[rleung]

Hi,

I am confused as to why reaction of methyl bromide with H2S yields CH3SH instead of CH3S(+)H2?  Since when can the nucleophile of an SN2 be deprotonated?  Thanks.

Ryan

[Mitch]

You'll be making HBr though.

[rleung]

Thanks.  I guess I am confused more so about why this reaction occurs in the first place via SN2.  The nucleophile (H2S) is a weak nucleophile, is it not?  The same reaction with H2O would not likely occur, right?  So, I am having a hard time understanding why this reaction would occur, especially since H2O is a stronger base than H2S, and H2O itself is not strong enough a nucleophile to foster a SN2 reaction, so why would rxn with H2S occur.  Thanks.

Ryan

[Mitch]

I don't know. Are you sure water wouldn't do the same?

[rleung]

Hmm, well I am pretty sure water wouldn't do the same, but then again, I am just an Orgo I student.  But CH3Br + H2O ---> CH3OH doesn't seem like it would happen since H2O is such a weak nucleophile, and since this reaction has a primary alkyl halide, it would necessarily occur via SN2, which would require a strong nucleophile, right?  Thanks.

Ryan

[Albert]

I apologize for interrupting you, guys

In order to get CH3SH, basic conditions are required:

CH3Br + KOH + H2S -> CH3Br + K⁺HS^- + H2O -> CH3SH + KBr + H2O

[rleung]

Thanks,

So, when you have basic conditions, does the reaction CH3Br + H2S ---> CH3SH still occur via SN2, or does it occur through SN1?  My book never explicitly talked about any kind of reactions of these types.  I thought that the nucleophile can usually only be deprotonated in SN1 rxns?

Also, if basic conditions are required, then does that mean that, in neutral conditions, the reaction would be CH3Br + H2S ---> CH3S(+)H2 instead?  Thanks.

Ryan

[Albert]

Thanks,

So, when you have basic conditions, does the reaction CH3Br + H2S ---> CH3SH still occur via SN2, or does it occur through SN1?

SN2
That's it...to be frank with you, I'm going out now   (I live in Europe) and I cannot answer you better...but guys here are extraordinary and they won't leave you alone

[kkjc2]

Remember that protonation and deprotonation occurs rapidly. Very little energy is needed for these proton transfers and most compounds exist in an equilibrium with its protonated/deprotonated form in a protic solvent, depending on its K_a. You would favour the MeS⁺H₂ in acidic conditions and the MeSH in alkaline conditions.

Hydrogen sulfide is a good soft nucleophile. It reacts well through orbital interactions. The S lone pairs are in large 3p orbitals. Water is hard and reacts through charge attractions. The C-Br bond is not that polarised (Pauling's electronegativity C-2.5; Br-2., so it favours a soft orbital interaction than a hard, charge directed interaction.  

The answer to your question heading is that the initial product is deprotonated by something, this something may be bromide, water, another molecule of H₂S, who knows? Proton transfer just happens extremely rapidly and reversibly. Protonated methanethiol and methanethiol co-exist in a protic solvent, like water.

[rleung]

Thank you SO muchf or your help  So, am I correct in saying these following statements:

1) H2O is a better base than H2S; therefore, H2O is a better nucleophile than H2S
2) However, since most SN2 reactions with H2S occur in an aprotic polar solvent, the solvation interactions will make H2S a better nucleophile than H2O, which is why H2S is able to react with primary alkyl halides via SN2 while H2O alone is not.

Is this correct, or did I just completely mess that up?  Thanks so much.

Ryan

[Mitch]

Best not to relate bascisity to nucleophilicity.

[rleung]

Hmm, but is what I said "correct" (altho it's best not to think of it that way)?

Ryan

[Albert]

H2O is a weaker acid than H2S but it is also less nucleophile (in an aprotic polar solvent). Why?

Nucleophily depends on: charge, basic character, solvent, polarizability and substrate's geometry.

H2S is more polarizable than H2O: S is bigger than O and so it has a bigger electronic cloud (sorry for my English! Is it correct?), more polarizable and more diffused. This means that orbitals are better superimposed in SN2 transition structure.

[Organishe]

hydrogen sulfide is more nucleophilic than water.  The rationale that I have for this statement is that S is larger than oxygen, so its valence electrons are held more loosely, therefore they could be more easily donated than those in the valence shell of oxygen.
Also, the sulfur would not have to get as close to the electrophile to form a bond as the oxygen would.

Also, statement one in your last post is true, to the extent that water is a stronger base than hydrogen sulfide, I believe.  ^-OH is a stronger base than ^-SH, so that fits with this theory.

As to your original question as to why this reactions goes SN2 and how, methyl iodide, for example, is a very toxic compound, simply because it methylates anything in sight in the body.  The things methylated here would be things like alcohols, amines, thiols, etc, most of which would likely be protonated when they act as nucleophiles, but the reaction still occurs readily because the carbon iodine bond is so weak, so MeI is a good electrophile (similar to MeBr).

[movies]

Great posts in this thread everyone!  Very nice explanations.