Methods Of Preparation
Chapter Name : Haloalkanes And Haloarenes |
Sub Topic Code : 102_12_10_04_01 |
Topic Name : Methods Of Preparation |
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Sub Topic Name : From Alcohols |
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Introduction
1.The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively. 2. Alkyl halides are best prepared from alcohols, which are easily accessible. 3. The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride. 4. Thionyl chloride is preferred because the other two products are escapable gases, hence the reaction gives pure alkyl halides. 5. Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively. 6.The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a solution of alcohol in concentrated aqueous acid. 7.The reactions of primary and secondary alcohols with HX require the presence of a catalyst, ZnCl2. 8.With tertiary alcohols, the reaction is conducted by simply shaking with concentrated HCl at room temperature. 9.Constant boiling with HBr (48%) is used for preparing alkyl bromide. 10.Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% phosphoric acid.
Pre-Requisites:
Elements, bond, alcohols
Activity:
1. Synthetic halogen compounds, viz. 2. Chloroquine is used for the treatment of malaria. 3. Halothane is used as an anesthetic during surgery.
Real Life Question:
What are haloarenes and haloalkanes ?
Key Words / FlashCards
| Key Words | Definitions (pref. in our own words) |
|---|---|
| Haloarenes | Haloarenes contain halogen atom(s) attached to sp2 hybridized carbon atom(s) of an aryl group. |
| Haloalkanes | Haloalkanes contain halogen atom(s) attached to the sp3 hybridized carbon atom of an alkyl group. |
Learning aids / Gadgets
| Gadgets | How it can be used |
|---|---|
| chloramphenicol | Chlorine containing antibiotic, chloramphenicol, produced by soil microorganisms is very effective for the treatment of typhoid fever. |
Real life uses :
1. Synthetic halogen compounds, viz. 2. Chloroquine is used for the treatment of malaria. 3. Halothane is used as an anesthetic during surgery.
Places to visit :
pharmacy
Practical examples around us
| Examples | Explainations |
|---|---|
| halothane | halothane is used as an anesthetic during surgery. |
| chloroquine | chloroquine is used for the treatment of malaria. |
What you learn in Theory:
1.The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively. 2. Alkyl halides are best prepared from alcohols, which are easily accessible. 3. The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride. 4. Thionyl chloride is preferred because the other two products are escapable gases, hence the reaction gives pure alkyl halides. 5. Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively. 6.The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a solution of alcohol in concentrated aqueous acid. 7.The reactions of primary and secondary alcohols with HX require the presence of a catalyst, ZnCl2. 8.With tertiary alcohols, the reaction is conducted by simply shaking with concentrated HCl at room temperature. 9.Constant boiling with HBr (48%) is used for preparing alkyl bromide. 10.Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% phosphoric acid.
What you learn in Practice:
Haloalkanes and Haloarenes are classified based on the number of halogen compounds contained in them