Acidity of H2O vs H2S: Formal Charge vs Electronegativity

[PhysicsCanuck]

Homework Statement

What is the Lewis dot structure for thiocyanic acid?

Relevant Equations

NA

I'm aware that the structure on the left is correct, but given that the formal charges are the same for each structure, I can't figure out why hydrogen is more likely to bond to the sulphur atom than it is to the nitrogen atom.

Thank you for your help here!

 

[TeethWhitener]

Both structures exist. The one on the left is thiocyanic acid and the one on the right is isothiocyanic acid. In equilibrium at STP, the molecules exist as a mixture of about 95% isothiocyanic acid (right) and 5% thiocyanic acid (left)

 

[PhysicsCanuck]

Both structures exist. The one on the left is thiocyanic acid and the one on the right is isothiocyanic acid. In equilibrium at STP, the molecules exist as a mixture of about 95% isothiocyanic acid (right) and 5% thiocyanic acid (left)

Hello and thank you very much for your response,

Could you also help me understand why the hydrogen is 20x more likely to bond with the sulphur than with the nitrogen?

Cheers

 

[TeethWhitener]

Hello and thank you very much for your response,

Could you also help me understand why the hydrogen is 20x more likely to bond with the sulphur than with the nitrogen?

Cheers

Read your question again and read my response more closely.

I actually think I gave you a little too much info the first time around (forgot I was posting in a homework thread).

 

[PhysicsCanuck]

Read your question again and read my response more closely.

I actually think I gave you a little too much info the first time around (forgot I was posting in a homework thread).

Oh I see I worded it backwards (which makes it even more confusing).

My resource only shows thiocyanic acid as an option--I was actually just trying to discover for myself if isothiocyanic acid can or does exist.

Could you help me understand why the H-N form of the tautomer is more common than the H-S form?

Thank you very much for your time and help here

 

[TeethWhitener]

Could you help me understand why the H-N form of the tautomer is more common than the H-S form?

What do you think? What do you know that can help you in this situation?

 

[PhysicsCanuck]

What do you think? What do you know that can help you in this situation?

Well the formal charges are the same, so I don't believe they will contribute.
My next thought is that electronegativity may contribute, but for the same reason as formal charge, I don't believe that should matter either.
I then wondered about tautomerization and have spent some time on Google and YouTube, but I couldn't locate anything that would suggest one of these forms should predominate.

Next I considered the deprotonated form (i.e. SCN- = thiocyanate = ||S=C=N|| ) and was trying to determine why the hydrogen atom would more readily form a bond with the nitrogen atom than the sulphur atom. Since the negative (formal) charge exists on the nitrogen, then I assumed it would thus more readily form a bond with the arriving positively-charge hydrogen atom.

 

[TeethWhitener]

My next thought is that electronegativity may contribute, but for the same reason as formal charge, I don't believe that should matter either.

I’m not sure I follow. You said the reason formal charge doesn’t matter is because the formal charge is the same for nitrogen and sulfur. Is this also true for electronegativity?

 

[TeethWhitener]

Next I considered the deprotonated form (i.e. SCN- = thiocyanate = ||S=C=N|| ) and was trying to determine why the hydrogen atom would more readily form a bond with the nitrogen atom than the sulphur atom. Since the negative (formal) charge exists on the nitrogen, then I assumed it would thus more readily form a bond with the arriving positively-charge hydrogen atom.

But that structure is only one resonance structure of the thiocyanic anion.

 

[PhysicsCanuck]

I’m not sure I follow. You said the reason formal charge doesn’t matter is because the formal charge is the same for nitrogen and sulfur. Is this also true for electronegativity?

Sorry, I was confusing there.
Nitrogen is more electronegative than is sulphur which implies that when involved in a chemical bond, nitrogen will draw (relatively) more electron density towards itself than would sulphur. But because both tautomer forms will produce molecules with formal charges of zero, then electronegativity should play no role.

Is that accurate?

 

[PhysicsCanuck]

But that structure is only one resonance structure of the thiocyanic anion.

It is only one form, but it would be the most common form as it has the lowest formal charge (a single negative charge) and the negative charge is on the most electronegative atom.

 

[PhysicsCanuck]

ahhhh, reasoning from my two previous responses, I suppose because nitrogen is more electronegative, and because it carries the negative charge on the deprotonated form (thiocyanate) then it is most likely to form a bond with the hydrogen atom

 

[TeethWhitener]

It is only one form, but it would be the most common form as it has the lowest formal charge (a single negative charge) and the negative charge is on the most electronegative atom.

What about:
$$S=C=N^- \longleftrightarrow {}^-S-C\equiv N$$

 

[TeethWhitener]

But because both tautomer forms will produce molecules with formal charges of zero, then electronegativity should play no role.

What does formal charge have to do with electronegativity?

 

[PhysicsCanuck]

What about:
$$S=C=N^- \longleftrightarrow {}^-S-C\equiv N$$

Because nitrogen is more electronegative, the resonance structure on the left will predominate

 

[PhysicsCanuck]

What does formal charge have to do with electronegativity?

I have no idea.

What do you think?

 

[TeethWhitener]

Ok but I hope you see that the formal charges are the same, just with the single negative charge on either sulfur or nitrogen.

 

[PhysicsCanuck]

Ok but I hope you see that the formal charges are the same, just with the single negative charge on either sulfur or nitrogen.

Yes! Thank you

 

[TeethWhitener]

I have no idea.

What do you think?

It’s your homework question. Here’s a hint: in comparing the acidity of H₂O with H₂S, which matters more, the formal charge or the electronegativity?