Class 12 Chemistry Haloalkanes and Haloarenes Methods of Preparation of haloalkanes

Methods of Preparation of haloalkanes

  • By halogenations of alkanes in presence of light, catalyst or heat:

  • By halogenations of alkenes with HX :

 

  • Hunsdiecker reactions:

The Hunsdiecker reaction is an instance of a halogenation reaction which involves an organic between silver salts of carboxylic acids and halogens resulting in the formation of organic halides.

  • Allylic halogenations:

 

  • Preparation of alkyl halides from alcohols:

Treatment of alcohol with HCl in the presence of anhydrous ZnCl2 phosphorous pentachloride, PX3(P4 + X2)or SOCl2 leads to the preparation of  alkyl halide.

  1. By the action of halogen acids:

1 and 2 degree alcohols require anhydrous ZnCl2 while alcohols do not require ZnCl2. Mixture of conc. HCl and anhydrous ZnCl2 is called Lucas reagent.

Primary and secondary alkyl chlorides are prepared from their respective alcohols by using HCl gas and anhydrous ZnCl2.

  1. By the action of phosphorous halides:

3ROH + PX3 à 3RX + H3PO3.

Alkyl chlorides can be synthesized by the action of PCl3 or PCl5.

 

  1. Darzen method:

Darzens halogenation is a chemical processs involving the preparation of alkyl halides from alcohols by treating with reflux of thionyl chloride or bromide (SOX2) in the presence of small quantity of a nitrogen base like tertiary amine or pyridine, or the equivalent hydrochloride.

Alkyl bromides and iodides cannot be prepared by this method. The reason behind this is that thionyl bromide is unstable and thionyl iodide does not exist. This method is preferred for preparing alkyl chlorides because here by-products are gaseous SO2 and HCl which escape easily. But this does not happen in the method involving phosphorous chloride; hence it is not used for preparing alkyl chlorides.

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