Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: UTTARA on November 12, 2009, 09:13:02 AM
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In the thiocyanate ion ,SCN - 3 RESONATING STRUCTURES ARE POSSIBLE WITH THE ELECTRON DOY METHOD AS SHOWN
S C N-1 S -1 C N S1+ C N2-
The decreasing order of % contribution in resonance hybrid is.. GIVE REASON
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Ok, so you have three possible resonance structures. The whole point of a resonance structure is to share the electric charge between atoms, instead of lumping it all on one, because that creates instability. The more "spread out" a charge is, the more "stable" a compound is, or in this case, the more likely one resonance structure will exist in a higher % than another.
The first two resonance structures, I can tell you, are the more common ones. The last structure with a -2 charge on nitrogen and a +1 charge on sulfur gives the least contribution to resonance structure because it is very unstable (2 charges on 2 atoms (unstable) vs a single -1 charge on a single atom (more stable).
So now you have to decide whether the structure with the -1 charge on N or S is more common. For the answer to that question, I suggest you search for an "electronegativity" table and find out which element is more electronegative, and that should give you your answer.
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So ans is S-1 C N > S C N-1 > S+1 C N2-
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Ok, so you have three possible resonance structures. The whole point of a resonance structure is to share the electric charge between atoms, instead of lumping it all on one, because that creates instability. The more "spread out" a charge is, the more "stable" a compound is, or in this case, the more likely one resonance structure will exist in a higher % than another.
The first two resonance structures, I can tell you, are the more common ones. The last structure with a -2 charge on nitrogen and a +1 charge on sulfur gives the least contribution to resonance structure because it is very unstable (2 charges on 2 atoms (unstable) vs a single -1 charge on a single atom (more stable).
So now you have to decide whether the structure with the -1 charge on N or S is more common. For the answer to that question, I suggest you search for an "electronegativity" table and find out which element is more electronegative, and that should give you your answer.
I agree that the charge-separated form is probably the one that contributes the least, out of the three given. I just wonder if bond strength arguments point to the same answer as electronegativities (don't have tables here). I acknowledge that most tables of bond strengths are average ones, so may not be 100% useful here..?
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I would say electronegativity plays a greater role than bond strength, because it is a measure of which atoms will "attract" that -1 charge. That being said, Nitrogen has a higher electronegativity than Sulfur, so I would guess the -1 charge on the nitrogen would contribute more, but Sulfur has a larger atomic radius, so the charge could be more easily spread out on the sulfur atom than on the nitrogen atom. I think this is a case where you really need to use experimental data (if possible to find/obtain) in order to get the right answer.
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The two most common forms are S-1CN and SCN-1. They are both major forms. Normally you would pick SCN-1 as the more prominent structure because electronegativity arguments tend to be most reliable in predicting the major resonance form, but in this case you have a problem because if you draw out the structures you will see that the double bonds in the SCN-1 resonance form are cumulated. "Cumulated" means (in English) that they are directly next to each other along the chain, and this typically is not a stable arrangement favorable to resonance (double bonds are much lower in energy if they are placed in a "double bond, single bond, double bond" pattern along a chain as opposed to in a "double bond, double bond" pattern as the two double bonds cannot resonate with one another if they are directly next to each other). Putting the lone pair on the sulfur also allows for the formation of a stable carbon nitrogen triple bond (no cumulation here, just a straight triple bond), and you gain some bonus as Grundalizer mentioned due to the large sulfur atom's ability to delocalize the negative charge. Hence, in this situation the most electronegative case does not win out and S-1CN is the major resonance form.
See the image here (the numbers show that 52% of the charge will be more on the sulfur as opposed to only 32% of the charge on the nitrogen): http://www.chemistry.mcmaster.ca/~aph/chem1a3/lectures/lec06/sld005.htm
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So , finally is Uttara's ans right??
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Yup.