Question

Propose a mechanism for the following reactions:

Short answer

(a)

(b)

Step-by-step solution

Electrophiles

Electrophiles are electron-deficient species, and they attack electron-rich regions.

In question, furan and disubstituted furan are shown and furan is electron rich species due to the lone pair of electrons on oxygen. ROH and HX are electrophiles.

Electrophilic aromatic substitution

As the attacking species are electrophiles, so electrophilic aromatic substitution occurs here. The base (:B) in the reaction mixture eliminates the proton from carbon that forms the bond with the electrophile.

Usually, the substitution occurs at C-2 preferentially as the intermediate is more stable than the C-3 intermediate, but in the case when C-2 is occupied, then substitution occurs at C-3.

The mechanism for part (a)

Here methyl group is present on C-2 so the electrophilic substitution can’t occur at it so the substitution will occur at C-3 and from HCl hydrogen ion will be added at C-3 followed by water addition. The mechanism can be shown as:

The mechanism for (a)

The mechanism for part (b)

Here no substituent is present on C-2, so the electrophilic substitution will occur at it as it gives a more stable intermediate and from Br2 the Br+ is added at C-2 followed by a reaction with CH3OH. The mechanism can be shown as:

The mechanism for (b)