Naming Aromatic Compounds



 

Learning Objective

  • Recognize the methods for naming aromatic compounds, including IUPAC nomenclature and historical names

Key Points

    • For substituted benzene rings where the substituent contains more than six carbons, the benzene ring is noted by using a phenyl prefix on the alkane name.
    • For substituted benzene rings where the substituent contains less than six carbons, the alkyl chain is added as a prefix with the ending changed to -yl.
    • For benzene rings with multiple substituents, the ring atoms are numbered to minimize the numbering of the substituents groups; alternatively, ortho/meta/para nomenclature can be used for disubstituted rings.

Terms

  • orthoA prefix used to name an aromatic ring with two adjacent substituents.
  • metaA prefix used to name an aromatic ring with two substituents separated by one carbon on the ring.
  • paraA prefix used to name an aromatic ring with two substituents directly across from one another on the ring.

Aromatic compounds are ring structures with unusual stability due to delocalized pi electron density that is shared between all of the carbon atoms in the ring.

There are a number of historically common names for aromatic structures. These names are frequently used in favor of standardized IUPAC nomenclature. For example, methylbenzene is often referred to as toluene, and dimethylbenzene is often called xylene.

Naming aromatic compoundsThis nomenclature tutorial video takes you through the IUPAC rules for benzene type molecules and includes the common names for substituted benzene.

Aromatic Compounds with a Single Substituent

When there is a single substituent on a benzene ring and the substituent contains six or fewer carbons, the substituent is included as a prefix to benzene. Alkyl groups are named according to the alkane series convention ending with -yl: methyl (for a single carbon), ethyl (for two carbons), propyl (for three carbons), etc.

If the substituent contains more than six carbons, the alkane portion is named first, and the aromatic ring portion is added as a suffix. For instance, an aromatic ring bonded to an 8-carbon chain would be 1-phenyloctane, and not octylbenzene.

Aromatic Compounds with Multiple Substituents

When there are multiple substituents, ring atoms are numbered to minimize the numbers assigned to the substituted positions.

Disubstituted benzene rings can be named based on the relative positions of the substituents: the prefix ortho– is used if the substituents occupy adjacent positions on the ring (1,2), meta– is used if the substituents are separated by one ring position (1,3), and para– if they are found on opposite sides of the ring (1,4).

p-XyleneChemical structure of 1,4-dimethylbenzene, better known as para-xylene.