Question

Predict the product(s) and provide the mechanism for each reaction below. What does each mechanism have in common?

Short answer

a.

b. No reaction possible.

c.

d.

Step-by-step solution

Introduction

The structures of reactants given above are acetals, these acetals can be converted back into corresponding aldehydes and ketones with aqueous acid.

It proceeds like the first start with an acetal and aqueous acid which results in protonation and gives a good leaving group ROH. This leaving group is displaced by an elimination reaction to give a new C-O pi bond.

The new C-O pi bond then undergoes the addition of water and oxygen is converted to its neutral form by deprotonation. Then again, the protonation of the OR group gives us ROH which after undergoing elimination gives protonated aldehyde and deprotonation gives us a neutral aldehyde.

Mechanism (a)

As we have already discussed the mechanism involved in these reactions, the representation of mechanism is given below-

Mechanism (b)

The reactant of the second reaction is not possible with trans stereochemistry, no reaction possible.

Mechanism (c)

Mechanism

Formation of benzaldehyde occurs due to the attack of acid catalyst on the oxygen five membered ring and then removal of water.

Mechanism (d)

Oxygen five-membered rings are attacked by acid catalysts and then water is removed from them, resulting in the formation of benzaldehyde.

Mechanism

Explanation for common feature

All these reactions are nucleophilic addition reactions where the nucleophilic part of the reagent adds to the electrophilic carbonyl carbon to form an intermediate and then the product is obtained.