Electrophilic aromatic substitution

An electrophile replaces a substituent, commonly hydrogen, on an aromatic system. Initial bond formation can make an arenium ion; loss of a proton then restores aromaticity.

Formation of a carbon–sulfur bond by reaction of an electron-rich carbon centre with electrophilic sulfur, followed where needed by deprotonation.

An electrophile adds at an aromatic carbon already bearing a substituent; rearomatization can expel that substituent instead of the newly added proton.

Aromatic nitration replaces an aromatic hydrogen with a nitro group. In a common pathway, aromatic attack on a nitronium ion forms a Wheland intermediate and deprotonation restores aromaticity.

Aromatic oxazolination introduces an oxazolinyl group at an aromatic C–H position. Activating 2-Oxazolidinone with triflic anhydride supplies an electrophilic carbon that an electron-rich arene can attack.

A Friedel-Crafts reaction introduces an alkyl or acyl group onto an aromatic ring by electrophilic aromatic substitution, commonly with activation by a Lewis acid. Alkylation and acylation are related reaction families but have different intermediate chemistry.

An alkylation pathway can involve carbocation-like rearrangements and can promote repeated substitution. An acylation transfers an acyl group through an activated electrophile, often described using an acylium ion, and its carbonyl substituent generally deactivates the product toward further substitution. These differences explain why the two variants are not interchangeable methods of adding a carbon substituent.

electrophilic aromatic substitution that installs an alkyl group on an aromatic ring.

Friedel-Crafts acylation attaches an acyl group to an aromatic carbon through electrophilic substitution. Lewis-acid activation of an acyl chloride is often represented by an acylium ion or strongly polarized contact pair.

An acylium ion is a positively charged acyl species, commonly written $\mathrm{RC}{\mathrm{O}}^{+}$, with charge and bonding described by resonance forms $\mathrm{R}-\mathrm{C}\equiv {\mathrm{O}}^{+}$ and $\mathrm{R}-{\mathrm{C}}^{+}=\mathrm{O}$. Its carbon is strongly electrophilic.

Acylium-like species explain carbonyl-group transfer in Friedel-Crafts acylation. Unlike an ordinary alkyl carbocation, the acylium ion is stabilized by oxygen donation and usually does not undergo the same skeletal rearrangements. The precise reactive species can include a counterion or Lewis-acid complex, so the isolated ion drawing is a mechanistic model.

An arenium ion is a positively charged intermediate in which electrophilic addition has interrupted the original aromatic delocalization. Lone-pair donors can stabilize contributors that place positive charge at adjacent positions.