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Aromatic Hydrocarbons

Structure and properties of compounds with a benzene ring.

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Aromatic Hydrocarbons

Kekulé was the first to propose the cyclic structure of benzene

Credit: Courtesy of the National Library of Medicine
Source: http://commons.wikimedia.org/wiki/File:Frkekul%C3%A9.jpg
License: CC BY-NC 3.0

Can a dream affect reality?

Friedrich Kekulé was a German chemist in the 1800s. He supposedly was thinking about the structure of the benzene ring as he fell asleep. While asleep, he dreamed of a snake eating the ring's tail. He used this idea to propose the cyclic structure for benzene. Whether or not he actually had this dream has been debated ever since. Whatever really happened, the tale has persisted until today.

Aromatic Hydrocarbons

Benzene is the parent compound of the large family of organic compounds known as aromatic compounds. Unlike cyclohexane, benzene only contains six hydrogen atoms, giving the impression that the ring is unsaturated and each carbon atom participates in one double bond. Two different structures with alternating single and double bonds around the ring can be written for benzene.

Benzene can be written as the resonance hybrid of two structures with alternating double and single bonds

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

In benzene, the true bonding between carbon atoms is neither a single nor a double bond. Rather, all of the bonds are a hybrid of a single and double bond. In benzene, the pi bonding electrons are free to move completely around the ring. Delocalized electrons are electrons that are not confined to the bond between two atoms, but are instead allowed to move between three or more. The delocalization of the electrons in benzene can best be shown by showing benzene with a ring inside the hexagon, with the hydrogen atoms understood.

Benzene is often drawn as a circle in a hexagon

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Delocalization of the electrons makes for a more stable molecule than a similar molecule that does not have delocalized electrons. Benzene is a more stable and less reactive compound than straight-chain hexenes. The sp2 hybridization of the carbon atoms results in a planar molecule as opposed to the puckered structure of cyclohexane. Benzene rings are common in a great number of natural substances and biomolecules. The figure below shows the structural formulas for vanilla and naphthalene. Naphthalens is a chemical which is commonly used in mothballs.

Structures of common aromatic compounds

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Nomenclature of Aromatic Compounds

The simplest aromatic compounds are benzene rings with one substituent replacing one of the hydrogen atoms. If this substituent is an alkyl group, it is named first, followed in one word with “benzene”. The molecule shown below is therefore called ethylbenzene.

Nomenclature of aromatic compounds

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Substituents can be groups other than alky groups. If a chlorine atom were substituted for a hydrogen, the name becomes chlorobenzene. An –NH2 group is called an amino group, so the corresponding molecule is called aminobenzene, often referred to as aniline. An –NO2 group is called a nitro group and so the third example below is nitrobenzene.

Examples of substituted aromatic compounds

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

If more than one substituent is present, their location relative to each other can be indicated by numbering the positions on the benzene ring.

Numbered positions on a benzene ring

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

The number of the carbon location then precedes the name of the substituent in the overall name, with the numbers separated by a comma. As with branched alkanes, the system requires that the numbers be the lowest possible and that prefixes be used for more than one of the same substituent. If there are different substituents, the first in alphabetical order is given the lower number and listed first. The structures below are called 1,2-dimethylbenzene and 1-ethyl-4-methylbenzene.

Nomenclature of disubstituted aromatic compounds

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

An alternate system for naming di-substituted benzene rings uses three different prefixes: ortho, meta, and para. If two groups are in the ortho position, they are on adjacent carbon atoms. The meta positioning refers to being in a 1, 3 arrangement. The para positioning refers to being in a 1, 4 arrangement. Shown below are the three possibilities for dimethylbenzene, also called xylene.

Examples of nomenclature with ortho, para, and meta substitution

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Lastly, a benzene ring missing one hydrogen atom (−C6H5) can itself be considered the substituent on a longer chain of carbon atoms. That group is called a phenyl group and so the molecule below is called 2-phenylbutane.

Example of nomenclature with phenyl as a substituent

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Summary

  • The structure and nomenclature of aromatic compounds are described.

Practice

Read the material at the link below:

http://www.chemguide.co.uk/basicorg/conventions/names3.html

At the bottom of the page, click on “Questions on naming aromatic compounds” in the green box and do as many of the problems (both naming and structure drawing) as you can.

Review

Questions

  1. What is a characteristic of aromatic compounds?
  2. What is the rule for numbering substituents on the ring?
  3. How is numbering affected if there is more than one type of substituent group?

Image Attributions

  1. [1]^ Credit: Courtesy of the National Library of Medicine; Source: http://commons.wikimedia.org/wiki/File:Frkekul%C3%A9.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  4. [4]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  5. [5]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  6. [6]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  7. [7]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  8. [8]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  9. [9]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  10. [10]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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