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25.3: Aromatics

Created by: CK-12

Lesson Objectives

The student will:

  • describe the bonding in benzene.
  • define aromatic compounds.
  • name simple compounds containing benzene.
  • draw simple compounds containing benzene.

Vocabulary

  • benzene ring

Introduction

In this lesson, we will focus on another category of organic compounds called the aromatics. One common building block of aromatic compounds is benzene. Benzene is composed of a 6-membered carbon ring commonly represented by the resonance structures below.

Benzene was discovered in the 1820s by Michael Faraday, but it took more than 40 years to determine the actual structure of benzene. The German organic chemist Friedrich August Kekulé is credited for discovering the benzene structure in 1865. Legend has it that Kekulé discovered the benzene structure upon awaking from a dream of a snake biting the end of its tail.

A compound classified as an aromatic doesn't necessarily have to have a pleasant smell, although many aromatic compounds do have a pleasant odor. Vanilla (structure shown below) is an example of a pleasant-smelling aromatic compound. In comparison, napthalene (structure shown below) is the chemical found in moth balls with an unpleasant smell. In short, aromaticity has to do with structure and not smell.

Benzene Structure

As indicated in the introduction, there are two possible structures of the benzene molecule (\mathrm{C}_6\mathrm{H}_6).

Both of these structures represent resonance structures of the benzene molecule. Recall from the chapter “Covalent Bonds and Formulas” that resonance is the theory that describes compounds whose electronic structure is not represented by any one Lewis structure, but by an average of several structures. This theory, proposed by American chemist Linus Pauling, can be used to explain the structure of benzene. Although the structure shows alternating double and single bonds, all of the bonds are actually the same length. Carbon-carbon single bonds are 154 \ \mathrm{pm} long, while carbon-carbon double bonds are 134 \ \mathrm{pm} in length. For benzene, however, all of the carbon-carbon bonds in benzene are 139 \ \mathrm{pm} in length, so they are neither single nor double bonds. As a result, you may sometimes see a benzene structure represented as follows:

A benzene ring, then, is a structure that contains a ring represented by equivalent resonance structures. Benzene is an important compound in organic chemistry because of its properties and the vast number of compounds in which a benzene ring is an integral part of the structure.

Benzene is a part of many everyday items. Examples of structures where the benzene ring is an integral part of the structure include aspirin, saccharin, and aspartame (structures shown below). Aspirin is a common painkiller, while both saccharin and aspartame are artificial sweeteners. Aspartame is commonly used in soft drinks and diet drinks.

Naming Benzene Structures

All of the carbon atoms in the benzene ring have one hydrogen atom attached. If we were to replace one of these hydrogens with a branch from Table below, the naming is simply the name of the branch plus “benzene.”

Branches
Group Name
-\mathrm{CH}_3 methyl-
-\mathrm{CH}_2\mathrm{CH}_3 ethyl-
-\mathrm{F} fluoro-
-\mathrm{Cl} chloro-
-\mathrm{Br} bromo-
-\mathrm{I} iodo-
-\mathrm{NO}_2 nitro-

Example:

Name the following structure.

Solution: ethylbenzene

When there is more than one substituent on the benzene ring, then there has to be a way to determine the position of each branch. We number the carbon atoms the same as we had done for the alkanes, alkenes, and alkynes.

By numbering, we have a way to methodically determine the location of each branch on the benzene ring.

  • The branches are both methyl groups
  • The locations of the methyl groups are on carbons 1 and 2.
  • The name of the molecule is 1,2-dimethylbenzene.

An alternate naming system is given for molecules with two substituent groups (two branches). Alternate names are provided in Table below.

Alternate Naming System for Disubstituted Benzene
Location of branches Prefix Prefix symbol
carbons 1 and 2 ortho- o-
carbons 1 and 3 meta- m-
carbons 1 and 4 para- p-

The prefix “ortho-” indicates that the two substituents are on adjacent carbon atoms. The prefix “meta-” indicates that the two substituents are on carbon atoms with one carbon atom between them. The prefix “para-” indicates that the two substituents are on opposite carbon atoms. Therefore, 1,2-dimethylbenzene could also be named ortho-dimethylbenzene, 1,3-dimethylbenzene could also be named meta-dimethylbenzene, and 1,4-dimethylbenzene could also be named para-dimethylbenzene

Example:

Name the following structure.

Solution:

  • The branches are one methyl group and one ethyl group
  • The locations of the methyl groups are on carbons 1 and 4.
  • The name of the molecule is 1-ethyl-4-methylbenzene or, according to Table above, p-ethylmethylbenzene.

You may have noticed that in Table above, the number of alkyl groups acting as branches is more limited for benzene than for alkanes, alkenes, and alkynes. When the alkyl group becomes larger, the benzene ring is sometimes considered the branch, and the alkyl group is considered the parent chain. As a branch, benzene is known as a phenyl group.

In this case, butane is the parent chain, and the benzene ring is a side chain. This molecule would be named 2-phenylbutane.

Drawing Benzene Structures

Drawing benzene structures works in a similar way as drawing alkanes, alkenes, and alkynes. Consider the following example.

Example:

Draw the structure of p-dinitrobenzene.

Solution:

Lesson Summary

  • A benzene ring is an organic structure that contains a 6-carbon ring with alternating double bonds.
  • Resonance refers to a condition occurring when more than one Lewis structure can be written for a particular molecule. The actual electronic structure is not represented by any one of the Lewis structures, but by the average of all of them.
  • The electrons in the pi bonds of the benzene molecule are delocalized because they are not confined to a particular pair of carbon atoms.
  • As a substituent of a hydrocarbon chain, benzene is known as a phenyl group.

Further Reading / Supplemental Links

  • Chemistry and Chemical Reactivity, Kotz, Truchel, Weaver; Thompson, 2006.

Review Questions

  1. Define aromaticity.
  2. When is the benzene ring referred to as a phenyl group?
  3. Name the following structure.
    1. dibromocyclohexene
    2. 1,3-dibromocyclohexatriene
    3. 1,3-dibromobenzene
    4. 2,4-dibromobenzene
  4. Which formula represents an aromatic compound?
    1. \mathrm{C}_2\mathrm{H}_2
    2. \mathrm{C}_6\mathrm{H}_6
    3. \mathrm{C}_6\mathrm{H}_8
    4. \mathrm{C}_6\mathrm{H}_{14}
  5. How many different possible structures of trichlorobenzene exist?
    1. 1
    2. 2
    3. 3
    4. 4
  6. Name the following structures.
  7. Draw the following structures.
    1. fluorobenzene
    2. p-diethylbenzene
    3. 3-phenylhexane
    4. 2-methyl-1,4-diethylbenzene

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