Effect on Rate and Orientation

While it's rather trivial to prepare monosubstituted benzenes since the electrophile can react at any of the 6 carbons in benzene.  All 6 carbons are identical.  But what if there is already a substituent present.  Suppose we would like to prepare di or tri-substituted benzenes.


We need to consider the effect of the substituent (R) on the

  1. Rate of EAS (Electrophilic Aromatic Substitution)

    For example, the rate of bromination of trifluoromethyl benzene is about 40,000 times SLOWER than benzene. 

    Take NoteWe say that the CF3 group "deactivates" the ring for electrophilic attack.  CF3 is considered an Electron Withdrawing Group (EWG).  In general EWG's deactivate aromatic rings (i.e. slow down EAS) and EDG's activate or speed up EAS reactions.
  2. Orientation (regiochemistry) of the incoming E+ attack.

    In the nitration of toluene three possible products are possible.  Notice how the ortho and para products are produced the most with 63% and 34% of products respectively.  The meta position hardly reacted producing only 3% of the products.

    Take NoteWe say that the methyl group is a Ortho/Para director, since it directs electrophiles to mostly the ortha and para positions.