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
Rate of EAS (Electrophilic Aromatic Substitution)
For example, the rate of bromination of trifluoromethyl benzene is about 40,000 times SLOWER than benzene.
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.