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Solution: Which one of the following is not a permissible contributing...

Question

Which one of the following is not a permissible contributing structure? Why? 

Video Transcript

So, let's take a look at these four compounds here and figure out which of them is actually not a resonance contributor, okay? So if we go ahead and look at these, guys we know our bonding purposes, right? We know that carbon likes to have four bonds, nitrogen likes to have three bonds and a lone pair and oxygen likes to have two bonds and two lone pairs, so now let's see what's in violation or in a different arrangement, okay? So, right here we can see at this carbon has only three bonds and no lone pairs, so it's got a positive charge, down here this nitrogen is in good shape, three bonds lone pair perfectly fine, this oxygen has one bond and 3 lone pairs, so it's got a negative charge, then this carbon has four bonds total, in fact, in all of these this bottom carbon has four bonds, so I'm not going to talk about them anymore. Up here, in B we've got a nitrogen with four bonds and no lone pairs, we've got this carbon that has four bonds. So, no formal charge and then we've got this oxygen with three bonds, three lone pairs and one bond, right? So it's got a negative charge, but C, guys look at our nitrogen, we know that it's in group five, right? So it has five valence electrons with which to bond or to have a lone pair, right? That's why we have three bonds and a lone pair, that's its preference, because if we look at this and we have five bonds we're violating nitrogen's octet, right? Because look we've got two boss here, so let's, we've got two bonds, two electrons in each of, right? so that's 2, 4, 6, 8, 10, so it's got not a, it's got as the ten electrons versus eight in its octet, right? So this actually is an invalid structure, this last one over here is perfectly fine, right? Four bonds. nitrogen is got a negative charge, two bonds to oxygen, it's got a zero charge and 3 bonds and a lone pair on that carbon gives it a negative charge, so now we can go ahead and actually rank them in order stability, so guys here we've got something interesting to worry about, right? We've got our electronegativity trans and while they're a good guideline they're not going to solve every problem that we have here and you'll see why in a second, so if we look at A, right? We've got a positive charge on carbon, which kind of makes sense considering that we know electronegativity increases as we move up into the right and that oxygen is fantastic, it's great having that negative charge, right? Perfect. Now, B also has the negative charge on oxygen, but it's got the positive charge on nitrogen and guys it actually turns out that B is more stable than A, in fact D is also more stable than A, okay? The reason for that is that B and D have more bonds than A does, okay? And guys, B is actually best, okay? Because the order is B, D, A in order to stability, so this is going to say, this is 1, 2 and this is 3, why? Well guys, here we've got the negative charge on our oxygen, so that's great, positive charge of our nitrogen, could be better right if we had a CH here of vinyl carbocation, that would actually be kind of unstable, okay? So, vinyl carbocations are notoriously unstable and then we have over here, we've got our D, okay? That has the positive charge on nitrogen, negative charge on this, on this carbon, so that's not that great, right? We'd rather have the negative charge on the oxygen, okay? And then over here we've got our negative charge on the oxygen and on the carbon. So, really it's just the bonding preference, it's just that we have more bonds in B and D than in A. Alright guys. So, please, let's go ahead and actually do the draw our resonances for these molecules, for these compounds, okay? So now let's go ahead and draw our resonance brackets, we know that those are very important, and here we can see something, right? we're going from this molecule, let me go ahead and spread this out actually, we're going between these molecules like that. So, how are we going to do this? Well, you can see that we have a lone pair here, we've got a positive charge here and then in the next molecule, right? We've got, or in the next resonance structure we've got a positive charge on our nitrogen and a double bond between that carbon and the nitrogen, so what do we do? Well, what we do is we kick electrons on to this bond and now we just created this, okay? And now to get from here to here, what can we do? well guys, we can do is shift electrons because we've got a double bond here and then we've got a lone pair on our carbon, so what happened here is that our oxygen kicked its lone pair onto this, onto this bond here, creating a double bond but now this nitrogen has too many bonds, so we kick these electrons on to that carbon, okay? And then we end up with this here, it's kind of difficult to go back to this configuration without going through this resonances structure so we're probably not going to go ahead and do this because we need to create another chain. Alright guys, so if you have any questions please feel free to let me know, if not, let's move on.

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