Bondline / Reactions

Friedel–Crafts alkylation

A carbocation (made from an alkyl halide and AlCl₃) is attacked by the ring, then H⁺ is lost. Problems: rearrangements and over-alkylation, because the product is more reactive.

Aromatic substitution · 2 steps

Conditions

RCl with AlCl₃. Here 2-chloropropane gives the isopropyl cation.

Mechanism in 2 steps

Red curly arrows show where each pair of electrons moves.

HHClAl−ClClCl++
Step 1The ring attacks the carbocation.
HClAl−ClClClHClClAlClClAlCl₄⁻+++
Step 2The proton is removed and the ring becomes aromatic again.

About aromatic substitution reactions

Benzene keeps its stable aromatic ring by substituting rather than adding. An electrophile attacks, then a proton is lost to restore aromaticity.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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