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Video Solution: Of two possible structures A and B for the conjugate acid of...

Problem

Of two possible structures A and B for the conjugate acid of guanidine, the more stable is the one that is better stabilized by electron delocalization. Which one is it? Write resonance structures showing this electron delocalization. 

Video Transcript

So, let's say we've got this molecule here and we go ahead and add some acid into the solution to protonate it. Now, there are 2 different locations where we can protonate, right? We can either go ahead and add a hydrogen to one of these locations, right? Which is what we have here or we can go ahead and add it to the location up that's got a double bond to that nitrogen, which is what we have over here, so let's go ahead and figure out which of them is more stable by drawing our resonance structures, right? And not only, if it's more stable, right? That's going to be our dominant form in the solution, so now down here we can go ahead and resonate, what can we do? well we know that we have a lone pair here, so we can go ahead and kick electrons on to here, kick electrons from this double bond on to this nitrogen since this carbon won't have too many bonds, so now will we get is H2N, double bond, positive charge since it has got four bonds, kick up right? We know that we've got an NH but now you, let me go ahead and get rid of that, we've got two lone pairs, right? So we've got actually a negative charge and then we've still got our NH3 positive and actually that's the last resonance structure we can make, right? the only thing we can do is go back to this one up here, by kicking electrons like that and then back to that nitrogen, is that very helpful? no actually, right? this is more stable by far because here we've got still a positive net charge as we should expect but then we've got a separation of charge, which is not very good, so now this next one over here on the right, can we do something? absolutely, we can go ahead and start the same way and kick electrons here and now we kick electrons on to that N but now what we get is a little bit different, right? Let's say we've got now our NH2 and then we've got one lone pair and 3 bonds, which is exactly nitrogen's bonding preference, so now we've got a single bond there but now we've got a double bond over here, H2N, and our positive charge is now over here on the left, then we've got this NH2 with a lone pair, can we do anything from here, can we continue resonating? yeah actually, we can go ahead and use this nitrogen, kick electrons over here, kick electrons down that way and then what we get is our H2N, single bonded, single bonded up here, NH2 neutral, right? With our lone pair, this also has its lone pair so it's neutral and then we've got this NH2 positive, let me move out of the screen. Okay, so now this is much more stable, right? Because we've got, we don't have a separation of charge in any of them and what we've got is three resonance structures versus just two. So, by far this molecule, this protonated version is much more stable. Alright guys, let me know if you have any questions about any of the chapter, if not guys, let's go ahead and move on.

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