In short, the only way aromatic and antiaromatic compounds differ is the number of electrons they have in the conjugated system. All the other criteria-being cyclic, planar and fully conjugated are a must for both categories:. There is also the third class of compounds we need to discuss: These are the nonaromatic or not aromatic compounds. As the name suggests, nonaromatic compounds have really nothing to do with the aromaticity well, almost.
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Antiaromatic Compounds and Antiaromaticity. Our last post in this series on aromaticity went through the 4 conditions a molecule must fulfill in order to be aromatic. In that post we tried to explain what each of those rules meant — so if any of these individual items seem unclear to you, it might be a good idea to go back to that post.
Make a table. You need to know the few exceptions that come up — we covered that last time. The easiest example to start with is benzene, and it demonstrates how to use the table. It has zero lone pairs that contribute to aromaticity. Therefore, for the purposes of determining aromaticity, we can ignore the lone pair.
The cyclopentadiene cation below is cyclic and conjugated that positive charge represents a carbocation with an empty p-orbital. There are two pi bonds and zero lone pairs to contribute to the pi system. Pyrrole is cyclic and conjugated that lone pair on nitrogen can contribute to the pi-system.
There are two pi bonds and one lone pair of electrons that contribute to the pi system. This gives us 6 total pi electrons, which is a Huckel number i.
If pyrrole is protonated on the nitrogen, the lone pair can no longer participate in the pi-system. Therefore the molecule drawn below is not aromatic. Pyridine is cyclic, conjugated, and has three pi bonds. Therefore we can ignore the lone pair for the purposes of aromaticity and there is a total of six pi electrons, which is a Huckel number and the molecule is aromatic.
Thiophene, the sulfur analog of furan , is cyclic and conjugated all the way around the ring. It has two pi bonds. The other lone pair is in the plane of the ring, much like the lone pair on the nitrogen of pyridine, above.
Therefore thiophene has six pi electrons total, which is a Huckel number, and thiophene is aromatic. Cyclobutadiene is cyclic and conjugated. There are two pi bonds and zero contributing lone pairs. Therefore it is not aromatic, just like the cyclopentadienyl cation, above. Cyclobutadiene is not aromatic. If we somehow pump two electrons into cyclobutadiene, however, then the situation changes considerably.
The cyclobutadiene dianion is cyclic and conjugated. It has a single pi bond, and now two carbons bearing lone pairs which can contribute to the pi system, giving us a total of six pi electrons. Naphthalene has two rings and is thus a bicyclic compound. It is conjugated around the perimeter of the rings and there are a total of 5 pi bonds and zero lone pairs, giving us 10 pi electrons total. The pyrylium ion is a six-membered ring that, like benzene, has three pi bonds.
It also bears a lone pair on the oxygen. Like pyridine and the benzene anion, this lone pair is actually in an orbital at right angles to the pi system so it does not in fact count towards aromaticity. Therefore the molecule has only 6 electrons in the pi system and is in fact aromatic. Indole is a bicylic molecule that looks like a molecule of benzene fused to a molecule of pyrrole. It is conjugated; every atom around the perimeter of the two rings participates in the pi system.
It has 4 pi bonds and a single pair of electrons on the nitrogen that participates in the pi system, giving 10 pi electrons in total. This is a Huckel number and indole is in fact aromatic. There are five total pi bonds and zero lone pair electrons, giving a total of 10 pi electrons, which is a Huckel number.
And azulene is, in fact, aromatic. For example, in the resonance form below, you can think of azulene as being composed of the cycloheptatriene cation aromatic fused to the cyclopentadiene anion also aromatic.
Note that one ring bears a negative charge and the other one bears a positive charge. It turns out that this resonance form has a significant contribution to the hybrid, since azulene itself has a substantial dipole moment 1. Naphthalene, in contrast, completely lacks a dipole moment. The whole point of using a table like the one above is to organize your thoughts.
The most common part where students get tripped up is in counting the number of pi electrons. Just remember that each atom can at most contribute one p-orbital towards the pi system, and that the p-orbital can have at most two pi electrons. Leave a comment! Antiaromaticity and Antiaromatic Compounds.
Otherwise very helpful post!!! In furan,are the the two C-O bonds at right angles because they are covalent bond formed involving py and pz orbitals of oxygen. Also does the one lone pair of oxygen which is counted in pi electrons shows resonance because it is in px orbital which is perpendicular to py and pz and the molecular plane,while the other lone pair in s orbital does not involve in resonance because it lies in the molecular plane?
Above 1, 3 and 4 conditions, actually based on Huckel model, are long time ago obsolete. Loved this one. Helped a lot in understanding aromaticity. Still having problems in multiple ring questions what if one ring is aromatic and the other is antiaromatic? Cyclobutadiene dianion has 6 pi electrons. However, there is no conjugation covering ALL the atoms.
Is this not a violation of Huckel rule? Sure there is. All four carbons are conjugated through their p orbitals. Two of them are half-filled, and two of them are completely full. I have probably appreciated you before as well. I love you so much Sir James. You make organic chemistry so so so so so much easy. I thoroughly enjoy your lessons. God bless you more and more. Love, from Mumbai, India. I understand that if a bridge was present, it makes the molecular compound flat again, thus fulfilling a criteria of aromaticity.
Sorry, just a bit confused. Hi James, from down under in Oz. Thank you very much for your clear and concise explanations. My memory is not good, so understanding the mechanisms and the logic behind the chemistry really helps and saves taxing the memory. I used your table above to answer questions like this and it was very helpful. Hi Peter — The thiophene radical will not be aromatic.
It will be a radical cation. Thiophene has six pi electrons. Remove an electron from this and you have five electrons. The same is true for the benzene radical not to be confused with the phenyl or benzyl radical! Hope this helps! Cyclohexenyl dianion.
The two anions are in 1St and 4th carbon.. Hey James, for the Pyrrole conj. There is no way that the formal charge on nitrogen can be removed through pushing the other double bonds around, since the nitrogen has no available p orbitals that can be conjugated with the pi-system. Nitrogen in this case has all 8 of its electrons bound up in single bonds, and we know that resonance forms never involve breaking single bonds, right?
Is the conjugate base of Pyrrole aromatic? It does seem to have 6 pi electrons but I read somewhere that the Pyrrole anion is unstable? Yes it is. The N-H bond is in the plane of the ring along with the C-H bonds, and the lone pair liberated by deprotonating nitrogen does not affect aromaticity since it is at right angles to the pi system. How do we determine aromaticity of bicyclo compounds high , one ring is aromatic benzene and the other ring is a non planar cyclohexene ring?
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Next Antiaromatic Compounds and Antiaromaticity. Aromatic vs Antiaromatic vs Non Aromatic Practice Exercises Our last post in this series on aromaticity went through the 4 conditions a molecule must fulfill in order to be aromatic. First, it must be cyclic Second , every atom around the ring must have an available p-orbital Third , the number of electrons in the pi system must be 2, 6, 10, 14, 18, or a higher number in the set that increases from 18 in increments of 4 22, 26, 30….
Fourth , the molecule must be flat.
“Is This Molecule Aromatic?” Some Practice Problems
Antiaromatic Compounds and Antiaromaticity. Our last post in this series on aromaticity went through the 4 conditions a molecule must fulfill in order to be aromatic. In that post we tried to explain what each of those rules meant — so if any of these individual items seem unclear to you, it might be a good idea to go back to that post. Make a table. You need to know the few exceptions that come up — we covered that last time.
17.5: Aromaticity and Huckel's Rule
Valence bond theory uses Lewis diagrams to depict structure and bonding of covalent entities, such as molecules and polyatomic ions, henceforth molecules. The Lewis diagram of many a molecule, however, is not consistent with the observed properties of the molecule. The Lewis diagram of some molecules suggests a ring bearing a fully conjugated pi-electron system, or loop of pi electrons, provided each atom in the ring is sp 2 — or sp-hybridized. As evident from the stability of pi electrons however, only some such rings actually have a loop of pi electrons. Of the above examples, only the rings in 2 , 4 , 5 , and 7 have a loop of pi electrons.
How to Determine the Aromaticity of a Ring System
You can determine whether a ring system is aromatic, anti-aromatic, or non-aromatic by determining whether it meets certain conditions. To be aromatic, a molecule must meet the following four conditions:. In other words, no atom in the ring can be sp 3 hybridized. If the molecule meets the first three conditions, but only has 4 n pi electrons, the molecule is anti-aromatic. If the molecule fails any or all of the first three conditions, then the molecule is non-aromatic. Following are some details about each of these points. The second condition involves the shape of the ring.
Aromaticity Practice Problems for Aromatic, Antiaromatic, Nonaromatic
Here, I have tried to hub three 03 time economic mnemonics by including two 02 formulae for the prediction of hybridization of hetero atom, aromatic and anti aromatic behavior of heterocyclic compounds. The conventional methods for determination of hybridization state of hetero atom planarity of molecule , prediction of aromatic and anti aromatic nature of heterocyclic compound is time consuming. Hetero atom atom containing lone pair of electron which is directly attached with single bonds only from all ends is to be considered as DLP containing hetero atom and its lone pair is to be treated as DLP. In Pyrrole lone pair of N atom is to be treated as DLP because it is directly attached with three single bonds only. Hetero atom atom containing lone pair of electron which is directly attached with single and double bonds with the ring system is to be considered as LLP containing hetero atom and its lone pair is to be treated as localized lone pair of electron LLP.