Nucleophilic Aromatic Substitution

Let’s review electrophilic aromatic substitution for a moment.

In electrophilic aromatic substitution:

• the aromatic ring is the nucleophile
• we add an electrophilic reagent
• we form a carbocation intermediate
• electron donating groups increase the rate of reaction (since they stabilize the carbocation)
• electron withdrawing groups decrease the rate of reaction (they destabilize the carbocation)

There’s a second class of aromatic substitution called nucleophilic aromatic substitution. In this reaction, we’re still forming and breaking a bond on a carbon of the aromatic group, but the polarity of the reagents is completely switched. 

In nucleophilic aromatic substitution:

• the aromatic ring is the electrophile
• we add a nucleophilic reagent
• we form a carbanion intermediate
• electron donating groups decrease the rate of reaction (they destabilize the carbanion)
• electron withdrawing groups increase the rate of reaction (since they stabilize the carbanion)

This is more like the SN2 reaction that you learned in first-semester organic chemistry. Except here we’re not doing a backside attack, we’re attacking the aromatic ring. And then the negative charge that forms can expel the leaving group.
Here’s a comparison of each.

Thanks for reading! James