Aromaticity

Definition and Criteria

Aromaticity is a specific property of planar ring molecules that contain delocalised pi electrons.
Aromatic compounds must satisfy Huckel’s rule, meaning they have to contain a continuous loop of 4n+2 pi electrons (where n is any whole number, including zero).
This rule is based on quantum mechanics and is the basis for the unusual stability of these compounds.
Aromatic molecules are cyclic, planar, and conjugated.
Cyclic means they form a ring.
Planar means all the atoms lie in a flat plane.
Conjugated refers to the alternating pattern of single and double bonds, allowing the pi electrons to spread out or ‘delocalise’ over all the atoms in the ring.

Aromatic compounds are pleasantly scented (hence their name).
The most common aromatic compound is benzene. Benzene has six carbons in a ring, with three double bonds.
Aromatic compounds exhibit greater stability than similar compounds that do not meet the criteria for aromaticity.
A consequence of this stability is that aromatic compounds usually undergo substitution reactions rather than addition reactions typical of alkenes.

Besides benzene, other examples of aromatic compounds include toluene, naphthalene, aniline, and many more.
Toluene is benzene with a CH3 group attached.
Naphthalene is made from two benzene rings merged together.
Aniline is a benzene ring attached to an NH2 group.
Note that not all aromatic compounds contain only carbon and hydrogen. Purine, pyrimidine, and indole are aromatic compounds that contain nitrogen.

Compounds that are cyclic, planar, and conjugated, but contain 4n pi electrons, are anti-aromatic. They are actually less stable than similar compounds that are not conjugated.
If a compound is not cyclic, not fully conjugated, or nonplanar, it will be non-aromatic. These compounds do not benefit from any particular stability.

Aromaticity isn’t just a curiosity or exception for a small group of molecules. Many biologically important molecules such as DNA, RNA, and many pharmaceuticals contain aromatic subunits.
Understanding aromaticity and how aromatic compounds react is therefore an important aspect of drug discovery and development.

A typical example of a reaction involving an aromatic compound is the Friedel-Crafts alkylation of benzene.
In this reaction, an alkyl halide (R-X) reacts with a Lewis acid such as AlCl3. This generates a highly electrophilic species, which then reacts with benzene.
The result is a substitution of a hydrogen atom on the benzene ring with an alkyl group, creating an alkylbenzene.
Note that the product is still aromatic. The mechanism of this reaction is a key learning area for penetrating the depth of aromaticity.