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?

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.