Explain the un-ionized form of vanillin
It is the un-ionized form of vanillin that has the typical odor. In NaOH solution, the phenol group ofvanillin ionizes to its conjugate-base phenoxide ion; because vanillin is no longer present, and because ionic compounds like the conjugate-base phenoxide are not volatile, the odor disappears. (A compound has to be volatile-that is, it must have a significant vapor pressure-to enter the gas phase and therefore reach the nostrils.) Acidification of the solution brings about protonation of the phenoxide and regeneration of vanillin and, as, the characteristic odor.