Huckels Rule?

[LSN]

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I don't quite understand Huckels Rule? In the Kaplan book, it talks about "anti-aromaticity". Is that the same as non-aromatic? The requirements for anti-aromatic are 4n pi electrons? It gives a triangle with one double bond, 2 lone pairs at the tip, and a negative charge. What do these mean? Do you count the lone pair, and negative sign? 😕

 

[DrTacoElf]

I don't quite understand Huckels Rule? In the Kaplan book, it talks about "anti-aromaticity". Is that the same as non-aromatic? The requirements for anti-aromatic are 4n pi electrons? It gives a triangle with one double bond, 2 lone pairs at the tip, and a negative charge. What do these mean? Do you count the lone pair, and negative sign? 😕

Anti and non aren't the same

A good example of non is a system that is not conjugated. Anti results from 4N pi electrons in a conjugated system. The triangle with 1 double bond and 1 lone pair at the top (2 e-) has a total of 4 pi e-.

 

[Sprgrover]

Huckel's rule is simply 4N + 2, where n is an integer (0, 1, 2, 3, and so on). Thus, the aromatic numbers are 2, 6, 10, and so on. Second, you need to examine the system of electrons: only closed systems qualify and they must fit one of those aromatic numbers. For example, lets say that you have cyclo-hexene and there are six conjugated pi electrons, except one carbon is sp3 hybridized and has two hydrogens hanging off of it. Although the number of pi electrons fits, this compound is not aromatic because it is not closed: that sp3 carbon 'breaks' the compound's aromaticity. That's a rough example, but the gist of Huckel's rule is to look for the electron magic numbers (2, 6, 10, 14, etc.) and also make sure the system is closed and conjugated.

 

[grikmok]

What does the 'n' stand for. I know it says that it can be any integer, but does it represent the number of pi-bonds?

Thanks!

 

[NyCzPeter]

What does the 'n' stand for. I know it says that it can be any integer, but does it represent the number of pi-bonds?

Thanks!

N is the variable that you try to solve for. If you're using 4N+2=#pi electron, then the compound is aromatic if N turns out to be an integer. If you're using 4N=#pi electrons, and N turns out to be an integer when you solve for it, then its anti. Any others are non-aromatic which is NOT the same as antiaromatic. N stands for nothing but an number you solve to see if its an integer or not.

Non aromatic is anything normal like cyclohexane. Aromaticity in molecules like benzene stabilizes it but antiaromaticity actually weakens the stability of the molecule.

So in order of stability
Aromatic>Non-Aromatic>Anti-Aromatic

 

[Sublimation]

If the molecule is planar and all the carbons are SP2 so that you have an empty porbital, and you find that the number of pi electrons is a multiple of 4, then u got urself a anti aromatic, however anything that isnt a multiple of 4 and all of the previous conditions are met you have a aromatic. now if you have and sp3 carbons then u got a nonaromatic.
Rules

1) look for conjugation.
2) is it continuous, in other words do all the carbons have available p-orbitals (i just make sure thay are all sp2) and remeber a heterocyclic compound with R-NH-R is planar the lone pair will be included in the ring. They usually try to get u with those.
3) Huckels rule. Multiple of 4, anti . Not a multiple of 4 Aromatic.

 

[SKation]

Huckel's rule is simply 4N + 2, where n is an integer (0, 1, 2, 3, and so on). Thus, the aromatic numbers are 2, 6, 10, and so on. Second, you need to examine the system of electrons: only closed systems qualify and they must fit one of those aromatic numbers. For example, lets say that you have cyclo-hexene and there are six conjugated pi electrons, except one carbon is sp3 hybridized and has two hydrogens hanging off of it. Although the ?number of pi electrons fits, this compound is not aromatic because it is not closed: that sp3 carbon 'breaks' the compound's aromaticity. That's a rough example, but the gist of Huckel's rule is to look for the electron magic numbers (2, 6, 10, 14, etc.) and also make sure the system is closed and conjugated.

Hey!

For Huckel's rule I understand the concept however I believe that I learned that it is used for monocylic compounds however my EK book is using it for anthracene which has 3 cyclic components? Is there a reason as to why this might be?
Thanks

 

[1Overdrive]

Hey!

For Huckel's rule I understand the concept however I believe that I learned that it is used for monocylic compounds however my EK book is using it for anthracene which has 3 cyclic components? Is there a reason as to why this might be?
Thanks

It's not a very good idea to revive a nine-year-old thread but I'll bite. Huckel's rule can be used for a polycyclic compound, so long as the system is conjugated and all the rings are planar (i.e. no ring has more than 7 (I believe it's 7, could be wrong) carbons making it up). In the case of anthracene, it is conjugated, with 7 pi bonds, meaning 14 pi electrons participate in the conjugation. 14 electrons fits the (4N + 2) criteria, because if you set N to 3, it's 4(3) + 2 = 12 + 2 = 14. So, yes, anthracene is definitely aromatic.

 

[Dotoday]

Yes. You simply count the pi electrons ( double bonds or lone pairs), then plug it into the formula to solve for n. If n comes out to be any number 0,1,2,3 and so on, then the molecule observes Huckel's rule.