Benzene and Aromatic Compounds

What is it?

Benzene, $\ce{C6H6}$ , is a ring of six carbon atoms, each bonded to one hydrogen. It is the parent of the aromatic compounds (arenes), a huge family that includes aspirin, paracetamol, many dyes and polystyrene.

In 1865 August Kekulé proposed a ring of alternating single and double bonds. But benzene does not behave like a molecule with three C=C bonds:

all six C–C bonds are the same length, 139 pm, between a single bond (154 pm) and a double bond (134 pm);
it is much more stable than expected;
it does not decolourize bromine water, and it reacts by substitution, not addition.

The modern picture: each carbon is sp² hybridized, and the six leftover p orbitals overlap side-on all around the ring. The six π electrons are delocalized above and below the ring, shared by all six carbons. This is drawn as a hexagon with a circle inside.

Key idea

Delocalizing the six π electrons over the whole ring makes benzene unusually stable (by about 152 kJ/mol). Reactions that keep the ring intact (substitution) are therefore favoured over reactions that would destroy it (addition).

Why does it matter?

Medicines and materials. Aspirin, paracetamol, ibuprofen, many antibiotics, dyes, explosives (TNT) and plastics (polystyrene, PET) all contain benzene rings.
Biology. Several amino acids (phenylalanine, tyrosine, tryptophan) contain aromatic rings, as do the bases of DNA.
Health. Benzene itself is toxic and causes cancer, so it has been replaced as a solvent by methylbenzene (toluene) in most uses.

How does it work?

1. Evidence from enthalpies of hydrogenation

Adding hydrogen to one C=C in cyclohexene releases 120 kJ/mol. If benzene had three separate C=C bonds, hydrogenating it to cyclohexane should release about $3 \times 120 = 360$ kJ/mol. The measured value is only 208 kJ/mol, so benzene is about 152 kJ/mol more stable than the Kekulé structure: the delocalization (resonance) energy.

2. Naming aromatic compounds

Formula	Name	Common name
$\ce{C6H5CH3}$	methylbenzene	toluene
$\ce{C6H5OH}$	phenol	phenol
$\ce{C6H5NH2}$	phenylamine	aniline
$\ce{C6H5COOH}$	benzoic acid	benzoic acid
$\ce{C6H5NO2}$	nitrobenzene	nitrobenzene
$\ce{C6H5Cl}$	chlorobenzene	chlorobenzene

With two groups, number the ring carbons: 1,2- (adjacent), 1,3- or 1,4- (opposite), for example 1,4-dimethylbenzene.

3. Electrophilic substitution

The electron-rich ring attracts electrophiles (electron-pair acceptors, E⁺). An electrophile replaces a hydrogen atom, and the ring stays aromatic:

\ce{C6H6 + E+ -> C6H5E + H+}

Reaction	Reagents and conditions	Product
Nitration	concentrated $\ce{HNO3}$ + concentrated $\ce{H2SO4}$ , about 50 °C	nitrobenzene
Halogenation	$\ce{Br2}$ with an $\ce{FeBr3}$ (or Fe) catalyst	bromobenzene + HBr
Friedel–Crafts alkylation	haloalkane with an $\ce{AlCl3}$ catalyst	alkylbenzene
Friedel–Crafts acylation	acyl chloride with an $\ce{AlCl3}$ catalyst	aromatic ketone

Unlike alkenes, benzene needs a catalyst to react with bromine, because its stable delocalized ring is reluctant to give up its electrons.

Think of it like this

The six π electrons in benzene are like six people sharing a round table instead of sitting in three fixed pairs. Shared around the whole table, everyone is more comfortable (more stable). Swapping one diner for a newcomer (substitution) keeps the table intact; breaking the table apart (addition) costs too much.

More precisely

A ring is aromatic when it is planar, cyclic and fully conjugated, with $4n + 2$ π electrons (Hückel’s rule; benzene has 6, with $n = 1$ ). In nitration, the electrophile is the nitronium ion, $\ce{NO2+}$ , formed when sulfuric acid protonates nitric acid. Groups already on the ring change both the rate and the position of further substitution: $\ce{-OH}$ and $\ce{-CH3}$ make the ring more reactive and direct new groups to the 2- and 4-positions, while $\ce{-NO2}$ makes it less reactive and directs to the 3-position.

Visualise it

Left: two Kekulé structures with alternating double bonds, and the delocalized structure, a hexagon with a circle inside. Right: an energy diagram for hydrogenation. Cyclohexene releases 120 kilojoules per mole. A hypothetical cyclohexa-1,3,5-triene with three double bonds would release 360. Benzene actually releases only 208, so it lies 152 kilojoules per mole lower: the delocalization energy. — Benzene is more stable than three separate double bonds would suggest.

Worked example

Worked example: The delocalization energy

Question: Use ΔH(hydrogenation) of cyclohexene, −120 kJ/mol, and of benzene, −208 kJ/mol, to estimate how much more stable benzene is than the Kekulé structure.

Expected for three C=C bonds: $3 \times (-120\ \text{kJ/mol}) = -360\ \text{kJ/mol}$
Difference:
$\small\begin{aligned} &-208\ \text{kJ/mol} \\[4pt] &\quad - (-360\ \text{kJ/mol}) \\[4pt] &= +152\ \text{kJ/mol} \end{aligned}$
Benzene releases 152 kJ/mol less energy, so it is about 152 kJ/mol more stable.

Worked example: Alkene or arene?

Question: Bromine water is shaken separately with cyclohexene and with benzene. What is seen, and why?

Cyclohexene decolourizes bromine water quickly: bromine adds across the C=C. Benzene gives no change at room temperature without a catalyst: addition would destroy the stable delocalized ring.

Worked example: Yield of nitrobenzene

Question: 7.80 g of benzene is nitrated: $\ce{C6H6 + HNO3 -> C6H5NO2 + H2O}$ . Calculate the theoretical yield of nitrobenzene, and the mass obtained at an 80.0 % yield.

Moles of benzene:
$\begin{aligned} &n = \frac{7.80\ \text{g}}{78.11\ \text{g/mol}} \\[4pt] &= 0.09986\ \text{mol} \end{aligned}$
1 : 1, so the theoretical yield is:
$\small\begin{aligned} &0.09986\ \text{mol} \\[4pt] &\quad \times 123.11\ \text{g/mol} \\[4pt] &= 12.3\ \text{g} \end{aligned}$
At 80.0 %: $0.800 \times 12.29\ \text{g} = 9.83\ \text{g}$

Common mistake

Common mistake: Drawing benzene as cyclohexene with three double bonds

The Kekulé structure is a useful drawing, but benzene does not have three short and three long bonds. All six bonds are identical; the circle notation shows the delocalized π electrons.

Common mistake: Expecting addition reactions

Benzene reacts mainly by substitution. Addition (for example, hydrogenation to cyclohexane) is possible but needs harsh conditions.

Common mistake: Forgetting the catalyst

Bromine reacts with benzene only with a halogen carrier catalyst such as $\ce{FeBr3}$ ; nitration needs concentrated sulfuric acid to make the $\ce{NO2+}$ electrophile.

Notation note

$\ce{C6H5-}$ is the phenyl group (Ph). Phenol is $\ce{C6H5OH}$ ; phenylamine is $\ce{C6H5NH2}$ .
“Aromatic” in chemistry means a stable conjugated ring, not a pleasant smell (though many aromatic compounds do smell).

Remember this

Benzene: planar ring of 6 sp² carbons, 6 delocalized π electrons; all C–C bonds 139 pm.
Evidence: equal bond lengths; ΔH(hydrogenation) −208 kJ/mol instead of −360 kJ/mol: about 152 kJ/mol extra stability.
Benzene reacts by electrophilic substitution, keeping the ring intact; it does not decolourize bromine water.
Nitration: conc. HNO₃ + conc. H₂SO₄, about 50 °C. Bromination: Br₂ + FeBr₃. Friedel–Crafts: AlCl₃ catalyst.

Practise this topic with flashcards and a quiz at chemistryclarity.com/chemistry/benzene-and-aromatic-compounds/