Physical and chemical property of Haloarenes
By Anup Pokhrel
Haloarenes are colourless liquid or crystalline solids.
Haloarenes are the non-polar compound which cannot form the hydrogen bond with water molecules. As a result, there are insoluble in water but soluble in organic solvents.
c. Melting and boiling point.
Boiling points of haloarenes increase with an increase in their molecular weight.
Among the isomeric haloarenes para isomer due to its symmetrical structure,fits more property in its crystal lattice than O-and M isomer. Hence, P-haloarenes have the highest Melting point.
The chemical property of Haloarenes.
1. Nucleophilic substitution reaction.
It is typical of haloarenes that they go nucleophilic substitution reaction only with extreme difficulty.
They show reactivity of haloarenes with nucleophiles can be explained on the basis of following resonating structure.
This resonance structure indicates that haloarenes develop partial double bond characteristic in C-Cl bond. Due to these reasons, a halogen atom is not cleaved easily as halide ion by a nucleophile.
SP2 hybridised carbon of C-Cl bond in haloarene pulls electron strongly from halogen. Therefore release of halogen strongly ion is difficult. Similarly, SP2 hybridised leads to less polarity of C-Cl bond. Hence, haloarenes react very slowly towards nucleophile.
Some typical nucleophilic substitution reaction of haloarenes are as follows.
a. Substitution of -OH group.
When haloarenes are heated with caustic soda at 3000C and 200atm pressure, sodium phenoxide is formed phenol on acidification.
b. substitution of NH2 group.
Haloarenes produce aniline on heating with ammonia at 2000C under pressure in presence of Cu2O.
c.Substitution of -C.N group.
Haloarenes reacts with cuprous cyanide at 2000C under pressure in presence of pyridine or DMF to form benzonitrile.
2. Electrophilic substitution reaction.
Haloarenes react easily with electrophile due to the formation of negative charge in the benzene ring by the strong resonance effect.
Haloarenes are ortho and para directing towards electrophilic substitution reaction. This can be explained on the basis of the following resonating structure.
The resonance structure (I,II,III) show that a haloarene develops partial negative charge only at ortho and para positions i,e electron density is higher at these positions in comparison to meta position. Therefore, as incoming electrophile attacks at ortho position and para position. Hance, haloarene are O-P directing towards electrophilic substitution reaction.
3. Reaction with metal.
Haloarenes produce diphenyl on heating with sodium in presence of dry ether. This reaction is called fitting reaction.
2. Wurtz fitting reaction.
When haloarenes and haloalkanes are heated with sodium metal in presence of dry ether, alkyl benzenes are obtained. This reaction is termed as Wurtz fitting reaction.
b. With magnesium.
Haloarenes react with metallic magnesium in presence of dry ether to produce an organic metallic compound (ArMgX) as Grignard reagent.
Haloarenes are reduced to their corresponding aromatic hydrocarbon in alkaline medium.