PREPARATION OF ALKYL HALIDES

PREPARATION OF ALKYL HALIDES

Mono haloalkanes or Alkyl halides can be prepared by a number of methods.Some important synthetic methods are as follows:

METHOD No. 1

BY USING ALCOHOL
AS A MAIN REAGENT

A number of reagents such as halogen acids, PCl₅, PCl₃, SOCl₂ etc. react with aliphatic alcohols to produce corresponding alkyl halides.

FROM HALOGEN ACID

Alcohols react with halogen acids (HCl ,HBr or HI) to produce alkyl halides.

R-OH + HX ® R-X + H₂O
CH_3-OH + HCl® CH3-Cl + H₂O (in the presence of ZnCl2/H2SO4)
C₂H₅OH + HI® C₂H₅I + H₂O
C₂H₅OH + HBr® C₂H₅Br + H₂O
CH₃-CH2-CH2-OH + HCl ® CH₃-CH2-CH2-Cl + H₂O

FROM PHOSPHORUS PENTA CHLORIDE (PCl₅)

When alcohols are treated with phosphorus penta chloride, corresponding alkyl chlorides are obtained.

CH3-CH2-OH + PCl₅ ® CH3-CH2-Cl + POCl₃ + HCl

FROM THIONYL CHLORIDE(SOCl2)

CH3-CH2-OH +SOCl2 ® CH3-CH2-Cl + SO2 + HCl

Above reaction is carried out in the presence of Pyridine. Pyridine absorbs HCl as it is formed.

FROM PHOSPHORUS
TRI BROMIDE (PBr₃)

3C₂H₅OH + PBr₃ ® 3C₂H₅Br + H₃PO₃

FROM PHOSPHORUS
TRI IODIDE (PI₃)

3CH₃OH + PI₃ ® 3CH₃I + H₃PO₃

METHOD No. 2

BY THE HALOGENATION
OF ALKANES

Alkanes react with chlorine and bromine in presence of sunlight and undergo photo chemical series of substitution reactions to yield alkyl halides.

CH₄ + (Cl₂) ® CH_{3Cl +}CH_{2Cl2 +}CH_{Cl3 +}C_Cl4

REMEMBER: THIS IS NOT AN APPROPRIATE METHOD FOR THE LABORATORY PREPARATION OF ALKYL HALIDES
BECAUSE THE RESULTANT MIXTURE CONTAINS POLYHALOGEN DERIVATIVES WHICH ARE DIFFICULT TO SEPARATE.

METHOD No. 3

FROM ALKENE

we know that alkenes are unsaturated hydrocarbons.Therefore when alkenes are treated with halogen acid, addition reaction takes place and alkyl halides are formed. Additionof HX takes place according to MARKONIKOV’S PRINCIPLE.

CH₂ = CH₂ + HCl ® CH₃ ? CH₂ ? Cl

CH₃-CH=CH₂ + HBr ® CH₃-CHBr-CH₃

The order of reactivity of halogen acid is as follows:

HI > HBr > HCl