Final answer:
The carboxylic acid present in the reaction mixture is likely serving as the proton donor, facilitating imine formation when no explicit acid is added. Additionally, reaction by-products or catalytic amounts of an acid could play a part in the acid catalysis required.
Step-by-step explanation:
The process of imine formation is indeed typically acid-catalyzed, but in cases where no explicit acid is added, an alternative source of acid must be present in the reaction mixture to facilitate the formation of the carbinolamine. In the mechanism described where carboxylic acids react with amines to create amides, it is possible that the carboxylic acid itself is acting as the proton donor, thereby generating the acidic environment necessary for the imine formation. This is particularly plausible when working with relatively strong carboxylic acids or in situations where the reaction solvent or other components might have acidic protons available. Additionally, reaction by-products or the use of catalytic amounts of an acid present from the start or generated in situ could be responsible for the required acid catalysis.