Question

Caffeine is uncharged over a wide pH range. In contrast, pyridoxine hydrochloride (vitamin B6) has two acid-base active sites: the phenol (pKa = 9.0) and the protonated nitrogen (pKa = 5.0).

a.   Given this information, write the chemical formula of the most prevalent forms of vitamin B6 (pyridoxine) above each pH region below.

                        pH < 5:

                        5 < pH < 9:

                        pH > 9:

Answer 1

To answer this question I have included a drawing, it is difficult to draw compounds that are this complex without hand so bear with me. But before that, I am including a short explanation about it, mostly about chemistry.

Both the pK_a are provided which is nothing but the pH at which they lose their protons (K_a the acid dissociation constant and pK_a is its negative logarithm). So, if you know the structure of pyridoxine phosphate (B₆) you will see that it will have a cationic & an anionic form with the change of pH and that is because of the affinity of nitrogen to lose an electron is higher than that of phenol which can be proved by the lower pK_a value of nitrogen whereas phenol does not easily lose electron due to its high stability owing to resonance. So think about it like this you have taken pyridoxine phosphate and you start increasing the pH, when you reach pH-5 the nitrogen will become protonated and then you continue the process and when you reach a pH -9 you will find that phenol has finally released its hydrogen and you will get your zwitterionic form (chargeless). If you still increase the ph > 9 then the protonated nitrogen will release the proton and you get your anionic form.

pH < 5 = normal pyridoxine phosphate

5 < pH < 9 = cationic form

pH > 9 = anionic form

Note: If you have any problem understanding this feel free to refer to the comment section and I will get back to you.