B. Organic Chemistry · Organic Chemistry

Aromatic Hydrocarbons: Benzene and Its Relatives

B2.2 B3.1 B3.2 B1.1 A2.2About 65 minutes

Learning goals

I can describe the structure of benzene and explain why it is unusually stable.
I can name and draw substituted benzenes, including 1,2-, 1,3- and 1,4- (ortho, meta, para) isomers.
I can use the phenyl group to name compounds in which a benzene ring is a branch.
I can compare aromatic hydrocarbons with alkanes and alkenes, and describe some of their uses and risks.

Words to know

aromatic compound
A compound containing a benzene ring or a similar ring with shared (delocalized) electrons.

aliphatic compound
An organic compound that is not aromatic: alkanes, alkenes, alkynes and their rings and derivatives.

benzene
C₆H₆: a flat ring of six carbons, each bonded to one H, with six electrons shared evenly around the ring.

delocalized electrons
Electrons that are shared over several atoms instead of being held between two atoms.

phenyl group
A benzene ring attached as a branch, C₆H₅–.

ortho, meta, para
Older prefixes for disubstituted benzenes: ortho = 1,2-, meta = 1,3-, para = 1,4-.

A ring that refuses to behave

In 1825 Michael Faraday isolated a liquid from the oily residue of the gas used to light London's streets. It was benzene, C₆H₆. Its formula puzzled chemists for 40 years. Six carbons and only six hydrogens means a very unsaturated molecule, so it should react like an alkene, quickly adding bromine. It doesn't.

In 1865 the German chemist August Kekulé proposed that benzene is a ring of six carbons with alternating single and double bonds. That was close. Today we know that:

All six C–C bonds are the same length (139 pm), between a single bond (154 pm) and a double bond (134 pm).
The ring is flat, with 120° angles.
Six electrons (the "extra" electrons of the three double bonds) are delocalized: shared by all six carbons equally.

We draw benzene either as one of Kekulé's structures, remembering that the real molecule is a blend of both, or as a hexagon with a circle inside.

Kekulé's two structures differ only in where the double bonds are drawn. The real molecule is neither: its six shared electrons are spread evenly around the ring, which the circle shows.

Why benzene is so stable

Delocalized electrons lower a molecule's energy. Benzene is much more stable than a ring with three ordinary double bonds would be. As a result:

Benzene does not easily undergo addition reactions (which would destroy the shared ring of electrons).
Instead, it undergoes substitution: an H atom on the ring is swapped for another atom or group, and the ring stays whole. You will see this in Lesson 6.

Compounds that contain a benzene ring are called aromatic, because many of the first ones discovered had strong smells (vanilla, cinnamon, wintergreen). Today "aromatic" means the special ring structure, not the smell. All other organic compounds (chains and ordinary rings) are aliphatic.

Naming substituted benzenes

One substituent: name it as a prefix on benzene. No number is needed, because all six positions are the same.

C₆H₅–CH₃: methylbenzene (common name toluene)
C₆H₅–CH₂CH₃: ethylbenzene
C₆H₅–Cl: chlorobenzene

Two or more substituents: number the ring to give the lowest locants, and if there is a choice, give C1 to the substituent that comes first alphabetically. For two substituents, the older prefixes are still widely used:

ortho- (o-) = 1,2 (neighbours)
meta- (m-) = 1,3 (one carbon between)
para- (p-) = 1,4 (opposite)

Benzene as a branch: when the ring is attached to a longer chain, or to a chain with a functional group, name the ring as a phenyl group (C₆H₅–): CH₃CH(C₆H₅)CH₃ is 2-phenylpropane.

Common names you should know: a few aromatic compounds have names that IUPAC accepts: phenol (C₆H₅OH), benzoic acid (C₆H₅COOH), toluene and styrene (C₆H₅CH=CH₂, IUPAC ethenylbenzene).

The three dimethylbenzenes (xylenes). They have the same molecular formula, C₈H₁₀, but the methyl groups are neighbours (1,2), one carbon apart (1,3) or opposite (1,4). They are structural isomers with different properties: para-xylene melts at 13 °C, while the other two melt far below 0 °C.

Four common aromatic compounds. Each is a benzene ring with one substituent. Remember that the ring corners are carbons and each one without a substituent carries one H.

Worked example

Example 1: One substituent

Name C₆H₅CH₂CH₃ and give its molecular formula.

C₆H₅– is a benzene ring that has lost one H. The substituent is CH₂CH₃, an ethyl group.
One substituent: no number needed.
Formula: 6 + 2 = 8 C; 5 + 5 = 10 H.

ethylbenzene, C₈H₁₀. (Notice: an isomer of the xylenes.)

Worked example

Example 2: Two substituents

A benzene ring has a methyl group and, on the opposite carbon, an ethyl group. Name it.

Opposite carbons are positions 1 and 4 (para).
Alphabetical order: ethyl before methyl, so ethyl gets C1.
List substituents alphabetically.

1-ethyl-4-methylbenzene (para-ethyltoluene).

Worked example

Example 3: Three substituents

Name a benzene ring with Cl atoms on three carbons, each separated by one empty carbon.

Positions: 1, 3 and 5. (Starting anywhere else gives 2, 4, 6, which is higher.)
Three of the same: trichloro.

1,3,5-trichlorobenzene.

Worked example

Example 4: The phenyl group

Name CH₂=CH–CH₂–C₆H₅.

The ring is attached to a chain that has a functional feature (a C=C), so the chain is the parent and the ring is a phenyl branch.
Parent: CH₂=CH–CH₂–, three carbons with a double bond: prop-1-ene, numbered from the double-bond end.
Phenyl is on C3.

3-phenylprop-1-ene (3-phenyl-1-propene).

Properties, uses and risks

Benzene and its simple relatives are non-polar, insoluble in water, and less dense than water. Benzene boils at 80 °C and toluene at 111 °C.
They are excellent solvents for oils, paints and glues.
Many medicines, dyes, plastics and flavours contain benzene rings: ASA (aspirin), acetaminophen, polystyrene, vanillin (vanilla flavour) and the preservative sodium benzoate.
Benzene itself is a known human carcinogen: long-term exposure can cause leukemia. It was once a common lab and industrial solvent; safer solvents such as toluene have largely replaced it. Gasoline still contains a small percentage of benzene, which is one reason to fill up without breathing the vapour and to keep gas cans out of living spaces.
Molecules with several fused rings, such as naphthalene (C₁₀H₈), are polycyclic aromatic hydrocarbons (PAHs). Some PAHs form when wood, coal, tobacco or food burns or chars, and several are linked to cancer.

You will weigh these benefits and risks in the last lesson of this unit.

Hands-on activity

Aromatic detective: an ingredient-label hunt

You need: 6 to 10 product labels from around the house (soft drinks, sauces, medicines, sunscreen, acne cream, chewing gum, cleaning products: read only, do not open cleaners), a notebook and internet access.

Read the ingredients. List every name that contains benz, phen, phenyl or tolu, and well-known aromatics such as vanillin or acetylsalicylic acid.
For three of them, look up the structure on a reliable site (for example PubChem, from the U.S. National Library of Medicine). Copy the benzene ring and mark the substituents.
Classify each substituent you recognize (alkyl group, halogen, –OH, –COOH...). You will learn the others later in this unit.
Record your source for each structure (A1.7).

What to look for: sodium benzoate (a preservative) is the salt of benzoic acid; the label on diet drinks sweetened with aspartame includes a statement about phenylalanine, an amino acid with a phenyl group; some sunscreens and acne creams list compounds with "benz" in the name. Every one of these contains a benzene ring.

Just for fun

Kekulé later said that the ring idea came to him while he dozed by the fire and pictured a snake biting its own tail. Whether or not the story is true, it is the most famous nap in chemistry.

Your turn

What is the molecular formula of benzene, and how many H atoms are bonded to each carbon?

Name C₆H₅Br.

Name a benzene ring with two Cl atoms on neighbouring carbons. Give both styles of name.

Give the molecular formula of methylbenzene.

Why doesn't benzene react with bromine water the way an alkene does?

Name CH₃CH₂CH(C₆H₅)CH₂CH₃.

Draw (in words) 1-chloro-3-ethylbenzene.

More practice: warm-up

What does the circle inside a benzene hexagon represent?

What is the common name of methylbenzene?

What positions does "para" describe?

What is the formula of the phenyl group?

Is hexane aromatic or aliphatic?

More practice: on your own

Name the three isomers of dichlorobenzene.

Name a benzene ring with a methyl on C1, and nitro groups (–NO₂) on C2, C4 and C6. (Nitro is a substituent prefix, like chloro.)

Ethylbenzene and 1,4-dimethylbenzene have the same molecular formula. What is it, and what does that make them?

Bond lengths: C–C 154 pm, C=C 134 pm, benzene ring 139 pm. Explain what the benzene value tells you.

Why has toluene largely replaced benzene as a solvent?

Challenge

Hydrogenating cyclohexene (one C=C) releases about 120 kJ/mol. If benzene had three ordinary double bonds, how much would full hydrogenation release? The measured value is about 208 kJ/mol. What does the difference tell you?

Explain why there are three dimethylbenzenes, not more, and why 1,5-dimethylbenzene is not a correct name.

Polystyrene is made from ethenylbenzene (styrene), C₆H₅CH=CH₂. Which part of the molecule reacts to form the polymer, and why not the ring?

Check your understanding

Quiz

Which statement about benzene is correct?

What is the IUPAC name of toluene?

Which prefix describes substituents in positions 1 and 3 on a benzene ring?

What is the phenyl group?

Which type of reaction does benzene typically undergo?

Why must benzene be handled with special care in labs and industry?

For parents and teachers

Big ideas (B2.2, B3.1, B3.2): benzene's six delocalized electrons make it flat, symmetrical and unusually stable, so it undergoes substitution rather than addition. Substituted benzenes are named with lowest locants (or ortho/meta/para) and the phenyl group.

How to help: Draw a hexagon and two substituents, and ask your teen to name it two ways (1,3- and meta-). Ask why benzene does not decolourize bromine water.

Common difficulties:

Thinking the double bonds in benzene "flip back and forth". The real molecule is one fixed structure, a blend of the two drawings.
Forgetting that each ring carbon without a substituent carries one H when counting formulas.
Using numbers like 1,5 or 1,6 instead of the lowest set.
Confusing phenyl (C₆H₅–) with phenol (C₆H₅OH).

Safety: the label hunt involves reading only. Keep cleaning products closed and out of reach.