Organic Chemistry / Major and Minor Resonance Contributors
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Solution: Basic red 1 is a tetracyclic compound (it has four rings) th...

Question

Basic red 1 is a tetracyclic compound (it has four rings) that shares many structural similarities with the dyes in the previous problem (Chem. Rev. 1993, 93, 381–433). This compound has many significant resonance structures, and the positive charge is highly delocalized. While resonance structures can be drawn in which the positive charge is spread throughout all four rings, nonetheless, one of the rings likely bears very little of the charge relative to the other rings. Identify the ring that is not participating as effectively in resonance and suggest an explanation. 

Video Transcript

Alright guys. Let's take a look at this question, then we have this compound, a basic red one, right? And it looks something like this and notice that there's going to be a lot of delocalization of the positive charge on that nitrogen because we have alternating double and single bonds throughout our molecule and it wants to know of all these rings, which one's going to be the least likely to participate, okay? We have A, B, C and D, okay? Which ring do you think. Notice that ring A, B and C are all directly attached to each other, D is kind of coming off, right? there's a single bond in between them and notice that here's our the positive charge, we have a hydrogen coming off here and it looks like this substituent, this ring, right? And our group that it has is overlapping a lot with that other compound, there's a lot of sterics going on, okay? So, although this positive charge can delocalize on to all my rings, one's going to be just a little bit off, it's not going to be able to delocalize that positive charge as much. So, after explaining what I just did here, which ring do you think it's going to be? Remember, the three rings A, B and C are stuck the way they are, they can't move, but notice that ring D could actually rotate because it's separated by a single bond to the other rings and we know that Sigma bond can freely rotate. So, actually, what can occur is if, let's draw the structure again, we actually could have this ring D right down here, we can actually have it move out of plane and we can draw it by drawing something like this, right? Where now we have this structure completely out of plane, okay? that's indicated by this dash that we have at this wedge, right? So what's most likely to happen in sterics, in this big molecule is ring D actually rotate out of plane and remember that in order for this positive charge to delocalize we're going to need to use the P orbitals and it can only, our P orbitals are going to actually overlap, if our P orbitals aren't overlapping with each other we actually won't be able to delocalize this charge, they need to be in the same plane essentially. So, ring D since their steric hindrance going on, right? We have this hydrogen, it's going to rotate on a plane and therefore that positive charge is not going to be able to delocalize with it as much because we no longer have our p orbital on the same plane. Alright guys, so that is why ring D is most unlikely to carry that positive charge on it. Alright guys, hopefully this makes sense.