Some of the following compounds show aromatic properties, and ...

Question

Some of the following compounds show aromatic properties, and others do not.

1. Predict which ones are likely to be aromatic, and explain why they are aromatic.

(n)

(o)

Accepted Answer

All right. Hello everyone. So for this question, let's go ahead and identify whether the following compounds exhibit aromatic characteristics or not. If the compound is aromatic, explain why it is aromatic. And here we have two compounds given. So before we go ahead and talk about both compounds, individually recall that aromatic has four distinct criteria, these criteria can be treated as a checklist. So essentially compounds that have all four of them met are considered aromatic. So the first requirement is that the molecule or species in question must be cyclic, it must contain a ring. In addition to that, that ring must be planted, the ring must also have a conjugated pie system. And we'll talk a little bit more about what that means as we go through each given compound here. But last but not least, the fourth criterion has to do with the number of pi electrons in the conjugated system. Now, once you've counted the number of pi electrons in the system, the idea is to set that number equal to the expression for N added to two. If solving for N in that situation yields a nonne whole number including zero, then huels rule is met and the compound is ultimately aromatic. So let's apply that now to our first one here. So going through the checklist, we can see that our first compound here is cyclic because it contains a ring, not only does it contain a ring, it's also going to be cleaner because all carbons inside of the ring are sp two hybridized. Now, it's important to mention that for the sake of this problem, we're going to consider the compound exactly how it's drawn here where all of the double bonds are in the Z configuration. So with that in mind, it is planet. And so for the third criteria, does it have a conjugated pie system? And the answer here is yes because notice the positioning of the car bottle that's inside of the ring here, recall that the resonance form the other resonance form of a car bal is one in which the carbon atom has a positive charge where it's a carbo cion V. Carbo cion provides an empty P orbital. And in addition to that inside of the ring, we have these four pi bonds that are spaced apart from each other by one single bond in between them, the carbon carbon, which would contain the empty P orbital when it's a carbo caton is also one single bond apart from the closest double bonds inside of the ring. And so therefore, this is in fact a conjugated pie system. All right. So here it's a conjugated pie system. So now the question is, is the fourth criteria met. So first, the number of pi electrons that's going to be counted here are the number of pi bonds inside of the ring as well as any lone pairs. So in this particular case, we have four double bonds inside of the ring. So that's four double bonds with a total of two electrons per double bond giving a total oops of eight of eight pi electrons in the system. And so to try and see if this molecule is aromatic, we're going to take the expression for and add it to two. And we're going to try and make this equal to the number of pi electrons, which is eight. Unfortunately, though solving for this gives you a value of N equal to 1.5 which is not a whole number. And so huckle rule is not met. Cole's rule is not met, which means that this molecule is not aromatic. So just to go ahead and answer part one here, this molecule is not aromatic or rather it does not exhibit aromatic properties. So now let's do the same thing to compound two running through our list here, we can see that it's cyclic and it's also going to be cleaner because all carbons are all atoms inside of the ring are sp two hybridized. And also for a similar reason as the first molecule, the second compound does have a conjugated pie system. This is because when you consider the location of the car bal relative to the other py bonds, the car bono itself, as well as all pi bonds inside of the ring are spaced away from each other with one single bond in between. So it is conjugated. So this leaves us now with the fourth criteria. And so once more, the total number of pi electrons in the system is going to be based on the number of pi bonds within the ring as well as any lone pairs if you happen to have any. So looking at that, we have a total of four P bonds inside of the ring and each one has two electrons. So that's a total of eight. So here, because we have the same number of pi electrons as the first compound, if we do try and solve for the expression four and added to two, we're going to get the same result and is not going to be a whole number, a positive whole number to be specific, which means that it is not going to be aromatic either. So essentially compounds one and two do not exhibit aromatic properties and there you have it. So with that being said, thank you so very much for watching. And I hope you found this helpful.

Some of the following compounds show aromatic properties, and others do not.
1. Predict which ones are likely to be aromatic, and explain why they are aromatic.
(n)
(o)

Key Concepts

Aromaticity

Hückel's Rule

Conjugation

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