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.
Nucleophile, electrophile, does it remind you of yin and yang yet? Tomorrow let’s go into more details of this reaction.
Thanks for reading! James