Question

In the mechanism for acetal hydrolysis shown, the ring oxygen atom was protonated first, the ring was cleaved, and then the methoxy group was lost. The mechanism could also be written to show the methoxy oxygen protonating and cleaving first, followed by ring cleavage. Draw the alternative mechanism.

Short answer

Oxygen of methoxy group gets protonated in the presence of acidic medium due to which oxygen acquires positive charge and we know that, positive charge on an electronegative atom is not stable, thus, methoxy group will leave as leaving group and carbocation is formed which is stable due to mesomeric effect by the neighbouring oxygen. The, water acts as a nucleophile and attacks at the carbocation.

Protonation of methoxy group

Step-by-step solution

Protonation of methoxy oxygen:

Oxygen of methoxy group gets protonated in the presence of acidic medium due to which oxygen acquires positive charge and we know that, positive charge on an electronegative atom is not stable, thus, methoxy group will leave as leaving group and carbocation is formed which is stable due to mesomeric effect by the neighbouring oxygen. The, water acts as a nucleophile and attacks at the carbocation.

Protonation of methoxy group

Formation of hemiacetal and ring cleavage:

In the next step, formation of hemiacetal takes place when positive charge on oxygen is neutralized. Then, oxygen of hemiacetal gets protonated and acquires positive charge, then ring cleavage or ring opening occurs in order to neutralize positive charge on oxygen. After hydrolysis, we get our required product.

Formation of hemiacetal and ring cleavage