Preparing haloarenes and their properties

A halogen on a ring

A haloarene has the halogen bonded to a carbon of an aromatic ring. Chlorobenzene, $\text{C}_{6}\text{H}_{5}\text{Cl}$, is the simplest example. The mothball chemical 1,4-dichlorobenzene is another. In this lesson we make haloarenes and look at how they behave as substances. The next lesson looks at their reactions.

Method 1: halogenation of the ring

An aromatic ring is rich in electrons. It reacts with an electrophile in electrophilic substitution. Chlorine or bromine reacts with benzene in the presence of a Lewis acid catalyst, such as iron or iron(III) chloride, or aluminium chloride. The reaction is done in the dark, at room temperature.

$\text{C}_{6}\text{H}_{6}\ +\ \text{Cl}_{2}\ \xrightarrow{\text{FeCl}_{3}}\ \text{C}_{6}\text{H}_{5}\text{Cl}\ +\ \text{HCl}$

The catalyst polarises the halogen molecule so that one atom acts as an electrophile. Toluene reacts in the same way and gives a mixture of 2-chlorotoluene and 4-chlorotoluene, the ortho and para isomers. They have nearly equal boiling points, so they are separated with difficulty.

Compare this with the reaction of toluene with chlorine in light, which goes on the side chain and gives benzyl chloride. The conditions decide where the chlorine goes.

Iodine and fluorine. Direct iodination is reversible. The hydrogen iodide formed reduces the iodoarene back. An oxidising agent such as nitric acid or iodic acid is added to remove it. Fluorine is so reactive that the reaction is violent and cannot be controlled. These two halides are made another way.

Method 2: from diazonium salts

Aniline, $\text{C}_{6}\text{H}_{5}\text{NH}_{2}$, reacts with sodium nitrite and hydrochloric acid at $273$ to $278\ \text{K}$ to form benzenediazonium chloride. This is diazotisation. The mixture is kept cold, because the salt breaks down when warm.

$\text{C}_{6}\text{H}_{5}\text{NH}_{2}\ +\ \text{NaNO}_{2}\ +\ 2\text{HCl}\ \rightarrow \ \text{C}_{6}\text{H}_{5}\text{N}_{2}^{+}\text{Cl}^{-}\ +\ \text{NaCl}\ +\ 2\text{H}_{2}\text{O}$

The diazonium group is an excellent leaving group, since it leaves as nitrogen gas. It can be replaced by several groups.

• Sandmeyer reaction. Copper(I) chloride with HCl, or copper(I) bromide with HBr, gives chlorobenzene or bromobenzene. $\text{C}_{6}\text{H}_{5}\text{N}_{2}^{+}\text{Cl}^{-}\ \xrightarrow{\text{CuCl},\ \text{HCl}}\ \text{C}_{6}\text{H}_{5}\text{Cl}\ +\ \text{N}_{2}$
• Gattermann reaction. Copper powder with HCl or HBr gives the same products, but usually in lower yield.
• Iodine. Potassium iodide alone gives iodobenzene. No copper is needed. $\text{C}_{6}\text{H}_{5}\text{N}_{2}^{+}\text{Cl}^{-}\ +\ \text{KI}\ \rightarrow \ \text{C}_{6}\text{H}_{5}\text{I}\ +\ \text{N}_{2}\ +\ \text{KCl}$
• Fluorine (Balz-Schiemann reaction). Fluoroboric acid, $\text{HBF}_{4}$, precipitates benzenediazonium tetrafluoroborate. It is dried and heated: $\text{C}_{6}\text{H}_{5}\text{N}_{2}^{+}\text{BF}_{4}^{-}\ \rightarrow \ \text{C}_{6}\text{H}_{5}\text{F}\ +\ \text{BF}_{3}\ +\ \text{N}_{2}$.

Physical properties

Haloarenes are colourless liquids or solids with a characteristic smell. They are insoluble in water and dissolve in organic solvents. They are denser than water, except the fluoro compound, which is close to it.

The boiling point rises from fluorine to iodine, as with the haloalkanes, because the molar mass and the number of electrons rise.

Ortho, meta and para isomers. The three dichlorobenzenes have boiling points that differ by only a few kelvin. Their melting points differ a lot.

The para isomer has the highest melting point. Its molecules are the most symmetrical, so they pack closely in the crystal, and the lattice is hard to break. The ortho and meta isomers pack less well. NCERT prints $323\ \text{K}$ for the melting point of 1,4-dichlorobenzene. Reference tables give $327\ \text{K}$, which is used here.

The dipole moment. The C-Cl bond of chlorobenzene is shorter than that of chloromethane, about $169\ \text{pm}$ against $178\ \text{pm}$. The dipole moment of chlorobenzene, about $1.7\ \text{D}$, is smaller than that of chloromethane, about $1.9\ \text{D}$. The sp2 carbon holds electrons more tightly than an sp3 carbon, and it pushes less charge towards chlorine. The dipole of 1,4-dichlorobenzene is zero, because the two C-Cl dipoles are equal and opposite.

Try it in Python

The program lists all the ways to put 1 to 6 chlorine atoms on a benzene ring. It counts a pattern as new only if turning or flipping the ring cannot make it match another pattern.

Common mistakes

• Using light for ring chlorination. Light chlorinates the side chain of toluene. The Lewis acid, in the dark, chlorinates the ring.
• Expecting copper in every diazonium reaction. Potassium iodide needs none, and the fluoride needs heat.
• Calling the para isomer the one with the highest boiling point. The difference is in the melting point.
• Forgetting the temperature of diazotisation. It is $273$ to $278\ \text{K}$.

Remember these

• Ring halogenation: $\text{X}_{2}$ with $\text{Fe}$, $\text{FeCl}_{3}$ or $\text{AlCl}_{3}$, in the dark. Light gives side-chain substitution.
• Diazonium route: $\text{NaNO}_{2}$ and $\text{HCl}$ at $273$ to $278\ \text{K}$, then $\text{CuCl}$ (Sandmeyer), Cu (Gattermann), $\text{KI}$, or $\text{HBF}_{4}$ and heat (Balz-Schiemann).
• Boiling points rise from F to I. The para isomer melts highest because of its symmetry.
• 1,4-Dichlorobenzene has a dipole moment of zero.

Check yourself

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

1. How is bromobenzene prepared from aniline?

2. Why is iodobenzene not made by direct iodination of benzene?

3. Which dichlorobenzene has the highest melting point, and why?

4. Is the dipole moment of 1,4-dichlorobenzene zero or not?

References

• NCERT, Chemistry Part II for Class XII, the chapter Haloalkanes and Haloarenes, on the preparation and properties of haloarenes.
• Morrison and Boyd, Organic Chemistry, the chapters on aryl halides and on diazonium salts.
• OpenStax, Organic Chemistry, Chapter 16 (Chemistry of Benzene: Electrophilic Aromatic Substitution).