H2F+ or CH3SH2+ which is more acidic by looking at the structures?

[ngu9997]

Homework Statement

Which molecule is more acidic H2F+ or CH3SH2+

Homework Equations

None

The Attempt at a Solution

H2F is more acidic as the answer. However, I was told in class that there were several factors affecting acidity, with some having more weight such as resonance>atomic radius> electronegativity> inductive effect.

So comparing the fluorine and sulfur, fluorine is more electronegative which makes its conjugate base less basic making it more acidic in this regard. And for atomic radius, sulfur is larger making its conjugate base less basic and making it more acidic in this regard. Using this logic I would think that the molecule with sulfur is more acidic since atomic radius has more weight than electronegativity. Then the only reason why I might be wrong is because of the methyl group. Through the last factor, CH3 donates electron density to sulfur making its conjugate base more basic and therefore the acid less acidic than H2F in this regard. Can someone tell me if my reasoning is right as to why H2F is more acidic? Or if I'm wrong, perhaps point me in the right direction.

 

[mjc123]

I think this sort of thing is very difficult to reason from first principles. The explanations you find in books are largely rationalisation after the fact of what we know by observation to be the case.

 

[epenguin]

Are you sure you mean CH₃SH₂⁺?
I would have thought CH₃SH very hard to protonate, IOW its conjugate acid, very acid.
Whereas HF is just a weak, or you could say strongish acid with pK in the region of 3.
If instead the question is about the acidity of CH₃SH, well those thiols/mercaptans/sulphydryls are all weak acids, dissociating in a moderately alkaline range, yours at pH 10. They are much more acid than the corresponding alcohols - that is where you can make rational comparisons.

As mentioned, prediction is quite difficult. A dominant effect on dissociations is rearrangement of the water molecules around the ions.
You can do better with the systematic comparison of related substances, and students get quite a lot of questions on this, (though I think some of the rationalisations still dodgy!)

 

[chemprof]

Are you sure you mean CH₃SH₂⁺?
I would have thought CH₃SH very hard to protonate, IOW its conjugate acid, very acid.
Whereas HF is just a weak, or you could say strongish acid with pK in the region of 3.
If instead the question is about the acidity of CH₃SH, well those thiols/mercaptans/sulphydryls are all weak acids, dissociating in a moderately alkaline range, yours at pH 10. They are much more acid than the corresponding alcohols - that is where you can make rational comparisons.

As mentioned, prediction is quite difficult. A dominant effect on dissociations is rearrangement of the water molecules around the ions.
You can do better with the systematic comparison of related substances, and students get quite a lot of questions on this, (though I think some of the rationalisations still dodgy!)

LOL RSH is always trivial to protonate or alkylate...these are great Lewis/Bronsted bases and nucleophiles - stronger base = weaker conjugate acid..SMH...these websites...