Other hand, if too much acid is present (low pH), the attacking amine nucleophile is completely protonated so the initial nucleophilic addition step can’t occur.
Evidently, pH 4.5 represents a compromise between the need for some acid to catalyze the reat-limiting dehydration step but not too much acid so as to avoid complete protonation of the amine. Each individual nucleophilic addition reaction has its own specific requirements, and reaction conditions must be optimized to obtain maximum reaction rates.
Imine formation from such reagents as hydroxylamine, and 2,4-dinitrophenydrazine are useful because the products of these reactions-oximes and 2,4dinitrophenylhydrazones (2,4-DNP’s),respectively are often crystalline and easy to handle. Such crystalline derivatives are sometimes prepared as a means of purifying and characterizing liquid ketones or aldehydes.