Electrophilic Aromatic Substitution

by James

It’s often helpful to make tables of reactions that have a common feature.

Here’s a table of the most common electrophilic aromatic substitution reactions.

Note why we’re calling it substitution: we’re breaking C-H and we’re forming a different bond at C (e.g. Cl, Br, N, S, C, etc.)

Here’s some key points:

  • The aromatic ring is the nucleophile here. So if we make the electron ring more electron rich, we’ll increase the rate of reaction [groups like OH, OCH3, NH2, or alkyl groups are good examples]
  • The electrophile (e.g. Cl2, Br2, etc.) is usually not reactive enough by itself to react with the aromatic ring. That’s where the catalyst comes in. [There isn’t any significant difference between aluminum (Al) and iron (Fe) catalysts – it’s textbook dependent.
  • The catalyst (always an acid or Lewis acid) accepts a pair of electrons from the electrophile. Since we’re taking electrons away from the electrophile, it becomes more electrophilic! Then it becomes reactive enough for the aromatic group to add to it. 
  • A short-lived carbocation intermediate is formed. 
  • Finally, a weak base comes along and re-forms the double bond, restoring aromaticity. This step is a lot like the last step of the E1 elimination mechanism. 

 

For an example, see here for a walkthrough of electrophilic chlorination.

 

Tomorrow: we’ll focus more on the factors that help this reaction go faster.
Thanks for reading! James

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