Question

Question. One of these reacts with dilute aqueous acid and the other does not. Give a mechanism for the one that reacts and show why this mechanism does not work for the other one.

Short answer

The resonance stabilization which occurs in mechanism of part (b) is not possible in part (a), and for this reason, the reaction will not take place with dilute aqueous acid.

Step-by-step solution

Mechanism for part (b):

The given acetal undergoes hydrolysis in the presence of dilute aqueous acid and in the first step, protonation of oxygen occurs due to which it acquires positive charge and to neutralise it, ring opening reaction occurs due to which carbocation forms. This carbocation is resonance-stabilized and water as a nucleophile attacks at the carbocation, and on further charge neutralisation and protonation of oxygen leads to formation of cyclohexan-1,2-diol and formaldehyde as the leaving group which is formed as side product after charge neutralisation.

Mechanism of formation of product via acetal hydrolysis

Reason for why part (a) acetal does not undergo hydrolysis in presence of dilute aqueous acid:

Acetal in part (a) does not undergo hydrolysis in presence of dilute aqueous acid as there is no driving force available in this reaction which can proceed the reaction further in forward direction towards formation of product. Resonance stabilisation is absent in this case, which is the reason why this acetal does not undergo hydrolysis. Resonance stabilisation or any other stabilization factor such as increase in entropy is not available, hence, the acetal in part (a) does not undergo reaction.