[INeedSerotonin]

Hello everyone

I found this exercise that mentioned L-Metionina:



Then it proceeded to mention two propositions:

I) It has potentially nucleophilic functional groups; and
II) It has an acid-base character.

Specify which one of these is true. If both, mark both.

I think that "potentially nucleophilic functional groups" are groups that have free electrons, that is, they make covalent bonds, but they don't share all of the electrons on their valence shell. Also, they don't need electrons.

And I think that NH2 is basic and COOH is acid, so it has both characters, and I think that an aminoacid is both basic and acidic, that is, neutral.

So I think that both are correct. Am I correct?

Thank you

[Babcock_Hall]

Which groups have free electrons in methionine?

[INeedSerotonin]

I think this is it:

[Babcock_Hall]

What about the sulfur atom?  BTW, the number of valence electrons is less important than the presence of electron pairs in the molecule.

[INeedSerotonin]

What about the sulfur atom?  BTW, the number of valence electrons is less important than the presence of electron pairs in the molecule.

But how can the presence of electron pairs tell me whether these functional groups are nucleophilic or eletrophilic? Is my method correct?

My reasoning is, if an element has unused electrons, they will be attracted by cations/protons and vice-versa. Hence nucleophilic.

[wildfyr]

This sulfur is not nucleophilic. Thioethers do not react in this way.

[INeedSerotonin]

This sulfur is not nucleophilic. Thioethers do not react in this way.

But why is the amine and the carboxyl nucleophilic, while sulfur isn't? 

[clarkstill]

This sulfur is not nucleophilic. Thioethers do not react in this way.

I don't think you can say this. It's certainly non-basic (since the S-H bond formed on protonation is very weak), but second row elements are generally pretty nucleophilic since their electron pairs are high in energy. For example, it is very easy to methylate dimethylsulfide using e.g. iodomethane, to generate the trimethylsulfonium ion. Similarly, oxidation of thioethers to sulfoxides is typically done using electrophilic oxidation reagents (e.g. mCPBA), where the sulfur acts as a nucleophile.

[Babcock_Hall]

This sulfur is not nucleophilic. Thioethers do not react in this way.

I respectfully disagree.  S-adenosylmethionine (SAM) is synthesized from methionine, and the sulfur ends up with a positive charge in SAM.  The sulfur of methionine coordinates iron in cytochrome c (although this might be more fruitfully thought of as Lewis acid-chemistry).  The mechanism of SAM synthetase has been debated, but the sulfur looks to be nucleophilic here:  https://pubs.acs.org/doi/10.1021/bi035611t 

More generally any chemical species with a lone pair of electrons is potentially a nucleophile IMO.  However, some species are far more nucleophilic than others.