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
                                              CH3-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® CH3Cl + CH2Cl2 + CHCl3 + CCl4
     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