Question

Draw a stepwise mechanism for each reaction.

a.

b.

Short answer

a.

b.

Step-by-step solution

Stepwise reactions

Stepwise reactions are also called complex reactions. Catalysis reactions proceed by several steps and are regarded as a stepwise reaction.

Reaction of carbonyl compound with alcohol

Acetals are generated due to the combination of a carbonyl compound with alcohol. The reaction is catalyzed by an acid like p-toulenesulfonic acid.

Stepwise mechanism for the reactions

a. The first step of this reaction involves the abstraction of an acidic proton leading to the formation of oxonium ion. The second step is the formation of secondary carbocation. The third step is the reaction of water molecule with the carbocation. The fourth step is the proton transfer. This is followed by the elimination of ethanol molecule leading to the formation of a carbocation. The next step is the abstraction of a proton by the water molecule to form the given product. The mechanism can be given as:

Stepwise mechanism of reaction a

b. The protonation of carbonyl oxygen takes place in the first step. The next step is the reaction of the cyclohexane-1,2-diol with the resonance stabilized carbocation that is formed from the oxonium ion. The third step is the formation of a hemiacetal. The OH group is converted to a good leaving group in the fourth step. The formation of carbocation occurs in the fifth step.The lone pair of electrons on oxygen then attacks the carbocation leading to cyclization. The last step is the deprotonation leading to the formation of the product. The mechanism of the reaction can be given as:

Stepwise mechanism of reaction b