The structure for the DNA base cytosine is shown.

Which of the following is not a resonance structure of cytosine? (LO 7.13) (a)

(b)

(c)

(d)

Video transcript

welcome back everyone. One of the following is not a resonant structure. For timing, we need to identify it. So, for getting our first resident structure, we can imagine shifting one of our lone pairs on nitrogen to a bond between nitrogen and carbon where carbon would have five bonds, which it cannot allow. And so it would cause one of these bonds between itself and oxygen to shift to oxygen as a lone pair. And so this would create our first resident structure where if we analyze our anti choice is below, that would be choice A. So we can rule out choice A. Because we want to pick the answer choice, that is not a resident structure of timing. So going back to timing and figuring out a second resident structure, we can also imagine shifting this lone pair on nitrogen to a bond here between this nitrogen and carbon, where this carbon would have too many bonds, causing this pi bond to shift to this location, which would ultimately create too many bonds on this carbon, causing it to have to shift one of its bonds between oxygen as a lone pair on oxygen. And this would correspond to our answer choices where we would have the following structure in choice B as our second resonant structure for diamond. And now going back to timing. So, we would rule out choice B. Because of that reason. And going back to time. And we want to come up with our next resident structure where we can imagine instead shifting one of the lone pairs on this nitrogen as a bond between nitrogen carbon here, which would ultimately also shift one of our bonds between this carbon as a lone pair onto the oxygen here. And so, looking at our answer choices, we would find the resident structure where we have a positive charge on this nitrogen here, since it would have lost its lone pair to form that bond between nitrogen and carbon here and so now, this would be our third resonant structure we would expect from timing, meaning we can rule out choice D. And because we've ruled out choices A B and D. That means that choice C. Would be the only final answer to complete this example as the structure that is not a resonant structure of timing. So C would be our final answer. To complete this example. I hope everything I reviewed was clear. If you have any questions, please leave them down below and I'll see everyone in the next practice video.