Haloarenes
HaloarenesHaloarenes are the chemical compounds containing arenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. The nature of the C-X bond depends on both...
Chlorobenzene
Chlorobenzene
1. By direct chlorination :
Chlorobenzene can be obtained by the direct chlorination of benzene in cold, dark, and in the presence of halogen carrier Lewis acids such as AlCl3, FeCl3, etc.
2. From benzene diazonium chloride :
Sandmeyer reaction :
Chlorobenzene can be prepared by treating benzene diazonium chloride with cuprous chloride dissolved in HCl. This reaction is called the Sandmeyer reaction.
Gattermann reaction :
Chlorobenzene is prepared by treating benzene diazonium chloride with copper powder dissolved in HCl. This reaction is the modification of Sandmeyer’s reaction and is called the Gattermann reaction.
Chemical Properties of Chlorobenzene
A. Reactions involving halogen atom
1. Aromatic nucleophilic substitution reaction
Haloarenes do not undergo nucleophilic substitution reactions readily. This is due to the C–X bond in aryl halide is short and strong and also the aromatic ring is a center of high electron density.
The halogen of haloarenes can be substituted by OH–, NH2–, or CN– with appropriate nucleophilic reagents at high temperatures and pressure.
For ExampleThis reaction is known as Dow’s Process
2. Reaction with metals
a) Wurtz Fittig reaction
Haloarenes react with haloalkanes when heated with sodium in ether solution to form alkylbenzene. This reaction is called Wurtz fittig reaction.
b) Fittig reaction
Haloarenes react with sodium metal in dry ether, two aryl groups combine to give biaryl products. This reaction is called fittig reaction
B) Reaction involving aromatic ring
3. Electrophilic substitution reaction
Haloarenes undergo aromatic electrophilic substitution reactions. The rate of electrophilic substitution of halobenzene is lower than that of benzene. halogen is deactivating due to the - I effect of halogen. The lone pair of electrons on the chlorine involves in resonance with the ring. It increases the electron density at the ortho and para positions. The halogen attached to the benzene ring withdraw electron and thereby and hence the halogen which is attached to the benzene directs the incoming, electrophile either to ortho or to para position in an electrophilic substitution reaction
4) Reduction
Haloarenes on reduction with Ni-Al alloy in the presence of NaOH gives corresponding arenes.
5) Formation of Grignard reagent
Haloarenes react with magnesium to form Grignard reagent in tetrahydrofuran (THF).
