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Aromatics, which smell, are one of the more important chapters in biochemistry.

What is an aromatic molecule?

Aromatic compounds in organic chemistry are those which smell. They contain conjugated rings of carbons. However, not all conjugated rings are considered aromatic. To determine which compounds are aromatic, follow the Hückel Rule.

The Hückel rule states that an aromatic 4n + 2 pi electrons will be aromatic, if n is a positive integer. This means that cyclic planar molecules with 2, 6, 10, 14, 18, 22, 26...etc. pi electrons will be aromatic. Let's take a look.

naro1.gif (3107 bytes)

These are all aromatics. Remember that each double bond counts for two pi electrons. Let's take a look at the top left molecule. This molecule has 3 double bonds. 3 x 2 = 6 pi electrons. This fits in with Hückel's rule. You can try the calculations for all the other ones; they all work. The following are NOT aromatic.

naro2.gif (1388 bytes)

Notice how the left one only has 4 pi electrons (not enough!) and the right onw has 8 pi electrons (sorry!). So, using Hückel's rule, you can figure out almost all the aromatic compounds.

Benzene is symmetrical, planar, hexagonal strcture that boils at 80 C and melts at 5 C.

naro3.gif (1137 bytes)

Benzene is the simplest of the aromatics (n=1 for Hückel's rule). Although you may see benzene written like that, benzene actually exists as two different compounds.

naro4.gif (1522 bytes)
Kekulé structures for benzene

The benzene ring is constantly changing its form because of resonance. You can see the two structures that are possible for benzene.

Benzene Substituents

Two substituents that involve the benzene ring are phenyl- and benzyl- groups.

naro5.gif (1119 bytes)naro6.gif (1221 bytes)

The molecule on the left is the phenyl- group and the one on the right is the benzyl- group. Aromatic groups are sometimes written Ar- (which is short for aryl) while R- represents an alkyl group. So if you saw someting written as Ar-CH3, that would be a methylbenzene (or phenylmethane).


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