Electrophilic aromatic substitution (EAS, or SEAr) is a fundamental way to replace an atom or group attached to an aromatic ring, usually hydrogen, with an electrophile. It is widely used to make useful aromatic compounds and is the basis for reactions such as nitration and Friedel–Crafts alkylation.
In EAS, an electron-rich aromatic ring reacts with an electron-poor species called an electrophile. The electrophile becomes attached to the ring, while a group already attached to the ring—most often hydrogen—is removed.
All flashcards
What is electrophilic aromatic substitution (EAS)?
A reaction in which an electrophile replaces a group attached to an aromatic ring, most often hydrogen.
What is an arenium ion?
The delocalized, positively charged intermediate formed when an electrophile bonds to an aromatic ring.
How do halogens affect EAS?
They deactivate the ring by induction but direct substitution to ortho and para positions through resonance donation.
How can pyridine N-oxide help produce substituted pyridines?
It makes the ring more susceptible to electrophilic substitution; reduction afterward gives a substituted pyridine.
What is ipso attack?
Electrophilic attack at an aromatic carbon that already bears a substituent other than hydrogen.
Quiz questions
How do halogens influence electrophilic aromatic substitution on a benzene ring?Answer: They deactivate the ring but direct substitution to ortho and para positions.
Which strategy can make pyridine more susceptible to electrophilic aromatic substitution?Answer: Oxidize it to pyridine N-oxide, perform substitution, then reduce the oxide.
In ipso attack, where does the incoming electrophile initially attach?Answer: At a ring carbon already bearing a substituent other than hydrogen.
What can a chiral Lewis acid catalyst do in some EAS reactions with prochiral carbon electrophiles?Answer: Favor formation of one enantiomer over the other.
Why are pyrrole, furan, and thiophene susceptible to electrophilic attack?Answer: A heteroatom lone pair can stabilize the cationic intermediate.