[newbie!]

ok i dont understand why this compound isnt aromatic?



its satisfies the Huckel rule: 4n+2=6 therefore n=1.
(if n is an integer then it should be aromatic)


is it because its a birch reduction of napthlene?

[Åke]

A compound that follows "4n+2 rule" is not necessarily aromatic nor does it necessarily follow Hückel's rule. Certain other characteristics have to be present too. What are they? Which of these characteristics are not present in your compound? Read up on aromaticity and you will be able to solve the problem.

[Åke]

P.S. Your compound dosen't even follow Hückel's rule, why?

[newbie!]

how doesnt it follow huckels rule?
there are 6 pie electrons right?

[Åke]

Yes, you are right, it has 6 pi electrons. So, is the compound shown below aromatic? Does it follow Hückel's rule? It too has 6 pi electrons, right?

[Åke]

For a compound to obey Hückel’s rule it has to fulfil following criteria:

• 1.) It has to have one or more rings that are planar.
• 2.) In such ring system, the compound has to posses 4n+2 electrons.
• 3.) These electrons have to be distributed in a conjugated, delocalized pi-system.

If one of these 3 criteria is violated the compound does not obey Hückel’s rule. However, a compound can be aromatic even if it violates the 4n+2 rule (criteria 2) and thereby also Hückel’s rule. Pyrene is a compound that violates the 4n+2 rule - it has 16 pi electrons, n = 3.5 => not an integer. Yet, pyrene is still an aromatic compound. This is how I understand it, does it make any sense?


Further reading:

1.  http://en.wikipedia.org/wiki/Aromaticity (Aromaticity)
2.  http://en.wikipedia.org/wiki/H%C3%BCckel%27s_rule (Hückel's rule)

Regards,

Vedran

[rucik5]

Hey,
you're confusing Huckel's rule with aromaticity I think. [from wikipedia]: A cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4n+2 where n is zero or any positive integer, although clearcut examples are really only established for values of n = 0 up to about n = 6.

The other requirements for aromaticity, apart from Huckel's 4n+2 pi electrons are - like you said - are conjugation/delocalized system and being cyclic. I think planarity is actually the result of aromaticity but there probably are some examples of rings which have 4n+2 pi electrons, are conjugated and delocalized but their shape in space [how the pi orbitals align] does not allow for aromaticity.

[newbie!]

ok so how can i identify if its conjugated and planar then?
i'm not sure...

[rucik5]

Conjugation occurs when a p orbital overlaps with another p orbital over a sigma bond. As long as the system consists of alternating pi and sigma bonds, all the electrons will be delocalized [ R-C=C-C=C-C=C-R...]. In your molecule the double bonds are two sigma bonds from each other and are not conjugated.

[Sepelio]

To follow huckels rule, the molecule has to be conjugated. First look for conjugation, THEN look to apply huckels rule.

[newbie!]

so for it to be conjuageted it needs to be alternating single and double bonds?

[Åke]

Hey,
you're confusing Huckel's rule with aromaticity I think. [from wikipedia]:

I tried to show that: a compound that obeys Hückel's rule must be aromatic but an aromatic compound doesn't have to obey Hückels rule and furthermore that a compound obeying 4n+2 rule dosen't have to be aromatic nor does it have obey Hückel's rule.

 

[fledarmus]

so for it to be conjuageted it needs to be alternating single and double bonds?

Not necessarily - it is more accurate to say that the atoms have overlapping p orbitals. Alternating single and double bonds is just one way to get there. Lone pairs of electrons on nitrogens, atoms, or carbocations can also participate in a conjugated system.

There is a good short description in Wikipedia on conjugated systems - http://en.wikipedia.org/wiki/Conjugated_system

[fledarmus]

However, a compound can be aromatic even if it violates the 4n+2 rule (criteria 2) and thereby also Hückel’s rule. Pyrene is a compound that violates the 4n+2 rule - it has 16 pi electrons, n = 3.5 => not an integer. Yet, pyrene is still an aromatic compound.

No fair throwing in red herrings    As I understand it, the rings in polycyclic aromatic hydrocarbons such as pyrene have more or less aromatic character depending on the number of resonance structures of isolated benzene rings the system can be imagined to include. This falls more under Clar's aromatic pi-sextet rule than under the Huckel rules.

[Åke]

Hi! I realise that aromaticity is difficult to define, as a synthesis chemist I see it as a useful and convenient device for grouping the reactivities of many compounds. I guess NMR spectroscopy is the most convenient method for confirmation of aromaticity in a molecule.¹

1. Merino, G., Heine, T., and Seifert, G. Chem.—Eur. J. 2004, 10, 4367. http://www.ncbi.nlm.nih.gov/pubmed/15352120

Quote from the abstract: "Aromatic, anti-aromatic, and non-aromatic molecules can be characterized by the appearance of the magnetic response. We also show that the magnetic response is directly connected to nucleus-independent chemical shifts (NICS)."