Chapter 23: Aryl Halides

Nomenclature:
Functional group suffix = -halobenzene
Functional group prefix = halo-
Numbering of the ring begins at the halogen-substituted carbon and proceeds in the direction of the next substituted carbon that possesses the lower number.

Ortho, meta or para ? (review)
Mono-substituted aryl halides are characterised using the prefix ortho (o-), meta (m-) or para (p-) depending on the placement of the substituent from the halogen or the halogen from a higher priority functional group: 1,2-, 1,3- or 1,4- respectively.
 

Physical Properties:

• Aryl halides have similar properties to alkyl halides
• The polar bond creates a molecular dipole that raises the melting points and boiling points compared to similar hydrocarbons.
• Aryl halides tend to be less polar than alkyl halides (since an sp² C is more electronegative than an sp³ C)

• Insoluble in water (low polarity, no hydrogen bonding)
• More dense than water.

• An aromatic sp² hybridised C atom is bonded to the halogen via a σ bond.
• The result is a C-X bond in an aryl halide that is shorter and stronger than those found in alkyl halides.

Fluorobenzene

The image to the left shows the electrostatic potential for fluorobenzene and to the right chlorobenzene  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  The halogen atom is a region of high electron density (red) due to the lone pairs (red).  The strength of the C-X bond causes aryl halides to react very slowly The polarity of the C-X bond makes the C atom electrophilic and prone to attack by nucleophiles.

Chlorobenzene

© Dr. Ian Hunt, Department of Chemistry