Aromatic Compounds: Definition and Examples

[MycelliumMan]

At first I read that an aromatic compound is a compound that contains a ring with alternating double bonds in which the electrons are delocalized and the ring exhibits a greater stability than regular conjugated systems. That makes sense to me.

In Organic Chemistry by Clayden I read something different. In this book they say an aromatic compound is any compound that contains a benzene ring or other ring of atoms. Is that correct? If I'm not mistaken some rings have double bonds between their carbons and are not conjugated systems. Can an aromatic compound be a compound with a non conjugated ring system? In other words is any old cyclic compound considered an aromatic as long as it has a ring somewhere?

 

[LtStorm]

At first I read that an aromatic compound is a compound that contains a ring with alternating double bonds in which the electrons are delocalized and the ring exhibits a greater stability than regular conjugated systems. That makes sense to me.

In Organic Chemistry by Clayden I read something different. In this book they say an aromatic compound is any compound that contains a benzene ring or other ring of atoms. Is that correct? If I'm not mistaken some rings have double bonds between their carbons and are not conjugated systems. Can an aromatic compound be a compound with a non conjugated ring system? In other words is any old cyclic compound considered an aromatic as long as it has a ring somewhere?

There's a set of parameters that define whether or not a ring is aromatic, actually.

Most obviously, it must be a of some sort ring of covalently bonded atoms with a delocalized group of pi electrons. It doesn't have to even be a single ring, but could be a fused ring or (I believe) even a bridged ring. For the delocalized electrons the easiest way to get them is alternating between single and double bonds.

Additionally, the ring needs to be coplanar, meaning it would lie flat if placed on a plane.

Perhaps most importantly, the delocalized cloud of electrons must contain and even number of electrons not divisible by four. This is determined by http://en.wikipedia.org/wiki/H%C3%BCckel%27s_rule" .