Chemical Reactions
Chapter Name : Haloalkanes And Haloarenes |
Sub Topic Code : 102_12_10_06_06 |
Topic Name : Chemical Reactions |
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Sub Topic Name : Nucleophilic Substitution Reactions |
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Introduction
• Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons: • (i) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with p-electrons of the ring and the following resonating structures are possible. • C—Cl bond acquires a partial double bond character due to resonance. • As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction. • Difference in hybridisation of carbon atom in C—X bond: • In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, carbon atom is sp2-hybridised. • Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres. • The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the reactivity of haloarenes. • The effect is pronounced when (-NO2) group is introduced at ortho and para- positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position.
Pre-Requisites:
Elements, bond, halides
Activity:
• Synthetic halogen compounds, viz. • chloroquine is used for the treatment of malaria; • Halothane is used as an anaesthetic during surgery.
Real Life Question:
What are haloarenes and haloalkanes?
Key Words / FlashCards
| Key Words | Definitions (pref. in our own words) |
|---|---|
| - Haloalkanes - Haloarenes | • Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group. • Haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group. |
Learning aids / Gadgets
| Gadgets | How it can be used |
|---|---|
| • Chlorine containing antibiotic, chloramphenicol, produced by soil microorganisms is very effective for the treatment of typhoid fever. |
Real life uses :
• Synthetic halogen compounds, viz. ? chloroquine is used for the treatment of malaria; ? Halothane is used as an anesthetic during surgery..
Places to visit :
Pharmacy.
Practical examples around us
| Examples | Explainations |
|---|---|
| • Synthetic halogen compounds, viz. ? chloroquine is used for the treatment of malaria; ? Halothane is used as an anesthetic during surgery. |
What you learn in Theory:
• Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons: • (i) Resonance effect: In haloarenes, the electron pairs on halogen atom are in conjugation with p-electrons of the ring and the following resonating structures are possible. • C—Cl bond acquires a partial double bond character due to resonance. • As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction. • Difference in hybridisation of carbon atom in C—X bond: • In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, carbon atom is sp2-hybridised. • Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres. • The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the reactivity of haloarenes. • The effect is pronounced when (-NO2) group is introduced at ortho and para- positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position.2) Fittig reaction
What you learn in Practice:
• Haloalkanes and Haloarenes are classified based on the number of halogen compounds contained in them.