Question

Why is the O-H and the N-H hydrogens not visible on the H NMR of p-aminophenol reactant and acetaminophen product?

Answer 1

Labile protons, such as alcohols and amines have protons which exchange with other labile protons (typically water), and what is observed in the NMR spectrum is an average chemical shift for these species. The actual position will depend on the relative concentrations of the exchanging species, and the exchange rates.

If you consider two protons in very slow (or no) exchange, you will observe two independent peaks. Two protons that exchange so fast that you cannot tell whether a proton is attached to one molecule or the other will appear as a single averaged peak, with a chemical shift of a population distributed average of those two exchanging peaks. For protons that exchange at an intermediate rate, peaks will become broadened and have their chemical shift move towards the peak that they are exchanging with.

Hence, alcohols, amines, amides in many common solvents will have peaks that appear broad or have coalesced with the residual water peak. So, for example, the normal residual water peak in chloroform comes at 1.6. As the amount of labile proton signal increases in the sample, either from more water or alcohols etc, the shift of residual water slowly moves to higher chemical shift. Initially, the water and other labile peaks will appear as separate but broadened peaks. As the concentration of these increase, the peaks broaden more and move together. At an infinite concentration of labile peaks exchanging with water, the peak will appear at 4.7ppm.

If you exclude all water from your sample and other pathways of exchange, these peaks will have very characteristic chemical shifts, just as other protons do.