Which of the following correctly describes the synthesis of an acetal from an aldehyde ($\mathrm{acetal} a$) and a ketone ($\mathrm{acetal} b$)?

• A. React an aldehyde with $H_{2}O$ and base to form $\mathrm{acetal} a$, and react a ketone with $H_{2}O$ and base to form $\mathrm{acetal} b$.
• B. React an aldehyde with a Grignard reagent to form $\mathrm{acetal} a$, and react a ketone with a Grignard reagent to form $\mathrm{acetal} b$.
• C. React an aldehyde with two equivalents of an alcohol in the presence of an acid catalyst to form $\mathrm{acetal} a$, and react a ketone with two equivalents of an alcohol in the presence of an acid catalyst to form $\mathrm{acetal} b$.
• D. React an aldehyde with ammonia (${\mathrm{NH}}_3$) to form $\mathrm{acetal} a$, and react a ketone with ammonia (${\mathrm{NH}}_3$) to form $\mathrm{acetal} b$.

Answer: (C)

Verified step by step guidance

1. Understand that acetals are formed by the reaction of aldehydes or ketones with alcohols under acidic conditions, not by reaction with water and base or Grignard reagents.
2. Recall the general mechanism: the carbonyl oxygen is protonated by an acid catalyst, increasing the electrophilicity of the carbonyl carbon.
3. Next, one equivalent of alcohol attacks the protonated carbonyl carbon, forming a hemiacetal intermediate after deprotonation.
4. Then, under acidic conditions, the hemiacetal hydroxyl group is protonated and leaves as water, generating a resonance-stabilized carbocation (oxonium ion).
5. Finally, a second equivalent of alcohol attacks this carbocation, and after deprotonation, the acetal is formed. This process requires two equivalents of alcohol and an acid catalyst for both aldehydes and ketones.