Question: Propose a mechanism for each of the following reactions:

Protonation is defined as the addition of a proton to an atom, molecule, or ion. It is different from hydrogenation because in protonation, a change in charge of protonated species occurs, while the charge remains unaffected during hydrogenation. For example, formation of ammonium group from ammonia .

In the first step of the reaction, protonation happens. The oxygen atom of carbonyl is protonated by mineral acid HB from the solution, then one OH group attacks the carbonyl carbon and cyclic structure is formed.

In the next step of the reaction, mineral acid ion deprotonates the oxygen atom inside the ring, followed by potonation of side chain hydroxyl (OH) group, which acts as a good leaving group.

In the final step of the reaction, OH₂⁺ group is replaced by methanol (CH₃OH) present in the solution, due to which O atom get deprotanated, and forms our desired product as shown in the figure below.

Mechanism of the reaction (a)

In this reaction mechanism, we use water as a nucleophile which attacks the electrophilic carbon atom bonded to the nitrogen atom, which acquires negative charge. OH₂⁺group is attached to that carbon as shown in the first step given below.

In the next step of the reaction mechanism, the positive charged OH₂⁺ gets deprotonated by the water, which acts as a proton donorand; also Nitrogen atom loses its negative charge after deprotonation by hydronium ion. The N atom is further protonated, because secondary amines show basic properties. To stabilize the charges, the ring opens up and formation of sp²carbonyl group takes place.

In the last step of the reaction, further deprotonation occurs due to which the NH₂group is protonated and the desired product is derived as shown in the figure below.