Halogenation, nitration and sulphonation reactions of aromatic ether

Ethers are relatively inert compounds inspite of the presence of oxygen atom carrying two lone pairs of electrons in their molecules. These are not easily attacked by alkalis, dilute mineral acids, metallic sodium, PCl₅ etc. under ordinary conditions. It is because of this reason that these are used as solvents.

Cleavage of C - O bond in ethers

The cleavage of C - O bond in ethers takes placed under drastic conditions with excess of hydrogen halides.

The reaction of di alkyl ethers gives two alkyl halides molecules.

Alkyl aryl ethers are cleaved at the alkyl oxygen bond due to the low reactivity of aryl oxygen bond. The reaction yields phenol and alkyl halide.

Ethers with two different alkyl groups are also cleaved in the same manner.

The order of reactivity of hydrogen halides is as follows:-HBr->-HCl">

HI > HBr > HCl

In the cleavage of mixed ethers with two different alkyl groups the alcohol and alkyl iodide that form depend on the native of alkyl groups. When primary or secondary alkyl groups are present, it is the lower alkyl group that forms the alkyl iodide.

For e.g.

When one of the alkyl group is a tertiary group, the halide formed is a tertiary halide.

This is because the attack by I^- takes place at that carbon of the alkyl group, which has a greater electron pushing inductive effect and a lower electron density.

In case excess of HI is used, the alcohol formed further reacts with HI to form alkyl iodide. The reaction.

Electrophilic substitution in alkyl aryl ethers

The alkyl group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol.

The resonating structures are shown below.

(i) Halogenation

Phenyl alkyl ethers undergo usual halogenation in the benzene ring.

For e.g., Anisole undergoes bromination with Br₂ in ethanoic acid even in absence of Iron (III) bromide catalyst.

As the benzene ring is activated by methoxy group, the reaction occurs as such. Para isomer is obtained in 90% yield.

(ii) Fridel Craft's reaction

Anisole undergoes Friedel craft's reaction with alkyl halide and aryl halide in presence of Aluminium chloride (a Lewis acid) as catalyst.

The alkyl and aryl groups are introduced at ortho and para positions.

iii) Nitration

A mixture of conc. H₂SO₄ and conc. HNO₃ nitrate anisole to give a mixture of ortho and para nitro anisole.