Chemistry Chemical Reactions of Ethers

Topics Covered :

● Chemical Reactions
● Cleavage of `C-O` Bond in Ethers
● Electrophilic Substitution

Chemical Reactions :

Following are the chemical reactions shown by ethers :

Cleavage of C–O bond in ethers :

`=>` Ethers are the least reactive of the functional groups.

`=>` The cleavage of `color{red}(C-O)` bond in ethers takes place under drastic conditions with excess of hydrogen halides.

`=>` The reaction of dialkyl ether gives two alkyl halide molecules.

`color{red}(R- O - R + HX → RX + R - OH)`

`color{red}(R - OH +HX → R- X +H_2O)`

`=>` Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond.

● The reaction yields phenol and alkyl halide. See fig.1.

`text(Note :)` Ethers with two different alkyl groups are also cleaved in the same manner.

`color{red}(R -O-R' +HX → R-X +R' - OH)`

`=>` The order of reactivity of hydrogen halides is as follows : `color{red}(HI > HBr > HCl)`.

● The cleavage of ethers takes place with concentrated `color{red}(HI)` or `color{red}(HBr)` at high temperature. See Mechanism as shown in image.

● However, when one of the alkyl group is a tertiary group, the halide formed is a tertiary halide.

`color{red}(CH_3 - underset (underset(CH_3)(|)) overset( overset(CH_3)(|))C-O-CH_3+HI → CH_3OH+CH_3-underset( underset(CH_3)(|)) overset(overset(CH_3)(|))C-I)`

● It is because in step 2 of the reaction, the departure of leaving group `color{red}((HO–CH_3))` creates a more stable carbocation `color{red}([(CH_3)_3C^+])` and the reaction follows `color{red}(S_N 1)` mechanism. See fig.2.

`=>` In case of anisole, methylphenyl oxonium ion, `color{red}(C_6H_5 - underset (underset(H)(|)) overset(+)O-CH_3)` is formed by protonation of ether. The bond between `color{red}(O–CH_3)` is weaker than the bond between `color{red}(O–C_6H_5)` because the carbon of phenyl group is `color{red}(sp^2)` hybridised and there is a partial double bond character.

● Therefore, the attack by `color{red}(I^–)` ion breaks `color{red}(O–CH_3)` bond to form `color{red}(CH_3I)`.

● Phenols do not react further to give halides because the `color{red}(sp^2)` hybridised carbon of phenol cannot undergo nucleophilic substitution reaction needed for conversion to the halide.

Electrophilic substitution :

The alkoxy group `color{red}((-OR))` is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol.
Q 3032167032

Give the major products that are formed by heating each of the following ethers with `HI`.


Halogenation :

Phenylalkyl ethers undergo usual halogenation in the benzene ring, e.g., anisole undergoes bromination with bromine in ethanoic acid even in the absence of iron (III) bromide catalyst.

● It is due to the activation of benzene ring by the methoxy group. Para isomer is obtained in `90%` yield.

Friedel-Crafts reaction :

Anisole undergoes Friedel-Crafts reaction, i.e., the alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous aluminium chloride (a Lewis acid) as catalyst.

Nitration :

Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole.