Synthesizing Poly-substituted Benzene's

EAS Reactions of Disubstituted Benzenes

Electrophilic aromatic subsitution reactions on polysubstituted benzenes are governed by the same rules we learned for mono-substituted benzenes.

This is the simplest case when both groups are the same.  There's only one possibility since all four sites are equivalent.

In some cases, the directing effects reinforce one another.  For example, the CH₃ (ortho/para-director), reinforces that of the -NO₂ (meta-director).

If there is a difference between the activating ability of the two groups then regiochemistry is controlled by the more powerful activating group.  In this case the -OH group is a more powerful ortho/para director than the -CH₃.

Example Problem

At what position would an EAS reaction occur in the following molecule?

• Determine the most "activating" group.  The t-butyl group is the more powerful activator since it's a moderate ortho/para-directing activator while the acetyl group is a meta directing deactivator.
  
  
  
  
• Examine the positions associated with the most powerful activating group.  In this case, only two of the sites are available.  The position ortho to the -t-butyl group would suffer from steric hindrance if an electrophile were to react there, so that leaves the position para to the t-butyl.  The acetyl group is a meta-director so it would help/reinforce reactions meta to it.
  
• If this molecule were to undergo bromination the following product would be the major product.