Preparation of haloarenes

Preparative methods of haloarenes are:

Direct halogenation of aromatic ring

Direct chlorination or bromination of benzene or other aromatic hydrocarbon gives chloroarenes and bromoarenes. These reactions are carried out in the presence of Lewis acids such as ferric or aluminium halides (FeCl₃, FeBr_3, AlCl₃) in the dark, at ordinary temperatures (310-320 K).

The Lewis acid acts as a catalyst or a halogen carrier, as its function is to carry the halogen to the aromatic hydrocarbon. In actual practice, iron filings in the presence of chlorine or bromine are commonly used. The halogens react with iron filings to form corresponding Lewis acids.

Halogenation of arenes is an electrophilic substitution reaction. The function of the halogen carrier is to generate 'electrophile' which attacks the benzene ring to form the product.

With excess halogen, the second halogen also gets introduced in the ring at ortho and para positions with respect to the first halogen because halogens are ortho and para directing groups.

Similarly, chlorination of toluene gives a mixture of ortho and para chlorotoluene because -CH₃ group in toluene is ortho and para directing.

The reaction with fluorine is violent and cannot be controlled. Therefore, fluoroarene cannot be prepared by direct fluorination of aromatic hydrocarbon. Bromo compounds can be prepared in a similar way by reacting with Br₂ in the presence of FeBr₃. Iodoarenes are also difficult to prepare by direct iodination because the reaction is reversible and HI produced is a strong reducing agent to reduce iodobenzene back to benzene.

This reaction is carried out in the presence of an oxidizing agent like iodic acid or nitric acid, which oxidizes HI formed in the reaction to iodine enabling the reaction to proceed in the forward direction.

Preparation of aralkyls

Direct halogenation of aromatic hydrocarbon produces these compounds. For example, when chlorine gas is passed through toluene in the presence of sunlight, substitution takes place in the side chain.

The other two hydrogen atoms of side chain also get substituted by halogen atoms if Cl₂ is passed in excess.

Sandmeyer's reaction

This reaction consists of treating freshly prepared diazonium salt solution with cuprous chloride or cuprous bromide dissolved in corresponding halogen acids. Chloro and bromoarenes are formed. Diazonium salts required for this purpose are prepared by treating ice-cold solution of aniline in excess of dilute HCl with an aqueous solution of sodium nitrite at low temperature (0-5^oC). This reaction is known as diazotization reaction.

The benzene diazonium salt is used for preparing aryl halides as:

The Sandmeyer reaction has been modified to 'Gattermann reaction'. Copper powder is used in the presence of corresponding halogen acid (HCl or HBr) in place of cuprous halide (CuCl, CuBr).

Warming the diazonium salt solution with aqueous KI solution by the above method gives iodoarenes. This is the best method for introducing iodine into benzene ring.

Balz-Schienmann reaction

This method is used to prepare fluoroarenes by treating benzene diazonium chloride with fluoroboric acid. The reaction produces diazonium fluoroborate which, on heating, produces fluorobenzene.

Physical properties of haloarenes or aryl halides

• Haloarenes are colorless liquids or crystalline solids.

• Aryl halides are heavier than water.

• Being heavier than water, they are insoluble in water but soluble in organic solvents.

• The melting and boiling points of aryl halides are nearly the same as those of alkyl halides containing the same number of carbon atoms. Boiling points of monohalogen derivatives of benzene are in the order: iodo > bromo > chloro > fluoro

For the same halogen atom, the melting and boiling points increase as the size of the aryl group increases.

The boiling points of isomeric dihaloarenes are nearly the same, but their melting points are quite different because in symmetrical structure, molecules are closely packed in the crystal lattice. As a result, intermolecular forces of attraction are stronger and therefore, greater energy is required to break lattice. So it melts at higher temperatures.