in this video we're going to talk about how to tell if a molecule or if an ion is aromatic anti-aromatic or non-aromatic so let's talk about the conditions that must be met for a molecule to be aromatic so first that molecule it has to be cyclic every atom in that molecule must have a p orbital so therefore you can't have any sp3 carbons you could have an sp2 or an sp carbon but it can have any sp3 carbons now the molecule has to be conjugated so the pi electrons have to be free to move around in the ring and so all of the p orbitals they must be able to overlap with adjacent p orbitals the molecule must be planar it has to be flat it has to have a 2d structure and it has to follow huckel's number or huckel's rule for n plus 2. so when n is 0 you get 2 when n is 1 you get 4 plus 2 which is 6. so when n is 2 you get 10 when n is 3 you get 14 and these represent the number of pi electrons in the cyclic ring structure of the molecule in order for it to be aromatic now let's look at the next column the conditions for anti-aromaticity is very similar to aromaticity the molecule must be cyclic there can't be any sp3 carbons it has to be conjugated the pi electrons have to be free to move in the ring it has to be planar but it doesn't have to follow huckel's rule in fact it follows four n so when n is one you get four when n is two you get eight when n is three you get twelve and so if you have a molecule with four pi electrons or eight pi electrons with these conditions met then it's going to be anti-aromatic now if one of these conditions are not met it's automatically non-aromatic so if the molecule let's say is a linear molecule if it's not cyclic then it's non-aromatic if you do have an sp3 carbon in the ring it's not going to be aromatic if it's not conjugated it's not aromatic let's say if it's a non-planar if it has a 3d shape as opposed to a 2d structure it's not aromatic or if it has an odd number of pi electrons such as 3 5 7 9 it's going to be non-aromatic now you need to know that aromatic compounds are very stable anti-aromatic compounds are unstable and non-aromatic compounds are just like normal they're regular so aromatic compounds are more stable than non-aromatic compounds and non-aromatic compounds are more stable than anti-aromatic compounds now i'm going to give you a series of molecules and ions and i want you to determine if they're aromatic anti-aromatic or non-aromatic so let's start with our most common example benzene is benzene aromatic anti-aromatic or non-aromatic benzene is cyclic it's conjugated and it doesn't have any sp3 carbon atoms all of the carbon atoms are sp2 hybridized which means each carbon atom has a p orbital where the pi electrons can move now every double bond contains a pi bond and one pi bond contains two pi electrons so benzene has two four six pi electrons so therefore it follows huckel's rule where aromatic compounds can have 2 6 10 or 14 pi electrons and benzene has six so we could say that benzene is an aromatic compound and so all of the pi electrons are free to move in the ring if we want to we can draw the resonance structure of benzene so the resonance structure looks like this you just gotta move those double bonds so because benzene is aromatic it is very very stable now what about this molecule 1 3 butadiene is it aromatic anti-aromatic or not aromatic well this molecule is conjugated a conjugated molecule has alternating double and single bonds so here we have a double bond single bond double bond so it's conjugated the only problem with this is is that it's not cyclic it's not in a ring and so because it's not cyclic it can't be aromatic or anti-aromatic automatically it's non-aromatic and so that's the answer for this one now what about our next example cyclobutadiene is it aromatic anti-aromatic or non-aromatic so this molecule is cyclic it's conjugated we have alternating single and double bonds every carbon atom has a p orbital because all the carbon atoms are sp2 hybridized and it's planar it has a 2d structure so is it going to be aromatic or anti-aromatic so now let's count the number of pi electrons so this is two and four so it has four pi electrons this doesn't follow huckel's rule which is four and plus two for aromatic compounds it doesn't have this number rather it follows the rule for anti-aromatic compounds which has the numbers 4 8 12 and so forth so because cyclobutadiene has four pi electrons it's anti-aromatic which means it is not stable here's another example so this molecule is called naphthalene it has two rings and what do you think about the aromaticity of this molecule is it going to be aromatic anti-aromatic or non-aromatic so we can see that it's cyclic it's conjugated we have alternating single and double bonds all of the carbon atoms are sp2 hybridized and let's count the number of pi electrons so this is going to be 2 4 6 8 10. so this follows huckel's rule of 4n plus 2. so since we have 10 pi electrons and all of the conditions for aromaticity are met naphthalene is an aromatic compound let's try this example so this molecule that i'm currently drawn is called phenanthrene so do you think this molecule is going to be aromatic anti-aromatic or non-aromatic so we can see that it's cyclic conjugated planar it has all of those characteristics so all we need to do is count the number of pi electrons so we have 2 4 6 8 10 12 14. so is it aromatic or anti-aromatic let's see if it follows huckel's rule when n is zero we get two and then keep adding four so the next number is six ten and fourteen so these are one of the aromatic numbers so since we have 14 pi electrons and all of the other conditions are met we could say that phenanthrene is an aromatic compound now for those of you who might be studying for the organic chemistry final exam i have a video that can help you and it's on my patreon page if you go to patreon.com slash math science tutor you can access that page and if you scroll down there's a lot of other videos i have here too but let's say if you're taking the first semester of organic chemistry i have a six hour video that can help you with that if you decide to become a patron now on youtube i have a free two hour trailer version of this video but if you want the entire six hour video you can access it here or on vimeo as well and for those of you who are taking the second semester of organic chemistry i have an eight hour video that you can access as well and there's some other stuff here that you could find too if you're taking gen chem or physics i have stuff on that as well so that's it just in case you're interested for the next example let's discuss cyclopentadiene so do you expect this molecule to be aromatic anti-aromatic or non-aromatic so this molecule is cyclic we do have conjugation we have some alternating single and double bonds however it's not completely conjugated notice that we have an sp3 carbon here so that carbon atom it doesn't have a p orbital where the pi electrons can resonate through and so as a result it's not going to be aromatic or anti-aromatic so this molecule is non-aromatic so there's nothing special about it right now