Definition of Aromatic Compounds

[crays]

Hi guys, i am wondering what defines an aromatic compound? From what i know, benzene is an aromatic compound, but why is it called an aromatic compound? Is it because it has alternating Double and Single Bonds? If so can i say Cyclohexanone is also an aromatic compound?

thanks.

 

[pzona]

I'm not sure about any relevant equations or anything, but aromaticity has to do with a higher than expected stability due to unsaturated bonds, lone pairs, and certain orbitals being empty. I'm not in organic chem right now, so this is really all I know; hopefully someone else can explain further.

 

[Kaxa2000]

Cyclohexanone is not aromatic...it's a ketone and considered nonaromatic. The reason it's not aromatic is because it does not have electron delocalization all around the ring like benzene. You can draw resonance structures to see this delocalization in benzene. Aromaticity has 3 rules it must fall under:

1)It has to be cyclic
2)The p orbitals around the ring must not be interuppted
3)Must be planar

If any of these requirements are broken then the compound is non aromatic.

If benzene only had 2 pi bonds instead of 3(cyclohexadiene) it would be considered non aromatic b/c the 2nd rule is broken. Two of the carbons would be sp3 hybridized.

Use Huckel's rule to predict whether a compound is aromatic/antiaromatic.

4N + 2 = aromatic

benzene has 6pi electrons which is a Huckels number when N = 1. So it is aromatic.

4N = antiaromatic

cyclobutadiene has 4 pi electrons which is not a huckel number and would be considered anti aromatic (N=1)

 

[crays]

I see, thank you very much kaxa2000. So as long as its cyclic, planar with delocalised p orbitals it would be an aromatic compound.

so carboxylic acid is not an aromatic compound either right? This book is weird :( . Is there exception to the rules?

 

[Borek]

So as long as its cyclic, planar with delocalised p orbitals it would be an aromatic compound.

Assuming it will have correct number of electrons - that's the Huckel rule.

Is there exception to the rules?

In chemistry? Always

Well, some rules in quantum chemistry are never violated, but they are just an exception to the rule that all rules have exceptions...

--

 

[Kaxa2000]

No carboxylic acid is not aromatic because it is not cyclic. If you attach the carboxylic acid to a benzene ring then the compound can be considered aromatic.

I'm not sure about exceptions to the rule...but don't worry about those because it's highly unlikely you will be asked about an exception on the test. Just do problems to make yourself understand and be familiar with the rules I talked about.

Also just to add to my previous post...if the compound doesn't have planarity ...that just means it has to remain flat...if anything distorts(e.g. trans hydrogens) the ring from staying flat then it will lose it's aromaticity and become non aromatic.

 

[Kaxa2000]

So as long as its cyclic, planar with delocalised p orbitals it would be an aromatic compound.

No...when you have determined that a compound has met all these requirements...then you count the number of electrons it has to determine if it is aromatic/anti aromatic.

If it falls under all 3 rules and has 6pi, 10pi, 14pi, etc. electrons then you know its aromatic...this is the 4N + 2 rule where N = 1, 2, 3

If it falls under all 3 rules but has 4pi, 8pi, 12pi, etc, electrons then you know its anti aromatic...this is the 4N rule where N = 1, 2, 3

 

[crays]

Thanks for the help everyone :)

 

[nimmysnv]

Hi,

I must thank you for this too as I amalso wondering about and finally I got the solution here which is no doubt helpful for me in near future.


Thanks again.