In the presence of an acid catalyst, the nucleophilic addition reaction of ethanol with propanal proceeds through the following mechanism:1. Protonation of the carbonyl oxygen: The acid catalyst donates a proton H+ to the carbonyl oxygen of propanal, making it more electrophilic and susceptible to nucleophilic attack. This forms an oxonium ion intermediate.2. Nucleophilic attack: The oxygen atom of ethanol, acting as a nucleophile, attacks the electrophilic carbonyl carbon of the oxonium ion, forming a tetrahedral intermediate.3. Deprotonation: The tetrahedral intermediate loses a proton H+ to the conjugate base of the acid catalyst, regenerating the acid catalyst and forming the final product, which is a hemiacetal.The overall reaction is as follows:Propanal + Ethanol in the presence of an acid catalyst HemiacetalIn the presence of a base catalyst, the reaction proceeds through a different mechanism:1. Deprotonation of the alcohol: The base catalyst abstracts a proton H+ from the hydroxyl group of ethanol, forming an alkoxide ion.2. Nucleophilic attack: The alkoxide ion, acting as a nucleophile, attacks the carbonyl carbon of propanal, forming a tetrahedral intermediate.3. Tautomerization: The tetrahedral intermediate undergoes tautomerization, which involves the migration of a proton from the hydroxyl group to the oxygen atom of the former carbonyl group, resulting in the formation of an enolate ion and water.4. Protonation of the enolate ion: The enolate ion abstracts a proton H+ from the conjugate acid of the base catalyst, regenerating the base catalyst and forming the final product, which is an aldol.The overall reaction is as follows:Propanal + Ethanol in the presence of a base catalyst AldolIn summary, the nucleophilic addition reaction of ethanol with propanal in the presence of an acid catalyst results in the formation of a hemiacetal, while the same reaction in the presence of a base catalyst leads to the formation of an aldol. These products differ in their structure and properties, with the hemiacetal containing a hydroxyl group and an ether linkage, while the aldol contains a hydroxyl group and a newly formed carbon-carbon double bond.