Final answer:
Imine formation is hindered under strongly acidic conditions because the nitrogen in the imine gets protonated and loses its basic character, thus preventing it from acting as a nucleophile to form the imine bond.
Step-by-step explanation:
Imine formation will not occur under strongly acidic conditions because the basic character of the nitrogen in the imine renders it susceptible to protonation. Imines display basic properties due to the presence of a lone pair of electrons on the nitrogen, which can readily donate to a proton. Under acidic conditions, this nitrogen becomes protonated, forming an ammonium species, which lacks the required lone pair of electrons to act as a nucleophile. The presence of additional H+ ions stabilizes the protonated imine, preventing it from further reacting and forming the desired imine linkage.
The very nature of imines, being derived from the reaction of a primary amine with an aldehyde or ketone, implies that they would perform poorly in an environment where there is a high concentration of H+ ions because the N in the imine is less electronegative and holds onto its lone pair less tightly than O does in a carbonyl compound. When a base is present, it can neutralize the acid, but in a strongly acidic solution, the base would be consumed, and the imine formation would be impeded due to the lack of available nitrogen nucleophiles.
Furthermore, the fact that protonated amines in the ammonium salts are not nucleophiles, as they have no lone pair left, further elucidates why imine formation is unfavorable in strongly acidic conditions. Any amine present would quickly become protonated and therefore unable to partake in the nucleophilic attack necessary to form an imine.