Chemical Reactions Of Haloarenes
Haloarenes are extremely unreactive towards nucleophilic substitution reactions. Due to resonance effect, difference in hybridization of carbon- halogen bond, instability of phenyl cation and repulsion between the nucleophile and the arenes. Resonance effect: In haloarenes, resonance is possible due to the conjugation of the lone pair of electrons on halogen with π - electrons of the ring. Due to this resonance, the carbon-halogen bond acquires partial double bond character. So, it becomes shorter and stronger and cannot be easily replaced by nucleophiles. The sp2 hybridised Carbon in haloarenes can hold the electron pair of the Carbon-halogen bond more tightly than the sp3 hybridised Carbon in haloalkane with less s-character. Thus, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reactions. Haloarenes undergo nucleophilic substitution reactions only under drastic conditions. Ex: Chlorobenzene converted into phenol by heating in aqueous sodium hydroxide solution at 623K and a pressure of 300 atmospheres. The reactivity of haloarenes can be increased by introducing electron withdrawing groups such as the nitro group (-NO2) in the ortho and para positions. More the number of electron withdrawing groups at the ortho and para positions the higher, is the reactivity. Ex: 2,4,6-Trinitrochlorobenzenecan be converted into 2,4,6 -trinitro phenol (or) picric acid just on warming with water. Haloarenes undergo the usual electrophilic substitution reactions such as halogenation, sulphonation and Friedel-Crafts reactions. Halogen substituents on arenes tend to direct further substitution to the ortho and para positions. The halogen atom because of its negative inductive effect show tendency to withdraw electron density from the benzene ring and tend to deactivate the ring for electrophilic substitution. Although chlorine is an electron withdrawing group, it acts as an ortho para director in electrophilic substitution reactions. The resonance effect is more pronounced at the ortho and para positions than at the meta position. The inductive effect, being stronger than resonance, causes net deactivation of the ring. The deactivating effect is lessened at the ortho and para positions due to the opposition of the inductive effect by the resonance effect at these positions. Haloarenes can be further halogenated by treating a haloarene with excess chlorine (or) bromine in the presence of a Lewis acid such as anhydrous iron (III) chloride or iron (III) bromide. Ex: If chlorobenzene is reacted with chlorine in the presence of anhydrous iron (III) chloride 1,2-dichlorobenzene and 1,4-dichlorobenzne (major product) will be formed. Haloarenes also undergo Friedel-Crafts alkylation reactions carried out by treating the haloarene with an alkyl chloride in the presence of anhydrous aluminium chloride as a catalyst. Friedel-Crafts acylation can be carried out by treating the haloarene with an acyl chloride in presence of anhydrous aluminium chloride. Haloarenes reactions with metals: In the Wurtz-Fittig reaction, a mixture of a haloalkane and a haloarene is treated with sodium in dry ether to form an alkyl arene. In the Fittig reaction, aryl halides are treated with sodium in dry ether. A new carbon-carbon bond forms between the rings forming diphenyl.

Summary

Haloarenes are extremely unreactive towards nucleophilic substitution reactions. Due to resonance effect, difference in hybridization of carbon- halogen bond, instability of phenyl cation and repulsion between the nucleophile and the arenes. Resonance effect: In haloarenes, resonance is possible due to the conjugation of the lone pair of electrons on halogen with π - electrons of the ring. Due to this resonance, the carbon-halogen bond acquires partial double bond character. So, it becomes shorter and stronger and cannot be easily replaced by nucleophiles. The sp2 hybridised Carbon in haloarenes can hold the electron pair of the Carbon-halogen bond more tightly than the sp3 hybridised Carbon in haloalkane with less s-character. Thus, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reactions. Haloarenes undergo nucleophilic substitution reactions only under drastic conditions. Ex: Chlorobenzene converted into phenol by heating in aqueous sodium hydroxide solution at 623K and a pressure of 300 atmospheres. The reactivity of haloarenes can be increased by introducing electron withdrawing groups such as the nitro group (-NO2) in the ortho and para positions. More the number of electron withdrawing groups at the ortho and para positions the higher, is the reactivity. Ex: 2,4,6-Trinitrochlorobenzenecan be converted into 2,4,6 -trinitro phenol (or) picric acid just on warming with water. Haloarenes undergo the usual electrophilic substitution reactions such as halogenation, sulphonation and Friedel-Crafts reactions. Halogen substituents on arenes tend to direct further substitution to the ortho and para positions. The halogen atom because of its negative inductive effect show tendency to withdraw electron density from the benzene ring and tend to deactivate the ring for electrophilic substitution. Although chlorine is an electron withdrawing group, it acts as an ortho para director in electrophilic substitution reactions. The resonance effect is more pronounced at the ortho and para positions than at the meta position. The inductive effect, being stronger than resonance, causes net deactivation of the ring. The deactivating effect is lessened at the ortho and para positions due to the opposition of the inductive effect by the resonance effect at these positions. Haloarenes can be further halogenated by treating a haloarene with excess chlorine (or) bromine in the presence of a Lewis acid such as anhydrous iron (III) chloride or iron (III) bromide. Ex: If chlorobenzene is reacted with chlorine in the presence of anhydrous iron (III) chloride 1,2-dichlorobenzene and 1,4-dichlorobenzne (major product) will be formed. Haloarenes also undergo Friedel-Crafts alkylation reactions carried out by treating the haloarene with an alkyl chloride in the presence of anhydrous aluminium chloride as a catalyst. Friedel-Crafts acylation can be carried out by treating the haloarene with an acyl chloride in presence of anhydrous aluminium chloride. Haloarenes reactions with metals: In the Wurtz-Fittig reaction, a mixture of a haloalkane and a haloarene is treated with sodium in dry ether to form an alkyl arene. In the Fittig reaction, aryl halides are treated with sodium in dry ether. A new carbon-carbon bond forms between the rings forming diphenyl.

Previous
Next