Question

Explain why the trifluoromethyl (`CF_3`) group is meta directing in electrophilic aromatic substitution. What would you expect `CF_3` to be activating or deactivating? Why? Can some one PLEASE help me!!!!! Thanks?

Answer 1

To determine this you should draw out the resonance structure for the intermediate that forms after benzene participates as a nucleophile in electrophilic aromatic substitution (EAS).

I drew out the resonance structures of the intermediate after benzene nucleophilically attacks a `"Cl"_2` (though it could be anything, as this is a theoretical comparison).

When `"CF"_3` is adjacent to a carbocation like in the specified ortho and para resonance structures, note that:

• `"CF"_3` is VERY inductively electron-withdrawing (three fluorines on the same carbon, all quite electronegative...).
• Carbocations are inherently electron-deficient/electropositive.
• It's unfavorable to attempt to draw electron density away from wherever there isn't much.

From this we can see that the meta resonance structures, which never have `"CF"_3` adjacent to a carbocation, though not amazingly stable, are less unstable than the ortho and para resonance structures.

Furthermore, since `"CF"_3` is a VERY electron-withdrawing group as I said before, it withdraws electron density away from the ring, thus leaving less available for the ring to use in a nucleophilic attack. That is, it deactivates the ring quite a bit.

Therefore, `"CF"_3` is strongly deactivating and prefers to direct an incoming substituent (in this case it was a chlorine from `"Cl"_2`) to the meta position, just like a nitro group would.