Lesson 5 of 13 · 5 min
Preparing haloarenes
NCERT §6.5
The chlorobenzene bottle is nearly empty. Kavya asks whether she could just treat phenol with HCl, the way she made tert-butyl chloride.
The lesson in notes
In short
Aryl chlorides and bromides come from electrophilic substitution of an arene with Cl₂ or Br₂ and a Lewis acid catalyst such as iron or iron(III) chloride.
When ortho and para isomers form, they are easy to separate because their melting points differ widely.
Iodination is reversible, so an oxidising agent (HNO₃ or HIO₄) is added to destroy the HI formed. Fluoroarenes cannot be made this way because fluorine is far too reactive.
Sandmeyer's reaction: a primary aromatic amine in cold aqueous mineral acid is treated with NaNO₂ to give a diazonium salt; mixing the freshly made salt with CuCl or CuBr puts –Cl or –Br in place of the diazonium group.
Iodoarenes need no copper salt: shaking the diazonium salt with potassium iodide is enough.