Question

7 Which molecule would react faster to answer. (HINT: you need to show the SLOW step of the mechanism.)(10 pts) CH3. H-

Answer 1

The mechanism of acid-catalyzed cleavage of an acetal or ketal to a hemiacetal or a hemiketal is shown below. The slow step is the second step of the reaction, i.e, the addition of water to the protonated acetal/ketal. Water is a nucleophile and the acetal carbon is the electrophilic center. The higher is the electron deficiency of the acetal carbon, the more electrophilic is the carbon and consequently, easier is the attack by the water on the electrophilic carbon. The alkyl (-CH₃) groups in the second compound are electron-pushing groups and therefore, reduce the electron deficiency of the electrophilic carbon. Thus, the attack by the water is less favored. However, the –H atoms in the first compound have no electron density to push toward the electrophilic carbon and thus, the acetal carbon is highly electrophilic and is easily attacked by water. Therefore, the first molecule is expected to undergo acid-catalyzed acetal cleavage faster.

H30 R_C OH R--Hor -CH R H H2O: OH