Benzene: structure, resonance and aromaticity

A smell, a lamp and a puzzle

Mothballs, the smell of some oils and the black residue of coal tar all contain compounds with a special kind of ring. They were called aromatic because many of them smell. Michael Faraday first isolated the simplest of them in 1825, from the oily liquid that collected in the lamps that burned gas from coal. It is benzene.

Benzene is a colourless liquid. It melts at $5.5\ ^{\circ}\text{C}$ and boils at $80.1\ ^{\circ}\text{C}$. It is nonpolar, does not mix with water and floats on it. It dissolves most organic compounds. It burns with a sooty, smoky flame, because it is rich in carbon: $92.3$ per cent by mass.

The formula is $\text{C}_{6}\text{H}_{6}$. The degree of unsaturation is $\dfrac{2 \times 6 + 2 - 6}{2} = 4$. That fits a ring and three double bonds. So we expect it to behave like an alkene. It does not.

What benzene does not do

• It does not decolourise bromine water, and it does not react with cold alkaline permanganate. An alkene does both at once.
• It mostly undergoes substitution, not addition, so that the ring stays.
• Only one monobromobenzene is known.
• Only one 1,2-dibromobenzene is known. If benzene had fixed single and double bonds, there should be two: one with a double bond between the two carbon atoms that bear bromine, and one with a single bond.

Kekulé and resonance

In 1865 August Kekulé proposed a ring of six carbon atoms with alternating single and double bonds. To explain the single 1,2-dibromobenzene, he said the double bonds swap places very fast.

We now say that benzene is a resonance hybrid of the two Kekulé structures. As you saw in chapter 8, the two structures are not real. The molecule is one thing, a hybrid. It does not flip between two forms.

This picture fits the facts.

• All six carbon atoms are $\text{sp}^2$ hybridised. The ring is a flat regular hexagon, with all angles $120^{\circ}$.
• All six C-C bonds are the same length, $139\ \text{pm}$. That lies between a single bond, $154\ \text{pm}$, and a double bond, $134\ \text{pm}$.
• Each carbon has a $p$ orbital at right angles to the ring. The six $p$ orbitals overlap sideways all round the ring. The six pi electrons are delocalised in two doughnut-shaped clouds, one above the ring and one below.

We draw the hybrid as a hexagon with a circle inside.

How much more stable is benzene

Adding hydrogen to cyclohexene, with one double bond, gives out $120\ \text{kJ/mol}$. If benzene had three separate double bonds, it would give out $3 \times 120 = 360\ \text{kJ/mol}$. In fact it gives out only $208\ \text{kJ/mol}$. Benzene has less energy than we expect, by

This is the resonance energy or delocalisation energy. It is often quoted as about $150\ \text{kJ/mol}$. It is why benzene keeps its ring in its reactions: an addition would destroy this stability.

Practise with new numbers

Aromaticity and Hückel's rule

Benzene is the first of a large family. A compound is aromatic when its molecule is

1. cyclic,
2. planar,
3. fully conjugated, so that every atom of the ring has a $p$ orbital, and
4. has $(4n + 2)$ pi electrons, where $n = 0, 1, 2, 3, \dots$

The last condition is Hückel's rule. It allows 2, 6, 10 and 14 pi electrons.

• Benzene has 6 pi electrons ($n = 1$). Naphthalene has 10 ($n = 2$), and anthracene has 14 ($n = 3$).
• The cyclopentadienyl anion has 6. The tropylium cation has 6.
• Pyridine has 6 pi electrons in a ring with one nitrogen atom. The lone pair of nitrogen lies in the plane of the ring and is not part of the pi cloud.
• Pyrrole, furan and thiophene are aromatic too. The atom in the ring puts its lone pair into the pi cloud, and the total is 6.
• Cyclobutadiene has 4 pi electrons, which is $4n$. It is called antiaromatic, and it is very unstable.
• Cyclooctatetraene has 8. It avoids the problem by twisting into a tub shape, which is not planar. It behaves as an ordinary alkene.

Why $4n + 2$? The pi levels of a ring are one lowest level, and then pairs of equal levels. Two electrons fill the first. Each pair then takes four more: 2, 6, 10, 14. With $4n$ electrons, two electrons are left alone in a pair of equal levels. The program below shows this.

Other names and isomers

Substituents on benzene get the usual names: bromobenzene, nitrobenzene and methylbenzene. The common name of methylbenzene is toluene. For two groups, the positions are ortho (1,2), meta (1,3) and para (1,4), or the numbers can be used. The three dimethylbenzenes are the xylenes. A group such as the benzene ring itself, when it is a substituent, is called phenyl.

The formula $\text{C}_{8}\text{H}_{10}$ has four aromatic isomers: ethylbenzene and the three xylenes.

Making benzene

• From ethyne. Three molecules join when ethyne is passed through a red-hot iron tube at about $873\ \text{K}$.
• From sodium benzoate. Heating with soda lime removes the carboxyl group, as in the preparation of alkanes: $\text{C}_{6}\text{H}_{5}\text{COONa}\ +\ \text{NaOH}\ \longrightarrow \ \text{C}_{6}\text{H}_{6}\ +\ \text{Na}_{2}\text{CO}_{3}$.
• From phenol. Zinc dust removes the OH group: $\text{C}_{6}\text{H}_{5}\text{OH}\ +\ \text{Zn}\ \longrightarrow \ \text{C}_{6}\text{H}_{6}\ +\ \text{ZnO}$.

Benzene is harmful. Breathing it over long periods damages the blood and the bone marrow, and it can cause leukaemia. It is classed as a human carcinogen. Laboratories use toluene as a solvent in its place.

Try it in Python

The program fills the pi levels of seven rings, using Hückel's picture. It counts the unpaired electrons and compares the result with the $4n + 2$ rule.

Common mistakes

• Drawing benzene as an alkene. It is a delocalised ring, and it keeps the ring in its reactions.
• Thinking the structures flip. The molecule is one hybrid. The two Kekulé structures are pictures.
• Counting the lone pair of pyridine as a pi electron. It lies in the plane of the ring. In pyrrole the lone pair is in the pi cloud.
• Applying the rule to a ring that is not flat. Cyclooctatetraene has 8 electrons but twists out of the plane.
• Confusing $4n + 2$ with $4n$. $4n$ gives antiaromatic, $4n + 2$ gives aromatic.

Remember these

• Benzene: $\text{C}_{6}\text{H}_{6}$, flat, six $\text{sp}^2$ carbons, six equal C-C bonds of $139\ \text{pm}$, a delocalised cloud of six pi electrons.
• Resonance energy is about $150\ \text{kJ/mol}$, found from hydrogenation heats.
• Aromatic: cyclic, planar, fully conjugated, $4n + 2$ pi electrons.
• Benzene prefers substitution to addition. It is made from ethyne, from sodium benzoate with soda lime, and from phenol with zinc.
• Four aromatic isomers have the formula $\text{C}_{8}\text{H}_{10}$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why are all the C-C bonds of benzene the same length?

2. Is the cyclopentadienyl anion aromatic?

3. Why does cyclooctatetraene not behave like benzene?

4. How can you make benzene from sodium benzoate?

5. How many aromatic isomers does $\text{C}_{8}\text{H}_{10}$ have?

References

• OpenStax Chemistry 2e, chapter 20, section 20.1 "Hydrocarbons" (aromatic hydrocarbons).
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons" (aromatic hydrocarbons: structure, resonance, aromaticity).
• OpenStax Organic Chemistry (McMurry), chapter 15, "Structure and Stability of Benzene" (heats of hydrogenation). NCERT prints about $150.4\ \text{kJ/mol}$ for the resonance energy.