Benzene, \mathrm{C_6H_6}, is a colourless liquid with a characteristic smell that boils at 80 °C. It does not mix with water, and because its molecules contain a high proportion of carbon, it burns with a smoky flame.
In the benzene molecule, six carbon atoms form a regular hexagon, and each of them is also bonded to one hydrogen atom. All twelve atoms lie in one plane, and every angle between the bonds is 120°.
The six bonds between the carbon atoms are all the same length, 0.140 nm. This ring of six carbon atoms is called a benzene ring, and compounds that contain a benzene ring are called aromatic compounds.
In 1865, Kekulé proposed that the six carbon atoms of benzene form a ring joined by single and double bonds in turn.
If this were so, the single bonds would be about 0.154 nm long and the double bonds about 0.134 nm long. In fact, all six bonds are 0.140 nm long, between the two, because six of the electrons that join the carbon atoms are not held between particular atoms but are spread evenly over the whole ring.
For this reason, the benzene ring is often drawn as a hexagon with a circle inside it. In such formulas, the carbon atoms at the corners and the hydrogen atoms bonded to them are left out.
Unlike ethene, benzene does not react with bromine on its own. When iron or iron(III) bromide is added as a catalyst, one hydrogen atom of the benzene ring is replaced by a bromine atom, and bromobenzene forms.
\mathrm{C_6H_6} + \mathrm{Br_2} \longrightarrow \mathrm{C_6H_5Br} + \mathrm{HBr}
This is a substitution reaction, like the reaction of methane with chlorine. In a substitution reaction the benzene ring stays as it is, so benzene undergoes substitution reactions more easily than addition reactions.
When benzene is warmed to about 50 to 60 °C with a mixture of concentrated nitric acid and concentrated sulfuric acid, a hydrogen atom is replaced by a nitro group, -\mathrm{NO_2}, and nitrobenzene forms. This reaction is called nitration.
\mathrm{C_6H_6} + \mathrm{HNO_3} \longrightarrow \mathrm{C_6H_5NO_2} + \mathrm{H_2O}
Nitrobenzene is a pale yellow, oily liquid that is denser than water and smells like bitter almonds. It is used to make aniline.
When benzene is heated with concentrated sulfuric acid, a hydrogen atom is replaced by a sulfo group, -\mathrm{SO_3H}, and benzenesulfonic acid forms. This reaction is called sulfonation. Benzenesulfonic acid is a strong acid and dissolves well in water.
\mathrm{C_6H_6} + \mathrm{H_2SO_4} \longrightarrow \mathrm{C_6H_5SO_3H} + \mathrm{H_2O}
Benzene undergoes addition reactions only under special conditions. When benzene is heated with hydrogen at high pressure, using nickel as a catalyst, hydrogen adds to the ring and cyclohexane forms.
\mathrm{C_6H_6} + 3\mathrm{H_2} \longrightarrow \mathrm{C_6H_{12}}
When benzene and chlorine are exposed to ultraviolet light, chlorine adds to the ring and 1,2,3,4,5,6-hexachlorocyclohexane forms. This compound was once used as an insecticide.
\mathrm{C_6H_6} + 3\mathrm{Cl_2} \longrightarrow \mathrm{C_6H_6Cl_6}