22.4 Aromatic Compounds – Structure and Naming

Learning Objectives

By the end of this section, you will be able to:

  • Describe the bonding in benzene and the way typical reactions of benzene differ from those of the alkenes.
  • Recognize aromatic compounds from structural formulas.
  • Name aromatic compounds given formulas.
  • Write formulas for aromatic compounds given their names.

Next, we consider a class of hydrocarbons with molecular formulas like those of unsaturated hydrocarbons, but which, unlike the alkenes, do not readily undergo addition reactions. These compounds comprise a distinct class, called aromatic hydrocarbons, with unique structures and properties. Historically, benzene-like substances were called aromatic hydrocarbons because they had distinctive aromas. Today, an aromatic compound is any compound that contains a benzene ring or has certain benzene-like properties (but not necessarily a strong aroma). You can recognize the aromatic compounds in this text by the presence of one or more benzene rings in their structure. Some representative aromatic compounds and their uses are listed in Table 22.4a., where the benzene ring is represented as C6H5.

The simplest of these compounds. Benzene (C6H6) is of great commercial importance, but it also has noteworthy health effects.

The formula C6H6 seems to indicate that benzene has a high degree of unsaturation. (Hexane, the saturated hydrocarbon with six carbon atoms has the formula C6H14—eight more hydrogen atoms than benzene.) However, despite the seeming low level of saturation, benzene is rather unreactive. It does not, for example, react readily with bromine, which, is a test for unsaturation.

To explain the surprising properties of benzene, chemists suppose the molecule has a cyclic, hexagonal, planar structure of six carbon atoms with one hydrogen atom bonded to each. We can write a structure with alternate single and double bonds, either as a full structural formula or as a line formula as shown in Figure 22.4a.

The molecular structure of benzene showing all carbons and hydrogens (on the left) and the line ring structure of benzene with no carbons or hydrogens showing (on the right).
Figure 22.4a. Benzene ring structural arrangement versions (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

However, these structures do not explain the unique properties of benzene. Furthermore, experimental evidence indicates that all the carbon-to-carbon bonds in benzene are equivalent, and the molecule is unusually stable. Chemists often represent benzene as a hexagon with an inscribed circle as in Figure 22.4b.

Benzene ring represented as a hexagon with an inscribed circle
Figure 22.4b. Benzene ring represented as a hexagon with an inscribed circle (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

The inner circle indicates that the valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized, or spread out, over all the carbon atoms). It is understood that each corner of the hexagon is occupied by one carbon atom, and each carbon atom has one hydrogen atom attached to it. Any other atom or groups of atoms substituted for a hydrogen atom must be shown bonded to a particular corner of the hexagon. We use this modern symbolism, but many scientists still use the earlier structure with alternate double and single bonds.

Table 22.4a. Some Representative Aromatic Compounds
Name Structure Typical Uses
aniline C6H5–NH2 starting material for the synthesis of dyes, drugs, resins, varnishes, perfumes; solvent; vulcanizing rubber
benzoic acid C6H5–COOH food preservative; starting material for the synthesis of dyes and other organic compounds; curing of tobacco
bromobenzene C6H5–Br starting material for the synthesis of many other aromatic compounds; solvent; motor oil additive
nitrobenzene C6H5–NO2 starting material for the synthesis of aniline; solvent for cellulose nitrate; in soaps and shoe polish
phenol C6H5–OH disinfectant; starting material for the synthesis of resins, drugs, and other organic compounds
toluene C6H5–CH3 solvent; gasoline octane booster; starting material for the synthesis of benzoic acid,

Table source: “13.8: Structure and Nomenclature of Aromatic Compounds” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

Many aromatic based compounds have pleasant odours but they are generally toxic with some being carcinogenic.  Users should avoid inhaling any vapours.  Lighter weight aromatic hydrocarbons are highly flammable and have a sooty flame.  Like all hydrocarbons, aromatics are less dense than water and not soluble in water.  Their boiling points tend to increase in molar mass, but their melting points are independent on molar mass.  Symmetry in the molecule leads to much higher melting points. (Roberts & Caserio, 1977).

Spotlight on Everyday Chemistry: August Kekule and The Benzene Structure

August Kekule was the first one to depict the benzene structure as a ring-like structure with alternating single and double bonds. For more information refer to infographic 22.4a. below.

Infographic 22.4a.  Read more about “Today in Chemistry History: August Kekulé and the structure of benzene” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.4a [New tab].

Example 22.4a – Identifying Aromatic Compounds

Which compounds are aromatic?

There are 4 images from left to right: a chlorobenzene; 2-chlorocyclohexene; a propylbenzene; and lastly 3-phenyl-1-propene.
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Solution

  1. The compound has a benzene ring (with a chlorine atom substituted for one of the hydrogen atoms); it is aromatic.
  2. The compound is cyclic, but it does not have a benzene ring; it is not aromatic.
  3. The compound has a benzene ring (with a propyl group substituted for one of the hydrogen atoms); it is aromatic.
  4. The compound is cyclic, but it does not have a benzene ring; it is not aromatic.

Spotlight on Everyday Chemistry: Benzene Health Hazards and Connection to Common Medical Ingredients

Benzene is a liquid that smells like gasoline, boils at 80°C, and freezes at 5.5°C. Most of the benzene used commercially comes from petroleum. It is employed as a starting material for the production of detergents, drugs, dyes, insecticides, and plastics. It was formerly used to decaffeinate coffee and was a significant component of many consumer products, such as paint strippers, rubber cements, and home dry-cleaning spot removers. It was removed from many product formulations in the 1950s, but others continued to use benzene in products until the 1970s when it was associated with leukemia deaths. Benzene is still important in industry as a precursor in the production of plastics (such as Styrofoam and nylon), drugs, detergents, synthetic rubber, pesticides, and dyes. It is used as a solvent for such things as cleaning and maintaining printing equipment and for adhesives such as those used to attach soles to shoes. Benzene is now known to have both short- and long-term toxic effects. The inhalation of large concentrations can cause nausea and even death due to respiratory or heart failure, while repeated exposure leads to a progressive disease in which the ability of the bone marrow to make new blood cells is eventually destroyed. This results in a condition called aplastic anemia, in which there is a decrease in the numbers of both the red and white blood cells.

Though benzene alone has been shown to potentially affect your health, substances containing the benzene ring are common in both animals and plants. Plants can synthesize the benzene ring from carbon dioxide, water, and inorganic materials. Animals cannot synthesize it, but they are dependent on certain aromatic compounds for survival and therefore must obtain them from food. Phenylalanine, tyrosine, and tryptophan (essential amino acids) and vitamins K, B2 (riboflavin), and B9 (folic acid) all contain the benzene ring. Many important drugs, a few of which are shown in Table 22.4b., also feature a benzene ring.

Table 22.4b. Some Drugs That Contain a Benzene Ring (Image Credits: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).
Name Structure
aspirin Molecular structure of aspirin
acetaminophen a molecular structure of acetaminophen
ibuprofen a molecular structure of ibuprofen
amphetamine a molecular structure of amphetamine
sulfanilamide a molecular structure of sulfanilamide

 

Naming Aromatic Compounds

Naming Monosubstituted Benzenes

In the International Union of Pure and Applied Chemistry (IUPAC) system, aromatic hydrocarbons are named as derivatives of benzene. Figure 22.4c. shows four examples. In these structures, it is immaterial whether the single substituent is written at the top, side, or bottom of the ring: a hexagon is symmetrical, and therefore all positions are equivalent.

5 substituted benzene structures. Top row from left to right: chlorobenzene, bromobenzene and iodobenzene. Bottom row from left to right: nitrobenzene and ethylbenzene.
Figure 22.4c. Some Benzene Derivatives. These compounds are named in the usual way with the group that replaces a hydrogen atom named as a substituent group: Cl as chloro, Br as bromo, I as iodo, NO2 as nitro, and CH3CH2 as ethyl (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Although some compounds are referred to exclusively by IUPAC names, some are more frequently denoted by common names, as is indicated in Table 22.4a. and shown in Figure 22.4d.

Aromatic structures from left to right: toluene (a benzene ring with a methyl group), phenol (a benzene ring with an OH group) and aniline (a benzene ring with an amine group)
Figure 22.4d. Common aromatic structures representing toluene, phenol and aniline (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Naming Polysubstituted Benzenes

When there is more than one substituent, the corners of the hexagon are no longer equivalent, so we must designate the relative positions. There are three possible disubstituted benzenes, and we can use numbers to distinguish them (Figure 22.4e.). We start numbering at the carbon atom to which one of the groups is attached and count toward the carbon atom that bears the other substituent group by the shortest path.

The three isomeric dichlorobenzenes from left to right: 1,2-dichlorobenzene, 1,3-dichlorobenzene and lastly 1,4-dichlorobenzene.
Figure 22.4e. The three isomeric dichlorobenzenes (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

The structural differences in the three dichlorobenzene compounds in Figure 22.4e. lead to differences in physical properties.  Table 22.4c. highlights some of the key physical properties of dichlorobenzenes.

Table 22.4c. Physical properties of dichlorobenzenes (Data sources credit: National Center for Biotechnology Information, 2024a, b, c)
Compound Chemical Formula Molecular Weight Boiling Point Melting Point
1,2-dichlorobenzene

(o-dichlorobenzene)

 C6H4Cl2 147.00 g/mol 180oC -17oC
1,3-dichlorobenzene

(m-dichlorobenzene)

 C6H4Cl2 147.00 g/mol  173oC -25oC
1,4-dichlorobenzene

(p-dichlorobenzene)

 C6H4Cl2 147.00 g/mol  174oC 53oC

In Figure 22.4e., common names are also used: the prefix ortho (o-) for 1,2-disubstitution, meta (m-) for 1,3-disubstitution, and para (p-) for 1,4-disubstitution. The substituent names are listed in alphabetical order. The first substituent is given the lowest number. When a common name is used, the carbon atom that bears the group responsible for the name is given the number 1 as demonstrated in Figure 22.4f.

4 images of substituted benzenes using common names ortho, meta and para. Top left to right: p-bromochlorobenzene and 0-bromophenol. Bottom left to right: m-chloroaniline and m-bromotoluene
Figure 22.4f. Common names using ortho, meta and para and where the carbon atom that bears the group responsible for the name is given the number 1 (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Infographic 22.4b. shows a section of benzene derivatives with their IUPAC and common names.

 

Infographic 22.4b.  Read more about “Benzene Derivatives in Organic Chemistry” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.4b [New tab].

Example 22.4b

Name each compound using both the common name and the IUPAC name.

 

3 images from left to right: 1,2-dichlorobenzene; bromotoluene and lastly 1,3-dinitrobenzene
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Solution

  1. The benzene ring has two chlorine atoms (dichloro) in the first and second positions. The compound is o-dichlorobenzene or 1,2-dichlorobenzene.
  2. The benzene ring has a methyl (CH3) group. The compound is therefore named as a derivative of toluene. The bromine atom is on the fourth carbon atom, counting from the methyl group. The compound is p-bromotoluene or 4-bromotoluene.
  3. The benzene ring has two nitro (NO2) groups in the first and third positions. It is m-dinitrobenzene or 1,3-dinitrobenzene.

Note: The nitro (NO2) group is a common substituent in aromatic compounds. Many nitro compounds are explosive, most notably 2,4,6-trinitrotoluene (TNT).

The molecular structure of 2,4,6-trinitrotoluene
(credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Exercise 22.4a

Name each compound using both the common name and the IUPAC name.

  1. The molecular structure of 1,2-dibromobenzene or o-dibromobenzene
  2. The molecular structure of 1-amino-2-bromobenzne or o-aminobromobenzene or o-bromoaniline

Check Your Answers:[1]

Exercise and image credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0

Naming Compounds with an Aromatic Substituent

Sometimes an aromatic group is found as a substituent bonded to a nonaromatic entity or to another aromatic ring. The group of atoms remaining when a hydrogen atom is removed from an aromatic compound is called an aryl group. The most common aryl group is derived from benzene (C6H6) by removing one hydrogen atom (C6H5) and is called a phenyl group (Figure 22.4g.), from pheno, an old name for benzene.

The phenyl group consisting of 6 carbons and 5 hydrogens is represented as a condensed structure, line structure and shown in a compound as a substituent 2-phenylheptane
Figure 22.4g. Different ways to represent the phenyl group using a condensed structure, line structure and phenyl as a substituent (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

Exercise 22.4b

Name this compound.

The molecular structure of 2-fluoro-4-methyl-1-phenylpentane
(credit: Samantha Sullivan Sauer / Biovia Draw, CC BY-NC 4.0.)

Check Your Answer:[2]

Activity source: Exercise 22.4b is created by Samantha Sullivan Sauer, using images from Biovia Draw, licensed under CC BY-NC 4.0

 

Spotlight on Everyday Chemistry: Benzene Derivatives in Sunscreen and New Car Smell

A number of sunscreens that are UVA and UVB blockers are made with ingredients that contain benzene structures. For a detailed look at the various substituted benzene ingredients, see infographic 22.4c. below.

Infographic 22.4c.  Read more about “The Science of Sunscreen & How it Protects Your Skin” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.4c [New tab].

Not only are benzene compounds found in products we use such as sunscreen, but they can also be found in objects we handle. A new car for example will often have a “new car smell”. This smell is associated with numerous volatile organic compounds (VOCs) that are made up of compounds that contain a benzene ring. For more information about the types of VOCs found within a new car see Infographic 22.4d.

Infographic 22.4d.  Read more about “The Chemicals Behind the ‘New Car Smell’” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.4d [New tab].

Polycyclic Aromatic Hydrocarbons

Some common aromatic hydrocarbons consist of fused benzene rings—rings that share a common side. These compounds are called polycyclic aromatic hydrocarbons (PAHs). A few examples are shown in Figure 22.4h.

Polycyclic aromatic structures from left to right: naphthalene (two benzene rings fused together in a row), anthracene (3 benzene rings fused together in a row) and phenanthrene (3 benzene rings fused together but the 3rd ring going up on the right)
Figure 22.4h. Examples of polycyclic aromatic compounds naphthalene, anthracene and phenanthrene. (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0).

The three examples (naphthalene, anthracene and phenanthrene), shown here are colourless, crystalline solids generally obtained from coal tar. Naphthalene has a pungent odour and is used in mothballs. Anthracene is used in the manufacture of certain dyes. Steroids, a large group of naturally occurring substances, contain the phenanthrene structure.

Spotlight on Everyday Chemistry: Glow Sticks

Glow sticks are effectively used at night so that you can be visible or to add some fun to an event. They come in all sorts of colours such as red, orange, yellow, green and blue. Benzene containing compounds are responsible for the various colours produced. Additionally, the reaction that allows the glow stick to glow, is from the reactions involving benzene containing compounds. Next time you use a glow stick, you can thank the benzene rings! For more information, see infographic 22.4e. below.

Infographic 22.4e.  Read more about “The Chemistry of Glow Sticks” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 22.4e [New tab].

Attribution & References

Except where otherwise noted, this page is adapted by David Wegman, Adrienne Richards and Samantha Sullivan Sauer from “13.8: Aromatic Compounds and the Structure of Benzene” and “13.9: Naming Aromatic Compounds” In Map: Fundamentals of General Organic and Biological Chemistry (McMurry et al.) by LibreTexts, licensed under CC BY-NC-SA 3.0. / A derivative of Basics of GOB (Ball et al.), CC BY-NC-SA 4.0 a LibreTexts version of Introduction to Chemistry: GOB (v. 1.0), CC BY-NC 3.0.

References cited in-text

National Center for Biotechnology Information (2024a). PubChem Compound Summary for CID 7239, 1,2-Dichlorobenzene. Retrieved January 14, 2024. .

National Center for Biotechnology Information (2024b).PubChem Compound Summary for CID 10943, 1,3-Dichlorobenzene. Retrieved January 14, 2024.

National Center for Biotechnology Information (2024c). PubChem Compound Summary for CID 4685, 1,4-Dichlorobenzene. Retrieved January 14, 2024.

Roberts, J. D., & Caserio, M. C. (1977). Basic Principles of Organic Chemistry, (2nd ed.) W. A. Benjamin, Inc.

  1. 1) 1,2-dibromobenzene or o-dibromobenzene 2) 1-amino-2-bromobenzne or o-aminobromobenzene or o-bromoaniline

    ↵

  2. 2-fluoro-4-methyl-1-phenylpentane

    ↵

definition

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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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