Final answer:
An acid catalyst is used in nucleophilic addition to increase the electrophilicity of the carbonyl carbon, assisting the nucleophilic attack.
Step-by-step explanation:
An acid catalyst is necessary in nucleophilic addition when the reactants require a more reactive form to undergo the reaction efficiently. This catalyst is consumed in the early stage of the mechanism but is regenerated later, hence speeding up the reaction without being consumed itself.
In the acid-catalyzed nucleophilic substitution mechanism, an acid catalyst accelerates the reaction by assisting in the protonation of the carbonyl compound, which increases the electrophilicity of the carbonyl carbon, making it more susceptible to the nucleophile attack.
In the presence of an acidic medium, the nucleophile is often in a less reactive, neutral form, and the protonated form of the substrate is generally more reactive.
The acid catalyst facilitates the reaction by providing a suitable protons to the nucleophile or to the intermediate formed during the reaction, thereby stabilizing it and allowing the reaction to proceed more rapidly.
For example, when nucleophiles such as water, alcohols, or cyanide are added to aldehydes and ketones, acid catalysis may be required to increase the reactivity of the neutral nucleophile (HNu) by protonation, thus leading to a more efficient nucleophilic addition process.
Ultimately, the choice of using acid catalysis over base-promotion in such reactions depends on the specific conditions and reactants involved.