All aromatic compounds are based on benzene: C6H6. Note the resonant structure of the Benzene ring. Also note that there are two ways to represent this structure and that either one is acceptable.
When a Benzene ring is used a subunit in an aromatic compound, it is called a phenyl group. Just like the alkyl groups, a phenyl group is always attacched to something else.
There are certain cases where a compound's name is based on benzene as opposed to phenyl in order to avoid giving compounds awkward names. Among these compounds are nitrobenzene and methylbenzene:
Most names of aromatic compounds are based on phenyl. This example is would be called phenylethene
Benzene consists of 6 carbons in a ring with delocalised electrons above and below the plane of the carbon ring. This delocalization makes benzene very stable. Benzene is very resistant to any reaction which would break this delocalized orbit structure and thus destabilize the molecule. This delocalization also prevents a benzene ring from forming a permanent dipole. Instead, temporary dipoles are more easily established, resulting in a high boiling point of 80° C- much higher than most other hydrocarbons of similar molecular size. Benzene also has a melting point of 5.5° C.
Arenes are simple aromatic compounds based on benzene. The simplest arene is benzene itself, the next simplest is methylbenzene, and so forth.
Methylbenzene consists of a benzene ring with a methyl group attached in place of one of the hydrogens.
Methylbenzene exhibits a higher boiling point than benzene: 111° C because it is a bigger molecule. Its melting point, however, is -95° C, because it cannot pack efficiently into a solid due to the methyl group sticking out of the ring.
No arenes are soluble in water.
Aryl Halides are compounds composed of a halogen attached to a benzene ring.
The chemical properties of chlorobenzene are significantly affected by the fact that one
of the free electron pairs of the chlorine interacts with the delocalised electron structure of the benzene.
This interaction creates a stronger bond between the chlorine and carbon, which affects reactions with chlorobenzene.
Chlorobenzene is not very polar because the electronegativity of chlorine is offset by the natural movement of delocalised electrons toward the benzene ring.
Phenol consists of an alcohol bonded to a benzene ring (alcohol and phenol group)
Like chlorobenzene (and other aryl halides) there is an interaction between one electron pair on the oxygen and the delocalised electrons of the benzene ring. The increase in electron density makes the benzene more reactive than it would be otherwise. Also, the movement of electrons away from the hydrogen (the one in the alcohol) makes that hydrogen much more acidic than a hydrogen in other alcohols.