[Seks]

I don't understand the concept...including huckel's 4n+2 rule.  And also about "planarity" requirements.
Can someone point me to a website that explains everything clearly???

ps: Solomons Org Chem book is what we're using and it just sucks the big one.  The explanation is just too complex.  I don't understand why my school is still using Solomons Org Chem book when people in the past have complained about it during term-end evaluation.

[Astrokel]

Cyclic conjugated molecules that are more stable than their acyclic counter parts are called aromatic compounds.

Cyclie conjugated molecules that are less stable than their acyclic counterparts are called antiaromatic compounds.

Cyclie compounds that do not have p orbitals on every ring atom are called nonaromatic compounds.

Acyclic compounds are also termed nonaromatic.

[sdfsfgfdgdfdf]

In order to understand Huckel's rule and the concept of planarity, you need to start off with the basic definition of an aromatic compound. You really just need to make a simple internet search for either - aromatic compound - or - aromaticity - and a number of pages will pop up. So, anyway, what is an aromatic compound?

In short, it's a cyclic compound, similar to benzene that has delocalized electrons, that make it more stable and less reactive. The delocalized electrons are usually represented as possible resonant structures of alternating single and double bonds.

In order to have delocalized electrons within a cyclic structure, all the atoms in the molecule must participate with p - orbitals (the p - orbitals carry the electrons that are being delocalized, and form the delocalization ring, called a P system and can come from a carbon atom, or any other element, a heteroatom that is sp2 hybridised). Since all the atoms must have a p - orbital, they can't be sp3 hybridised.

If they were sp3 hybridized they could exist in several conformations, like the cycloalkanes for example and would not be planar, but because sp2 hybridization results in the formation of double bonds, and double bonds do not allow the functional groups to rotate around the C- atom, the molecule remains planar. Try to imagine this, visualize the double bonds not allowing the molecule to bend, or make models of benzene and cyclohexane and try to twist them, you'll see the mechanism that makes benzene stay planar.       

Huckel's rule is based on quantum mechanical calculations, and I would advise you to just remember it's formulation for now.

[Seks]

Starting from top left, going across...this is my answer...

Nonaromatic - not conjugated because of the carbocation.

Nonaromatic - not conjugated

nonaromatic - not conjugated

aromatic - pair of e- on sulfur can contribute

aromatic - conjugated

nonaromatic

Am I right?  I'm not sure if my explanations are correct.  How does the 4n and 4n+2 rule come into play for all the above?

[ebowman]

I've been having problems with this recently too.

i think on the last one that it is aromatic, because the oxygen is a Electron withdrawing group, and will help form a tropylium ion, with a negative charge on the oxygen and a positive charge on the ring, giving rise to a 4n + 2 system (6 pi electrons)