The formation of a hemiacetal from an aldehyde and an alcohol in the presence of an acid catalyst involves a series of steps in the reaction mechanism. Here's a step-by-step description of the mechanism:1. Protonation of the carbonyl oxygen: The acid catalyst donates a proton H+ to the carbonyl oxygen of the aldehyde, making it more electrophilic. This results in the formation of an oxonium ion.2. Nucleophilic attack by the alcohol: The lone pair of electrons on the oxygen atom of the alcohol acts as a nucleophile and attacks the electrophilic carbonyl carbon of the oxonium ion. This leads to the formation of a tetrahedral intermediate.3. Deprotonation of the tetrahedral intermediate: A nearby molecule of alcohol or water acts as a base and abstracts a proton from the oxygen atom that was originally part of the alcohol. This results in the formation of a hemiacetal.The overall reaction can be summarized as:Aldehyde + Alcohol + Acid Catalyst Hemiacetal + Acid CatalystIt's important to note that the acid catalyst is regenerated at the end of the reaction, so it is not consumed in the process.