF3 would have to carry a positive charge, which would be destabilized by the electron withdrawal, making this a high-energy intermediate. 

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Electron-withdrawing substituents give meta products... 

ELECTRON-WITHDRAWING SUBSTITUENTS GIVE META PRODUCTS... 

We start with electrophilic aromatic substitution. It was established in Chapter 21 that an electron-donating substituent favours ortho/para and an electron-withdrawing substituent favours meta substitution. Although meta substitution is usually slower than ortho/para substitution (because electron-withdrawing groups deactivate the ring), it usually gives the meta product alone. 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

The observations are that a donor substituent, as in the benzonorbornene 8.65 forms the first bond to the meta position, which is best pictured as giving the diradical 8.66, and hence the tricyclic product 8.67. However, an electron-withdrawing group in the benzonorbornene 8.68 forms the first bond to the para position, which is best pictured as giving the diradical 8.69, and hence the tricyclic product 8.70. Both of these reactions are highly regioselective, yet the diradicals 8.66 and 8.69 are probably the less stable of the two possible radicals in each case. 

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