Thin-layer-chromatography and the role of acid/bases (protonation deprotonation)

Thin-layer-chromatography and the role of acid/bases (protonation deprotonation). Protonated/Deprotonated Drug – General Understanding
I have a general problem in my understanding of protonated and deprotonated drugs and the role of pka.
Generally, a low pka indicted a strong acid.
When pka = pH we can expect to have a drug 50% protonated and 50% deprotonated.
However, when we go two unites above or below the pka value of a drug, then we can expect 99% of it to be undissociated and 1% dissociated.
For an acid it would imply that if: pH the protonated form exists predominately
When we have pH > pKa -> the deprotonated form dominates.
This now where I struggle when I try to apply these principles in practice.
From what pka range can I start classyfing drugs into “Basic” and acidic? Is a pka 10 and above considered a basic drug?
Lets take Paracetamol as an example. It has a pka value of 9,4, which I would interpret as weakly acidic.
– If I would add the paracetamol to an acidic environment, I would expect it to stay as it is with the OH. If I were to put it in a basic environment the OH would become deprotonated and instead of it there would be a negatively charged O-.
o But couldn’t at the same time the nitrogen become protonated from the acidic environment? Any why?
o ->How can I know which part becomes protonated or deprotonated?
What about nicotinamide? It has a pka of 3,3: Would it imply a deprotonation of NH2 in basic environments?
Also when looking at structures such as coffein. Is there any advice to quickly recognize the most basic nitrogen, which is most likely to become protonated?
Thin-layer-Chromatography Questions Worksheet A
Question 1:
You have paracetamol, coffein and acetylsalicylic acid. You want to perform 3 TLC with the different eluents.
TLC A: Ethyl acetate
TLC B: Ethyl acetate and formic acid
TLC C: Ethyl acetate and ammonia
Write the order of elution which you would expect for each TLC and explain your reasoning.
Question 2a) You have paracetamol, salicylamide, salicylic acid given. Write the order of elution for each TLC. Provide an explanation for each order.
TLC A: Ethyl acetate
TLC B: Ethyl acetate and acid
TLC C: Ethyl acetate and base
2b) In addition to the substances above, you now need to also analyse naphazoline nitrate. Is it possible to run a TLC with all these 4 substances together? Explain.
Question 3) How do you undergo analysis/detection in TLC? Explain the significance of the 254nm and 365nm values.

Thin-layer-chromatography and the role of acid/bases (protonation deprotonation)

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