In an exercise I have the following structures

Moclobemide: moclobemide

Paracetamol: paracetamol

Hexobarbital: hexobarbital

I know for example by heart that paracetamol is an acid. What is the best approach in determination of acidity of a compound based on it's structure?

I'd like to know for example if these compounds are:

  • Basic
  • Acidic
  • Amphoteric

I know for example that many metals (Zn, Pb, etc) can be amphoteric (act as $H^+$ donors or acceptors). But I don't know how to determine what a complex structure is.

Please offer a detailed explanation, with links if possible.

Best Regards,

  • $\begingroup$ Welcome! I think you can find some information in this question. In organic chemistry you should always take in account the stability of the compound resulting from a de-protonated or protonated (or accepting or donating electrons). If the resulting molecule is more stable (e.g. some resonance effect occur) the reaction is favorite. This is the case of acidity of the phenol! $\endgroup$ – G M Jan 5 '14 at 15:28

Panagiotis, in order to solve the exercise, one needs to identify the relevant functional groups and explore their environment a bit.

Supposed that we're talking about "standard conditions", i.e., what is likely to happen if the dissolve/suspend the compounds in water, the situation is as follows:

In the structure of Moclobemide, we spot an amide, and - in the morpholin moiety - an ether and a tertiary amine. Amides do have a lone pair on the nitrogen atom, but due to the conjugation with the carbonyl group, the electron density on the N atom is strongly reduced, as compared to an amine. Ethers are rather lousy bases, in fact, the $pK_a$ of protonated tetrahydrofuran is as low as -2.05.

The remaining tertiary amine thus is the functional group that influences the pH: tertiary amines show a $pK_b$ around 3.3.

In Paracetamol we identify an amide (that we've already excluded as irrelevant) and a phenol group. The latter, once deprotonated, will stabilize the negative charge by resonance with the $\pi$-system (G M has already mentioned this). Therefore, phenols are far more acidic than alkanols. Consequently, the compound definitely acts as an acid.

I'm sure that you've already noticed two imide units in the case of Hexobarbital and in fact, it is possible to abstract the proton from the nitrogen atom. However, you'd need a base such as a carbonate under anhydrous conditions to deprotonate here. As a result, Hexobarbital will hardly do anything at all in water.


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