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CNS- formal charge

Posted: Sat Nov 02, 2019 5:07 pm
In the lecture, we went over how the best version of CNS- is when the formal charge of -1 is on the S. Why is the formal charge of -1 on sulfur and not on nitrogen?

Re: CNS- formal charge

Posted: Sat Nov 02, 2019 5:39 pm
I don't recall seeing that structure in my notes, but assuming that carbon is the central atom, the most electronegative atom should carry the negative formal charge. In this case, N should have the negative charge since it's more electronegative than sulfur. So the most stable structure would go: S = C = N.

Re: CNS- formal charge

Posted: Sat Nov 02, 2019 9:01 pm
Yes, the example in the book also shows that the most stable structure is S = C = N. Nitrogen does have a -1 formal charge in this structure, and carbon and sulfur both have formal charges of 0. This example is on page 85 in the book. In the structure C = N = S, the formal charges are -2 for C, +1 for N, and 0 for S, and the structure C = S = N has the charges -2 for C, +2 for S, and -1 for N. In these examples, S never has a formal charge of -1.