Question

Write the mechanism of the reaction of HI with methoxybenzene.

Answer 1

In case of alkyl aryl ethers, the products are always phenol and an alkyl halide because due to resonance $C_6{H}_5$ - O bond has partial double bond character. The mechanism is given below
Mechanism Protonation of anisole gives methyl phenyl oxonium ion.
$C_6{H}_5-\stackrel{..}{\underset{..}{O}}C{H}_3+H^{+}\to C_6{H}_5-\stackrel{+}{\underset{\underset{H}{|}}{O}}:-C{H}_3$
In this ion, the bond between $O-C{H}_3$ is weaker than the bond between $O-C_6{H}_5$ which has a partial double bond character. This partial double bond character is due to the resonance between the lone pair of electrons on the 0 -atom and the $s{p}^2$ hybridized carbon atom of the phenyl group. Therefore, attack by $I^{-}$ ion exclusively breaks the weaker $O-C{H}_3$, bond forming methyl iodide and phenol.